Hadar

The coming attractions bin at Journal of Human Evolution includes a paper by Kaye Reed, reviewing the evidence of paleoenvironment in the Hadar formation:

Habitat reconstructions of 12 submembers of the Hadar and Busidima formations (˜3.8-2.35 Ma) are presented here along with faunal differences in these submembers through time. Habitats with medium density tree and bush cover dominated the landscape through much of the earlier time period in the Hadar Formation. The lowermost Sidi Hakoma Member is the most closed habitat. The Denen Dora Member shows the influence of frequent floodplain edaphic grasslands with high abundances of reducin bovids. There is an influx of ungulates in the Kada Hadar Member (˜3.2-˜2.96 Ma) that indicates a more arid habitat populated by mammals that were recovered from earlier deposits further south in Ethiopia and Kenya. In the younger deposits from the Busidima Formation at Hadar, the landscape was open wooded grassland with some floodplain environments. The fossil assemblages from the Busidima Formation show a substantial species turnover. Although high numbers of A. afarensis specimens are associated with the lower Sidi Hakoma Member, they clearly inhabited a variety of habitats throughout the entire Hadar Formation. Australopithecus afarensis from Laetoli through Hadar times appears to have been a eurytopic species.

This is a nicely detailed paper, focusing on the amount of wooded/bush habitat, the relation of the hominids to those habitats, and the relative lack of early faunal exchanges with areas further to the south.

The discussion focuses on the range of paleoecologies in which fossil A. afarensis has been found -- including not only Hadar but also nearby Maka and Dikika, and more distant Koobi Fora and Laetoli. Altogether, these localities cover a long time (from before 3.5 up to around 2.9 million years ago). From the range of paleoecologies reconstructed in this paper at Hadar, Reed concludes that A. afarensis did not have a "narrow" habitat preference. It is found in relatively closed woodland, open woodland/bush, and wet grassland/marshland.

There are some differences between localities. At Koobi Fora, relatively few specimens of A. afarensis have been found in the Tulu Bor Member, despite the fact that it occupies the same time as the Hadar sequence. Based on the paleoecological data, Reed suggests that Hadar was a wetter, more closed woodland habitat than Koobi Fora at that time -- Koobi Fora would have included more scrubland punctuated with wetlands and floodplains (here she cites her own 1997 paper).

The early end of the A. afarensis sample is represented at Laetoli. Reed gives a brief review of the paleoecology of that site, which has been interpreted differently by different authors but broadly appears to have had a fairly high amount of rainfall and some patches of forest amid closed woodland:

Thus, the earliest known A. afarensis material was found in deposits showing habitats in which trees and or bushes were fairly plentiful. It is also interesting to note that while the deposits of A. afarensis at Laetoli and Hadar share some perissodactyls, giraffids, suids, and proboscideans, the bovid taxa and those primates other than A. afarensis are not very similar.

Reed concludes that A. afarensis was a "eurytopic" species -- one that inhabited a wide range of habitats and moved broadly across space. It contrasts with the more habitat-selective ("stenotopic") species, which include most of the bovids.

White et al. (1993) suggested broad habitat tolerance for A. afarensis, and indeed, the species has thus far been recovered from regions in which the reconstructed habitat ranges from closed woodland through more open, but wet woodland and shrubland. There is no direct evidence that A. afarensis only existed in riverine forests or grassland habitats, or that they preferred one habitat over another. It is tempting to equate the aridification in the Kada Hadar Member with the extinction of A. afarensis. However, sediments at Hadar are sparse or missing altogether from ˜2.90-2.35 Ma thus obscuring details of the species' demise. All that can be said is that they are no longer present at 2.35 Ma and most of the fauna, including hominins, has been replaced.

References:

Reed KE. 2008. Paleoecological patterns at the Hadar hominin site, Afar Regional State, Ethiopia. J Hum Evol (in press) doi:10.1016/j.jhevol.2007.08.013

In the current Science, Elizabeth Pennisi reports on Ethiopia's efforts to bring more resources and tourism to its fossil heritage:

Then in 2003, that lab [at the National Museum, built in 1982] was razed to make way for a six-story, modern structure that includes a two-floor library, a 500-person auditorium, and 200 rent-free offices, plus storage and study space for more than a million specimens. The three wings are devoted to paleontology and archaeology; art and history; and administrative, conservation, and educational spaces.

...

Foreign aid is helping: France is supplying furniture, and Japan may outfit the hominid spaces. Everyone involved is thrilled and not just with the prospect of more space. "It shows how much emphasis has been given [to research]," says Ethiopian native and paleoanthropologist Yohannes Haile-Selassie of the Cleveland Museum of Natural History in Ohio. "In a country that has a lot of needs, the government could have easily used that money for something else."

The article includes a map showing the impressive array of 25 active archaeological or paleontological field sites across the country. The funding and effort are also being devoted to training students -- not only in paleontology but across the sciences -- including the establishment of new universities.

One of the more controversial areas is tourism -- including the encouragement of travel to active research areas:

At the same time, "one of the most important things that needs to happen is the integration of tourism and science," says [anthropologist Tim] White. And that, too, is happening. National Geographic has pledged support for an educational center at the village nearest to Hadar, home of the 3.2-million-year-old Lucy. With better roads under construction, "it could easily be a destination spot for tourists," [anthropologist Donald] Johanson predicts. Exhibit plans are still taking shape, but there likely will be casts of Lucy and other fossil hominids, as well as photographs from the site.

This raises fears (by some) that the sites will be more vulnerable to destruction by trampling and looting. On the other hand, bringing a better transportation infrastructure is bound to improve matters for the scientific research teams, possibly including the most enduring problem -- security.

Personally, I wonder whether such projects can be a financial success. Many of the paleontological museums in the American West are a great experience for visitors because you can get close to the original (and often still-active) sites, see original specimens, and do it all without the huge crowds of urban natural history museums. But that's the point: there are no huge crowds of people. These are substantial tourist draws for small towns in the West, but they aren't often making back the substantial federal or state grants that help to build them. It's an important cultural resource and a valuable investment, but it may not be realistic to expect small regional Ethiopian museums to draw premium paleo-tourist dollars.

References:

Pennisi E. 2008. Rocking the cradle of humanity. Science 319:1182-1183. doi:10.1126/science.319.5867.1182

I've been trying to spread the interviews across the field in various directions. I (virtually) talked with Mica Glantz about Neandertals, Adam Van Arsdale about early Homo, and Anne Weaver about human brain evolution, all the australopithephiles in the readership are probably feeling neglected.

So I wrote to Michelle Drapeau, who was very generous in answering questions about her work on the anatomy of early hominids and her recent field work in Ethiopia. Michelle is on the faculty of the Université de Montréal, in the Department of Anthropology. She serves as co-director of field operations in the Bala Paleoanthropological Research Area of southern Ethiopia.

Hawks: I will start out by asking about your dissertation work, which centered on the new partial skeleton from Hadar, A.L. 438-1. How did you get involved in that analysis?

Drapeau: It's a case of being at the right place at the right time. Bill Kimbel and Don Johanson had asked my advisor at the time, Carol Ward, to describe all the postcranial material recovered from the field in Hadar since 1990. Among those specimens was the partial skeleton of A.L. 438-1 which included associated fragments of the humerus, clavicle, radius, right ulna, mandible, and frontal as well as a complete left ulna, right and left second metacarpals and left third metacarpal. Considering the relatively numerous body parts from one individual, Carol thought the specimen deserved a more detailed analysis. I was Carol's Ph.D. student at the time and the 438-skeleton (as we started to call it) appeared like an ideal subject.

Hawks: What did you have to learn to be able to undertake the work?

Drapeau: I had to learn a lot! My master's thesis was in the history of science field, so all the functional anatomy, including the descriptive and comparative aspects were completely new to me. It was something I really wanted to do, however, so I really enjoyed immersing myself into it.

Hawks: A.L. 438-1 exhibits more curvature across its length than A.L. 288-1, an issue that you discussed in your analysis of the fossil. I have always been puzzled by the problem of ulna curvature -- mainly because I've always been puzzled by the comparison of later, larger, and more curved fossils like Omo L40-19 and OH 36 -- and then, of course, KNM-WT 15000 is a lot more like most recent humans. Do you have any insights about these contrasting morphologies?

Drapeau: Forearm bone curvature is an intriguing issue. Intuitively, it makes sense to assume that curvature reflects arboreality since the curvature of both the ulna and radius give greater area on the interosseous membrane for attachment of forearm muscle important for arboreal locomotion such as the finger flexors. However, orangutans and gibbons do not have the most curved forearm bones. It is an honor that goes to gorillas, definitely not the most arboreal animal of the bunch. If the area of muscle attachment is the variable that interests us, then it is important to take into account forearm length as well. When that is done, species generally sort by locomotor preference, with the most arboreal having the greater ‘area' for muscle attachment relative to body size and humans having the smallest (at least, when measured on the ulna). So gorillas appear to have very curved forearm bones because they also have relatively short forearms when compared to other apes.

The differences between A.L. 438-1 and A.L. 288-1 are fairly minor and probably reflect normal within-species variation. Neither is very curved and they may belong to a population with slightly more curved ulnae than modern humans but definitely less curved than any extant apes.

The KNM-WT 15000 specimen is pretty much what you would expect an ulna belonging to a completely terrestrial biped to look like, i.e., it is not particularly curved. Since it is a juvenile, it is difficult to compare it to other fossils, but there is nothing really surprising about it.

That said, what about the intriguing Omo L40-19 and OH 36? These specimens present combination of morphologies that are difficult to underscore in quantitative analyses. The former had a human-like proximal morphology but a really long and curved (ape-like) diaphysis. The latter, OH 36, has a general ape-like morphology with a pronounced curvature, but is unique in a few characters. The whole bone (proximal articulation and diaphysis) is very constricted medio-laterally, more comparable what is observed in monkeys (and it is not the result of distorsion). Despite its general ape-like morphology, it has an olecranon process that projects proximally like no other ape of its size. It is definitely much more human-like for that trait and it is generally agreed that it is a hominin. McHenry and colleagues argue in a recent article (AJPA, 134: 209-218) that these two fossils are very different and can hardly be accommodated into the same genus (Paranthropus) as it is usually done (probably by default). McHenry and colleagues argue that it may indicate Paranthropus is in fact a polyphyletic taxon. They also conclude, as I stated above, that OH 36 is unlike anything living today.

So, if curvature of the ulna reflects arboreality, does it mean that these fairly recent fossils were much more arboreal than A. afarensis? Remember that they are big ulnae, particularly L40-19, likely belonging to large individuals.... Maybe the Paranthropus clade (if indeed it is a clade) is more arboreal than A. afarensis? This would imply either reversal of behavior or that A. afarensis is not ancestral to Paranthropus. Or, alternatively, could the curvature in these individuals reflect forelimb muscularity but not necessarily related to arboreality? As you can see, I have many more questions than answers. All this variability suggests that the behaviors of fossil hominin species were much more variable than what we have been used to think and may have been (very?) different from the behaviors of extent species.

Hawks: Of course, the big debate about forelimb proportions is the idea that they may have been very different (and more apelike) in A. africanus compared to A. afarensis. (reviewed by Green, Gordon, and Richmond 2007) What do you think about the issue?

Drapeau: That idea first met with some resistance because it involved a reversal of proportions from A. afarensis to A. africanus and implied a more arboreal behavior in the latter than the former. Given that Homo habilis is often described has having more ape-like proportions than A. afarensis, it also implied that A. afarensis may not be the ancestor of the Homo lineage (an idea more recently suggested by Yoel Rak and colleagues based on mandibular data). Since I remain unconvinced of the primitive proportion of H. habilis, I am not so certain that the 'derived' proportions of A. afarensis exclude it from being an ancestor to the Homo lineage.

Back to the differences between the two australopithecine species. Despite original skepticism, the data appears to be robust and the differences in joint size between A. afarensis and A. africanus appear to be real. As observed in the previous question, this variability may reflect locomotor differences possibly related to differences in the environment. If A. afarensis was still occasionally arboreal, is it too hard to imagine that, if the environment is changed (more wooded, greater predator pressure, more resources found in trees, etc.), the percentage of arboreal behavior would increase and that the proportions would revert to being more chimp-like in A. africanus? Again, there is no reason to assume that all early hominins, because they were bipedal, were identical in their locomotor behaviors.

I want to underscore that these differences are in joint SIZE, not in limb length, and reflect relative loading of the limbs. Usually, the major source of loading of the limbs is related to locomotion, but it is an assumption that cannot be verified in early hominins. If, as stated above, OH 36 is unlike anything living today, maybe it did things that have no modern equivalent. And the same can be said of other hominin species including A. africanus with its 'apparent' primitive proportions.

Hawks: You have recently been involved in field research in the Bala-Weyto region of southern Ethiopia. Can you describe the site, and your role?

Drapeau: The Bala–Weyto basin is part of a series of small parallel rifts that link the northern limit of the East African Rift to the southern limit of the Main Ethiopian rift. These small rifts constitute today a string of many small basins. The Bala-Weyto basin is located east of the Omo river basin. It is a region more difficult to survey when compared to dryer region because of the vegetation coverage that limits exposures visibility and access. However, it is little-explored paleoanthropologically speaking. Work in the Konso, another small basin a few kilometers away, but at a higher altitude, has a fauna with a certain degree of endemism and an A. (P.) boisei specimen with unique morphological variations. Among other things, we want to know if this variation and the faunal endemism are due to the relative isolation of the basin or to its particular environment. These answers may be found in contiguous basins that vary in their physical characteristics, such as the Bala-Weyto basin.

I am co-director of that project with Elizabeth Harmon of the City University of New York. At this stage of the project, being co-director involves organizing the whole expedition, securing funding, and coordinating the work of other team members. I would say that the most time consuming aspect is coming up with money and getting everything moving in the field. As a director, I am responsible for the team's well-being and it is a pressure that can sometimes weigh heavily on my shoulders. It is nice to be able to share the burden with a co-director.

Hawks: Do you involve students in your work?

Drapeau: My funding is limited and field work in Ethiopia is not particularly cheap. However, I plan to bring one student in the field this summer. I look forward to share this experience with a highly motivated student!

Hawks: Many of us have heard about the difficulties of field research, particularly in East Africa. What are some of your biggest challenges?

Drapeau: Doing field work in Ethiopia can be a challenge for many reasons. As can be expected, there are numerous permissions, letters, official documents, etc., that are required and the bureaucracy is somewhat heavy. However, I find Ethiopians very helpful and professional and, usually, the quest for documents goes smoothly, particularly once you know what to do and in what order.

A second difficulty is the access to the sites. Ethiopia did not have one highway until relatively recently and road traveling remains an experience that can be frightening. A lot of work is being done on the roads, however, and I believe that things will keep improving. Access to the research area involves off-road traveling as well, with all the difficulties that it entails. When you leave for the field, you have to be a self-sufficient unit, relying on the local environment as little as possible. It is still necessary to get gasoline on a regular basis, but except that, we try to be as autonomous as possible. It is particularly important when you go to a new area and don't know what (if anything) will be available to you.

A third aspect of field work, particularly in Ethiopia, is the politics, the paleoanthropological politics that is. Although most scientists are polite and civilized to each other, I really feel that we had to walk on eggs when we were researching an area in which to conduct field work.

A final difficulty (and certainly not the least) in our situation, is to find an area that has fossiliferous exposures of a time period that interests us and in which we can work at least a few years. The numerous discoveries that are made in East Africa give the impression that finding hominin fossils is something easy to do, but it usually involves many years of surveying. We are still at the exploratory phase of our project, i.e., we are still actively looking for an area that could sustain scientific work for a few years. Hard work (and perhaps a little luck) is essential.

Hawks: You had a lot of field experience before going to Ethiopia. How did you get your start?

Drapeau: At the end of my undergraduate degree, I had the chance of getting a couple of paying jobs in prehistoric archaeology. It was the beginning of a series of jobs in field archaeology conducted in parallel to my studies. I used to think (and still do) that these were the best summer jobs an anthropology student could have. The pay check was very descent and it usually came with room and board. These jobs allowed me to see many regions of Quebec and Canada that I would otherwise have never visited and to do things I would probably have never done otherwise. I have flown in helicopters for hours (and even survived a major crash), piloted a hydroplane (just for a few minutes, but still!), hear wolves howl into the night while trying to sleep in a tent hundreds of miles from any road or civilization, dipped my foot in the arctic ocean (too chicken to swim), seen the midnight sun, and I could go on. This fieldwork experience, and a stint in the Caune de l'Arago in Tautavel, France, opened another door: to be invited to do field work in Hadar in 2000.

Hawks: Any interesting stories?

Drapeau: I have an anecdote that I find amusing, but mostly informative on the nature of humans. When we were doing field work in the Bala basin, our camp was set up about a 2-hour drive off the road. It was clear that the local people had seen very few foreign workers. For the whole time we were there, we had a constant group of people just sitting in the shade observing us like zoo animals, watching our every move, laughing when we did things unexpected, etc. We were quite the entertainment. The occupation of the local Mali people appeared to be tending their few sorghum fields, but mostly to take their sometime large herds of cows, goats and sheep a few miles down to the river for a drink every day. Even though it was not that hot, the men walk around wearing only colorful underwear (the Speedo-type) and it was sometimes literally falling apart. From our western perspective, they really seem to have almost nothing. Anyhow, after a few days in the field, some crew members were starting to crave fresh meat. We agreed to allow the cook to purchase one goat from a local herder. We didn't think it would be a problem given the large quantities of these animals around and our willingness to pay a fair price for it. It came as quite a surprise that no one was willing to sell us any! It turned out that goats, sheep and cows were not herded to be eaten or even milked, but were really just status items. One man from the village nearby apparently owned more than a hundred head of livestock but was still unwilling to sell. We were all quite shocked of the apparent frivolity of it all, particularly considering that food (for humans and beasts) did not appear to be particularly abundant in the region. But then, we couldn't miss seeing the connection to what we can observe in the western world: huge houses for one or two people, oversized and overpriced cars. These are just to show off. The same frivolities, although expressed slightly differently, can be found anywhere. I guess it really is in the human nature. We were finally able to convince someone to sell us a goat, but we paid a really high price.

Hawks: Congratulations! You seem to be a very busy person right now, both professionally and personally. What's next for you?

Drapeau: I just started one of the most challenging projects of my life, a project that will keep me busy for the rest of my life. His name is Henri and he is almost 8 months old. Professionally speaking, I am investigating manipulatory adaptations in the early hominin hands and the morphology of muscle markings. However, one of my main objectives in the next two years is to settle on a specific field research area with good scientific potential.

Despite all the trouble I had traveling (or maybe because of it), I got to have a really enjoyable time finishing Ann Gibbons' new book, The First Human. For a while I was really afraid I'd lost it in the backpack without knowing how it ends! But what a relief, it was in another suitcase so I can report on the whole thing.

I've read most -- not all -- of the recent trade books about paleoanthropology, and this is definitely one of the top few in terms of being fun to read. It follows a familiar form: the quest for the source of the Nile. The book even mentions Burton and Livingston, whose explorations were to some of the earliest anthropologists what the Leakeys discoveries were to the current generation. Like the quest for the solo transatlantic flight, the summit of Everest, or the race to the Moon, the paleoanthropologists here all are trying to capture the same prize: the earliest hominid.

The book appeals in large part because it is well-written. Instead of beginning with the long dry history of finding bones in old dry places, Gibbons' first chapter plunges us right into the middle of three discoveries of the mid-1990's -- all happened within six months of each other, but the events of January 1995 brought them together. The chapter even ends with a cliffhanger!

Then comes the long dry history, with the usual cast of characters: Haeckel, Dubois, Dart, Louis and Mary Leakey. I was apprehensive about this -- no book ever seems to skip this stuff, and it's usually the same boring slog -- but Gibbons adds some details that most people haven't seen before. She's mercifully light on the "Dart courageously fighting the scientific establishment" theme, and brings us a great description of Dart excitedly opening the crate containing the Taung fossils at a friend's wedding. We get rather less of Louis Leakey's long struggle for recognition and more of his behind-the-scenes support from LeGros Clark.

Most notably, Gibbons brings us sketches of many of the paleontologists that the usual accounts miss. We see Bryan Patterson find not one, but two of the earliest hominids, and the episode that caused him to leave Kenyan field work, with his site of Kanapoi lying fallow for 30 years. We are led down the blind alley of Ramapithecus with Elwyn Simons and David Pilbeam. And we follow Yves Coppens to the Omo, Hadar, and Chad. Indeed, one of the real highlights is the account of field research in Chad, which I haven't seen described elsewhere in English so well.

The soap opera really begins with the origins and education of the current fieldworkers, who are as interlinked as characters on Days of our Lives. Pilbeam plays a Kevin Bacon-like role connecting Michel Brunet, Andrew Hill, and Martin Pickford. Pickford and Richard Leakey were old schoolmates, and -- maybe or maybe not, according to the book -- Hill comes between them. The chief fossil hunter from Hill's team goes to work for Pickford. The son of the chief fossil hunter for Richard and Maeve Leakey goes to work for Hill.

We see quite a bit less of the soap opera in Ethiopia, which describes the current Middle Awash work extensively but has little to say about Hadar or other current field sites. Donald Johanson's perspective on events of the last twenty years is very noticeably absent. We see Mary Leakey's anger at White and Johanson for naming her Laetoli discoveries Australopithecus afarensis, but the section does not explain the justification for the anger -- attaching the name to LH 4 as the type specimen removed any chance of naming the Laetoli hominids anything else.

Ian Tattersall raised an important point in his Nature review of the book: Any reporter who depends on access to subjects faces a possible conflict of interest. Report bad things about the subjects, and they may restrict access. Gibbons has obviously received exceptional access to some of the book's subjects -- indeed, the book mentions the famous lack of journalistic access to some of the research teams. Has this exceptional access affected the narrative?

I think that the book has a fair account of many events, but omits other well-known incidents that might have been described. For most of these, there is little that Gibbons could have done -- after all, if some subjects don't talk to you, and others won't give details about certain events, then what are you going to write about? In fact, there must be an intense incentive for many people not to cooperate with a book like this, especially those hoping to continue fieldwork in Ethiopia or begin there in the future. The accounts that are in the book make quite plain that one misplaced word can result in field permits being revoked, or access to collections being revoked, or even worse. As a result, the book puts on the record many arguments that were aired in public -- like the dispute over the Galili field site, for instance -- but doesn't necessarily give the whole story.

There is pretty obviously one overarching prize that shapes the entire narrative. The introductory chapter ends with the world on Alan Walker's "tenterhooks" -- in 1995! -- waiting to see the Ardipithecus skeleton. The book describes on four occasions just how fragile the skeleton was. Twice we hear how the condition of the skeleton "tempered" the Middle Awash team's excitement, twice it is described as "the most fragile skeleton ever found," twice as "roadkill." Early in the book White emerges as a secretive Svengali; at the end -- during an event White himself describes as "theater" -- we see him casting aside the velvet curtains to show his specimen at last to his skeptical colleagues.

Except, well, we don't get to see it. A reader might be forgiven for thinking the obviously crushed skull on the book jacket is the centerpiece of the book -- its "crushed" skull is twice mentioned. Sadly, no, the cover shot is just Sahelanthropus. Ardipithecus is still locked in its fortress of solitude, unseen by the unwashed. This does raise some concern for me -- since Gibbons will undoubtedly be writing the story of this fossil when it at last surfaces.

But some of the best moments are those that shine light on the relationship of the science to journals and the media. Two of the major research teams make a point of rejecting the taint of National Geographic and its film crews. In counterpoint, the book repeatedly notes the long association between National Geographic and the Leakey family, including a direct contrast between the histories of Richard and Maeve Leakey and Tim White. Amid descriptions of media-savvy scientists, we see Henry Gee, editor of Nature, commenting on fossils, prognosticating on future discoveries, "prodding" researchers, and having one incredible meeting that was hard for me to believe even after reading it. If one wonders about possible conflicts of interest for Gibbons, how much more must one wonder about the chance of one of these papers being rejected by Nature's vaunted six "peer reviews"?

At its bottom line, the book really raises two substantive issues. The first is the real danger of today's field work. Paleoanthropology is not merely a game today, it is "the Great Game" replayed. Field teams divide up "Connecticut-sized" research territories, hem opponents into areas with younger sediments, and -- when bullying, scientific name-calling, and bureaucratic manouvers fail -- finally agitate local people, enlist bandits, or pull their guns. To me, the book's most touching moment is its description of Michel Brunet's feelings after losing a colleague on his field team. In another episode, a young graduate student (who deserves recognition for her science and not this) personifies a near-miss with violence in the field. The two cases together bear rereading: if paleoanthropology continues along its current path, then who can doubt that some people will be killed in the field?

The other issue is the relationship between these field teams and the science as a whole. As depicted in the book, they clearly do resemble explorers looking for the source of the Nile. They know what the goal is -- at one point, Pilbeam even sketches what the ancestor will look like, at another Henry Gee opines about it. It is still out there waiting to be found, and these teams will be searching until they find it. It's "the First Human" of the title.

But these fossils aren't human -- and it's darned hard to tell whether they are even the more humanlike kind of apes! In the book, we see that the science turns against the scientists sometimes. Ramapithecus is no longer considered hominid by anybody -- it's not even a valid taxon anymore. Louis Leakey's Kenyapithecus wasn't a hominid either.

Can it be that all of these new fossils are really hominids? Or have some of these scientists in their quest for older and older fossils overshot the mark? The current scientific debate over specimens is only glossed here -- the book sketches what the disagreements are, but gives no details to judge the arguments. (If you want those details, you'll need to read the blog!) Instead, the science appears as another forum for the scientists to misbehave -- accusing each other of holding "creationist positions" and the like.

Many readers will surely be puzzled to read how these men and women, who brave disease, bullets, broken families and years of denial, can be so poorly composed in the face of scientific examination. Again and again we see them squirrel the fossils away, withdraw them from the world, or give up on paleoanthropology altogether. How can it be that this story is repeated so many times? But the reader should consider: No one can take away Hillary and Norgay's summit photos. But even after all the years of work, the lowliest graduate student might turn one of these "hominids" into an ape.

Even I make a brief appearance in this book -- blink and you'll miss me dancing through to aggravate Brunet's heart condition.

Razib has been working over genetic drift real good (concerning effective population size and population history, and founder effects). It deserves it.

This post is about genetic drift applied to phenotypic -- not molecular -- evolution. The two are distinct for two important reasons: first, phenotypes are widely genetically correlated with each other while unlinked DNA sequences are not; and second, because the theoretical reasons for some nucleotides to have no correlation with fitness are very strong, but such theoretical reasons are nonexistent for phenotypes.

Personally, I think selection is more important than drift at the molecular level as well, for reasons having to do with those "genetically correlated" and "unlinked" assumptions. But to the extent that neutral evolution may be credible for many genes, it is much less credible for most phenotypes.

Here's what I tell my students:

To explain the evolution of a feature in ancient humans, genetic drift is my absolute last resort...right before sexual selection.

Why I don't like sexual selection is a topic for another day.

For now, on to genetic drift. Here's what Gould and Lewontin's famous "spandrels" paper has to say:

Have we not all heard the catechism about genetic drift: it can only be important in populations so small they are likely to become extinct before playing any sustained evolutionary role (but see Lande 1976) (Gould and Lewontin 1979:585-586).

With some math, we can show that the "catechism" is not literally true -- genetic drift can cause substantial phenotypic evolution in large populations. But ...

Some worked examples

Lande (1976) shows genetic drift can cause phenotypic evolution consistent with many examples in the fossil record:

This paper presents a statistical test for the hypothesis of evolution by random genetic drift, contingent on the effective population size. In examples from the fossil record, it is found that the rates of evolution equal to or greater than those observed have a significant probability of occurring by random genetic drift even in very large populations (Lande 1976:314-315).

Let's consider an example not examined by Lande. One of the most significant temporal trends in the early hominid species Australopithecus afarensis is a decrease in the length of the lower third premolar. This decrease in length is associated with a change in the morphology of the tooth, in which a more sectorial one-cusped form becomes less common and a more bicuspid form becomes more common. Lockwood et al. (2000) show that the P₃ length decreases from an average around 10.5 mm in the early, 3.5 Ma Laetoli sample down to an average around 8.5 mm in the latest 3.0 Ma Hadar sample. Estimating the standard deviation of the sample as a whole (including intermediate time periods at Hadar) is a bit complicated, but if we consider the mean as a moving average, then the standard deviation is between 1.0 and 1.5 mm. I'll assume 1.0 mm to be conservative.

Lande (1976) derives the distribution of phenotypic change due to genetic drift in a population with effective size N_e. The average change due to genetic drift is no change -- the most likely result of random sampling is no change at all. Populations can change in either direction (larger or smaller) due to random sampling, and larger amounts of change are increasingly less likely. More change is likely in smaller populations, so that the amount of change depends on the effective population size. Lande gives an expression for the effective population size N* at which the observed amount of change is at the 95 percent confidence limit:

If we assume h² = 0.5, t = 25,000 generations, and the standard deviation is 1, then N* is estimated as 12,000. Since the effective population size of all extant hominoid species is around 10,000, this estimate is fully consistent with the evolution of P₃ length by genetic drift alone.

In fact, it's pretty hard to find anything in human evolution that couldn't have evolved by drift alone, under these assumptions. For example, Wolpoff and I (2001) found that the Middle Pleistocene increase in cranial capacity was consistent with genetic drift in a population with N_e = 1.8 x 10⁶. The increase from early Neandertals to Würm Neandertals was less likely to occur by drift alone -- our estimate of N_e = 1.2 x 10³ is quite a bit less than 10,000. On the other hand, there would be many who would argue that the effective population size in Europe alone really was that small, and that therefore the Neandertals increased their cranial capacity by genetic drift also.

Now, the increase in endocranial volume in humans is one of the most impressive long-term evolutionary trends in mammals. If even that is explicable by genetic drift, then it is pretty clear that we don't ever need natural selection at all.

So I should really like genetic drift, right? I mean, it explains everything, doesn't it?

The fallacy

Of course, what is actually going on is that we have chosen a null hypothesis that is especially hard to refute. This means we should expect a lot of type II error: using this method, we can't reject the hypothesis of genetic drift even if it isn't the right answer.

What is worse, even if we were to show that the amount of phenotypic change is too great for a given effective population size, there will always be someone to argue that the effective population size was smaller in the past. So genetic drift is a moving target -- it is effectively impossible to reject.

The operative problems here are (i) a relatively small amount of change over (ii) a very long period of time. This combination will usually be consistent with Lande's derivation for genetic drift and reasonable effective population sizes -- particularly if we cannot establish in advance what effective population size is actually reasonable.

There is a big contrast between a long timescale and a short timescale in this comparison. Natural selection is certainly much faster than genetic drift on a short timescale -- the time to fixation of an adaptive allele by selection proceeds as the logarithm of population size, while the fixation time by drift proceeds linearly with population size. Genetic drift can change a population quickly, but only if the population is very small. Selection can change a large population quickly.

The fallacy is the assumption that the difference between selection and drift over short timescales also is a difference over long timescales. There is some ultimate limit on the evolution of any character. Selection may make a mouse the size of a cat in a few hundred generations, but even assuming that those cat-sized mice can stick around, there is no reason to think that dog-sized mice will be better! At some point, selection will stabilize -- and for most characters the amount of change permitted by stabilizing selection is not too great. Sampled at time intervals of many hundreds of generations, selection may look exactly like genetic drift.

What to do

People think about genetic drift because of its mathematical convenience. Sampling error is predictable, and the repeated occurrence of sampling error over many generations follows well-known probability distributions.

Selection is predictable too, but it requires you to actually know something about ecology. We often don't know anything, and when we do, we usually have some particular relationship in mind, which needs to be tested. So, we test the hypothesis of neutrality, with all its mathematical simplicity.

But remembering that the null hypothesis is sometimes -- maybe even often -- true doesn't mean that we should be satisfied with any particular test of that null. For neutral evolution of phenotypes, there are more powerful tests than evolutionary rates. The problem is that these "tests" are not in large part quantitative, but instead are logical or qualitative.

For example, it is very likely that human brains increased in size under selection because there should clearly have been selection against larger brains because of their energetic costs. The rate of evolution does not factor in here, and is in fact irrelevant to the assessment of selection.

The case does require a more complex model than simple directional selection -- instead it involved a structured model in which the force of selection is mostly stabilized by a counterforce of selection. It also begs an explanation for why the change should have proceeded at a given rate -- for example, was it slow because of environmental constraints? Such constraints would seem a likely explanation for the rate of change of dental size in Australopithecus afarensis.

But when most people talk about genetic drift, their reality doesn't seem to include the mathematical consequences of genetic drift.

For one thing, genetic drift is sloooooow. It affects allele frequencies on a time scale in generations proportional to the effective population size.

We sometimes hear that it is a bad thing to doubt the power and ubiquity of genetic drift. This doubt is sometimes equated with adaptationism, taken as the uncritical assumption of selection as a null hypothesis.

References:

Gould SJ, Lewontin RC. 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proc R Soc Lond B 205:581-598.

Lockwood CA, Kimbel WH, Johanson DC. 2000. Temporal trends and metric variation in the mandibles and dentition of Australopithecus afarensis. J Hum Evol 39:23-55.

Hawks J, Wolpoff MH. 2001. The accretion model of Neandertal evolution. Evolution 55:1474-1485.

Rex Dalton has a great two-page article in Nature about the bush vs. ladder dispute. It keys off of the Middle Awash Australopithecus anamensis article by White and colleagues from a couple of weeks ago.

If you recall that one, White et al. posited that Ardipithecus was likely ancestral to Au. anamensis, and that the two did not overlap in time. Here's the key exchange in the Dalton piece:

This month's Nature paper makes a bold argument, and shows the Awash team seeking to put its mark on the record. Others in the
field are impressed. "When you find 30 new hominid fossils, you are allowed a certain amount of conjecture," says Bernard Wood, a palaeoanthropologist at George Washington University in Washington DC. "As always, they have done a fantastic job."

But he and others are unconvinced by the Awash team's conclusion: "This is only the first half of the rugby match," says Wood. Meave Leakey, lead author on the Au. anamensis discoveries in Kenya, is more blunt. "I don't believe this," she says. "We do not have the specimens to fill the gaps."

Leakey and Wood are among those who believe that other, as yet undiscovered hominid species may have lived at this time, from 4.4 million to 2.9 million years ago. The existence of other species would cloud or eliminate the argument for a direct lineage. "My prejudice is there are more lineages rather than fewer -- more diversity," says Wood. "I have to concede these new data are dramatic. But we should beware coming out with a complete explanation when we don't have all the
evidence."

This argument frustrates White. "There were Martians there back then too," he says. "And spacecraft all over the Pliocene -- we just haven't found them yet."

Waiting for Monte Cassino

In a series of articles since 2000, White and colleagues have laid out a systematic attack on the "bushy" phylogeny model. Their arguments have extended across four million years and seven species, with a breadth that rivals the Allies breaking the Winter Line.

Consider the angles of attack:

1. Au. anamensis -- Au. afarensis. Everyone basically accepts that Au. anamensis is a direct ancestor of Au. afarensis. And the two species are really not very different from each other -- for instance, they are more alike than either is to Ardipithecus. The transition between these species would look to be a simple case of anagenesis, except...

...for Kenyanthropus (Leakey et al. 2001). This small-toothed, flat faced hominid needs an ancestor, too. Au. anamensis might have been the common ancestor of Kenyanthropus and Au. afarensis. If so, then both these later species originated by cladogenesis from Au. anamensis. A similar argument might be made for other species, like Australopithecus bahrelghazali (Brunet et al. 1996) or the Sterkfontein Member 2 hominids. But Au. bahrelghazali is only known from a partial mandible and only differs from Au. afarensis by a three-rooted premolar, which is considered by many to be weak evidence, and the Sterkfontein Member 2 sample has not yet been taxonomically assigned -- they might turn out to be Au. afarensis, for example. Kenyanthropus remains the strongest case for cladogenesis (i.e., a "bush"). Yet...

...White (2003) denied that the Lomekwi skull KNM-WT 40000 was a distinct species. In particular, he argued that the extensive postmortem deformation of the skull made it impossible to substantiate an anatomical difference from Au. afarensis, and even if it was different, the anatomical diversity of living hominoid species is so great that it would probably encompass the difference between KNM-WT 40000 and known Au. afarensis crania.

2. Earliest hominids. At the moment, the earliest putative hominids include three genera: Orrorin (Senut et al. 2000), Sahelanthropus (Brunet et al. 2002), and Ardipithecus, represented in the Late Miocene by Ar. kadabba (Haile-Selassie 2001, Haile-Selassie et al. 2004). Evidence for obligate bipedality has been challenged (by different researchers) for each of these three (I'm one of those who has questioned bipedality for Sahelanthropus).

So far the only comparable anatomical parts from all three samples are teeth...

...which were examined by Haile-Selassie, Suwa and White (2004). They concluded that the variation among these three genera

is no greater in degree than that seen within extant ape genera. Despite claims of molar enamel thickness differences among these late Miocene fossils, we question the interpretation that these taxa represent three separate genera or even lineages. Given the limited data currently available, it is possible that all of these remains represent specific or subspecific variation within a single genus (Haile-Selassie et al. 2004:1505).

Additionally, Ohman, Lovejoy and White (2005) challenged the interpretation of the internal anatomy of the Orrorin femur, which had been suggested to be more derived than that of Au. afarensis. They wrote:

We agree that the Lukeino femur's external morphology suggests some form of bipedality. Yet the more detailed original scans appear to show a distinct superior cortex different from Australopithecus and humans, with the cortex distribution being more primitive than that seen in any other hominid, including Australopithecus.

The relevance of this argument to the phylogenetic diversity of early hominids depends on the anatomy of the Ardipithecus femur, which none of the rest of us are in a position to know. But one may speculate that if all these early "hominids" had femora with similar morphology, it would further reinforce the interpretation that they belong to a single lineage.

3. Ardipithecus -- Au. anamensis. This is the current example. Here's how Dalton discusses it:

The latest Afar discovery is exciting experts because it shows that the three hominids existing in the same area, but in successive time periods. Tim White of the University of California, Berkeley, co-leader of the Awash team, believes this points to a direct lineage between the three -- a process called phyletic evolution. The new Au. anamensis fossils are only 300,000 years younger than Ar. ramidus, meaning that if one became the other, the changes would have had to happen that fast. But the key point, says White, is that fossils of Au. anamensis and Au. afarensis have never been found in sediments the same age as those containing Ar. ramidus. If fossils of the different species were found together, that could show that they belonged to multiple lineages existing simultaneously.

Finding remains of all three species in the same area but not from the same time period suggests they did not coexist, says White.

...

The specimens also provide anatomical clues to evolutionary history. "The new Au. anamensis fossils are anatomically intermediate between the earlier Ar. ramidus and the later Au. afarensis," says White. For example, the teeth of the newly discovered Au. anamensis fossils seem adapted to chew tougher and more abrasive foods than Ar. ramidus. The researchers believe this shows that Au. anamensis had a broader diet. "All this strengthens the view that there is phyletic evolution from Ar. ramidus through Au. anamensis," says White. He believes he has nailed down the relationship between the two later species, although he says that further specimens are needed to prove the earlier link (Dalton 2006:1100).

Of course, it would help matters if we knew in more detail what Ardipithecus looked like. But one must imagine that the stage is being set for its revelation. The unilineal interpretation places Ardipithecus at the critical point as an ancestor to the major mid-Pliocene australopithecine lineage. Extending the unilineal interpretation earlier into the Late Miocene would make Ardipithecus the earliest hominid as well.

It is not necessary to think that taxonomic uniformity means anatomical uniformity, though. Ardipithecus already encompasses a trend of decreasing canine size and less sectorial P₃ for example. A trend toward fuller skeletal adaptation to bipedality may also be imagined. But in that context, it is important to note that the time interval between the Orrorin femur and the unpublished Aramis skeleton is longer than the time between Aramis and Hadar. Those relative times may become quite important in thinking about the evolution of those postcrania.

The Winter Line was broken at Monte Cassino, after many failed attempts from different approaches. The Aramis fossils are either the heavy shoe waiting to drop, or they are the uncomfortable foot that all this talk about phyletic evolution is meant to shoehorn into place.

Commentary

If all these cases are added together, they imply a single evolving lineage encompassing at least four anagenetic taxa, Ar. kadabba -- Ar. ramidus -- Au. anamensis -- Au. afarensis. This last would presumably be followed by a cladogenesis into a robust australopithecine species (Australopithecus aethiopicus) and Australopithecus africanus.

One could add Homo erectus to this list, since White and colleagues argued in their description of the Daka skull (Asfaw et al. 2002) that the Asian and African samples represent one cosmopolitan species.

But then one species sticks out as a surprising exception to the pattern: Australopithecus garhi (Asfaw et al. 1999). It will be interesting to see a close argument showing why this species is really different from South African Au. africanus. Say, more different than KNM-WT 40000 is from the Hadar crania. It's quite glaring, really, that this species should be there mucking up such a simple phylogeny.

I have to say, after reviewing all these papers in one sitting -- this entire bush vs. ladder thing is getting very tiresome! I mean, isn't there something else that we could organize early hominid discoveries by? These are all papers in the top journals, and this is the (fairly specialized) discussion that has been promoted as the central issue in the field!

The subtitle of the Dalton piece suggests that it is merely a philosophical difference:

Deciding whether our ancestors evolved as a single lineage may depend more on philosophy than fossils.

But that's not really true. There is a clear null hypothesis here, quite directly drawn from William of Ockham:

entia non sunt multiplicanda praeter necessitatem

Which of course means:

Sometimes fossil samples really do form ancestor-descendant relationships.*

(*) It doesn't really. It means "Entities should not be multiplied beyond necessity."

References:

Asfaw B, Gilbert WH, Beyene Y, Hart WK, Renne PR, WoldeGabriel G, Vrba ES, White TD. 2002. Remains of Homo erectus from Bouri, Middle Awash, Ethiopia. Nature 416:317-320. DOI link

Asfaw B, White T, Lovejoy O, Latimer B, Simpson S, Suwa G. 1999. Australopithecus garhi: A new species of early hominid from Ethiopia. Science 284:629-635. DOI link

Begun DR. 2004. The earliest hominins -- is less more? Science 202:1478-1480. DOI link

Brunet M. and 37 others. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418:145-151. DOI link

Brunet M, Beauvillain A, Coppens Y, Heintz E, Moutaye AHE, Pilbeam D. 1995. The first australopithecine 2,500 kilometres west of the Rift Valley (Chad). Nature 378:273-275. DOI link

Dalton R. 2006. Feel it in your bones. Nature 440:1100-1101. DOI link

Haile-Selassie Y. 2001. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412:178-181. DOI link

Haile-Selassie Y, Suwa G, White TD. 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303:1503-1505. DOI link

Leakey MG, Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN, McDougall I. 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410:433-440. DOI link

Ohman JC, Lovejoy CO, White TD. 2005. Questions about the Orrorin femur. Science 307:845. DOI link

Senut B, Pickford M, Gommery D, Mein P, Cheboi K, Coppens Y. 2001. First hominid from the Miocene (Lukeino formation, Kenya). Comptes Rendus 332:137-144.

White T. 2003. Early hominids -- diversity or distortion? Science 299:1994-1996. DOI link

Michelle Drapeau and colleagues (2005) report on the AL 438-1 specimen from Hadar. The specimen consists of "part of the mandible, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, plus parts of the clavicle, humerus, radius, and right ulna" (1). At 3 million years, the specimen is one of the youngest of the A. afarensis sample.

The AL 438-1 individual was evidently relatively large compared to the rest of the Hadar sample. The ulna length is 278 mm, which is larger than the mean for any of the human samples examined by Aiello et al. (1999) in their comparative study of the OH 36 ulna. It is about average for a chimpanzee, although chimpanzees have relatively longer forelimbs than would have been true of A. afarensis, so again this is evidence of a relatively large body size.

Drapeau et al. (2005) make a point of the proportion of the ulna and the mandible being similar to that found in AL 288-1 (Lucy), which they take as evidence that large teeth in this late specimen may be attributed to larger body size rather than greater megadonty:

Both Australopithecus afarensis mandibles have a larger corpus (breadth and height at M₁ relative to the ulnar size surrogate than those of African apes. Similarly, mandibular corpus shape (breadth/height x 100 at M₁) is similar in the two fossils (A.L. 288-1, 57%; A.L. 438-1, 60%). This difference in mandibular size corresponds to what would be expected from two extant ape conspecifics with ulnae of such different sizes. Since there are no differences between the two Hadar skeletons in mandibular to ulnar proportions, there is no evidence for an increase in mandibular size relative to the rest of the skeleton between the points in time represented by these two individuals. We cautiously offer this as support for Lockwood et al.'s (2000) suspicion that the observed temporal trend toward larger mandible size reflects a body size increase late in the Hadar time span of A. afarensis (Drapeau et al. 2005:41-42).

The paper has a substantial discussion of the morphology and comparative anatomy of the ulna. The bottom line of this analysis is that the ulna is similar to that of AL 288-1 in most respects, except for its larger size and somewhat greater curvature. It is, however, smaller and somewhat less curved than the later Omo L40-19, and substantially less curved than the OH 36 ulna. The authors write this about its similarities to other homionids:

While phenetically A.L. 438-1 presents a mix of ape-like and human-like morphology, when considered in a phylogenetic context, the Australopithecus afarensis forelimb shares synapomorphies exclusively with humans among extant hominoids taxa. It resembles non-hominins only in plesiomorphic character states. In this context, it is apparent that A. afarensis forelimb anatomy reveals the results of selection for a more human-like humeroulnar joint, larger thumbs, and altered carpometacarpal joints that reflect an emphasis on manipulative aptitude at the expense of forelimb-dominated climbing ability (Drapeau et al. 2005:43).

That is a relatively powerful statement of the adaptive qualities of the A. afarensis forelimb, which appers more or less necessary to explain the differences between early hominids and apes in this respect. If the early hominids were really climbing a substantial proportion of the time, then we might hypothesize that their forelimbs ought to look more like ape arms. But they don't; there are clear differences that make the early hominid arms look more similar to human arms. Thus, the authors turn to the hypothesis that the A. afarensis forelimb is additionally adapted to "an emphasis on manipulative aptitude."

At the moment, this hypothesis remains to be strongly tested. Most of the human-like features of the A. afarensis arm are arguably the result of not being used in quadrupedal weight support. Thus, the fact that "the Australopithecus afarensis elbow joint appears to reflect habitual loading the elbow at or near 90 degrees, rather than optimization for loading in a more extended posture as in extant apes" (43-44), as well as the anatomy of the joint and the form of the carpometacarpal joints may all be explained by the fact that early hominids were not knuckle (or fist) walkers. The large thumbs are the strongest piece of evidence for any kind of manipulative behavior in A. afarensis.

On the subject of retained similarities with apes, Drapeau et al. (2005:46) have this to say:

The retention of African ape symplesiomorphies in A. afarensis may be attributed to either stabilizing selection fore a partially arboreal locomotor repertoire, or to lack of selection against these traits (see discussion in Stern, 2000; Ward, 2002). It is inherently difficult to test these alternative hypotheses. Thus, the significance of these retained traits for reconstructing the behavior of A. afarensis is difficult to determine with certainty in the context of demonstrable selection for a human-like elbow and hand joints. Australopithecus afarensis shares some apomorphies with humans that suggest emphasis on use of the forelimb in flexed postures, and improved grip capability relative to apes. The presence of these synapomorphies suggests similarities in forelimb function among hominins, likely reflecting selection for expanded manipulative capabilities and flexed forearm postures relative to that found in apes and a diminished capacity for ape-like arboreal behaviors. Only later did humans display evidence of further selection for manipulation coupled with reduced forelimb robusticity. We conclude that in Australopithecus afarensis, selection for natural manipulation outweighed selection for arboreal activities, but that selection for refined manipulative ability had not yet come into play in human evolution.

A fine balance, if it is true, and fitting within the generally understood picture that, with regard to its arm and hand functions, A. afarensis was either Homo habilis nor a chimpanzee.

References:

Aiello LC, Wood B, Key C, and Lewis M. 1999. Morphological and taxonomic affinities of the Olduvai Ulna (OH 36). Am J Phys Anthropol 109:89-110.

Drapeau MSM, Ward CV, Kimbel WH, Johanson DC, and Rak Y. 2005. Associated cranial and forelimb remains attributed to Australopithecus afarensis from Hadar, Ethiopia. J Hum Evol Advance before print.

Lockwood CA, Kimbel WH and Johanson DC. 2000. Temporal trends and metric variation in the mandibles and dentition of Australopithecus afarensis. J Hum Evol 39:23-55.

Mark Weiss from NSF appeared at the AAPA business meeting to discuss recent changes in the funding guidelines from the Physical Anthropology program. The most significant change, effective in the upcoming (July 2005) funding cycle, is the requirement to file and follow a data access plan with every grant. This change is the NSF response to the questionaire circulated last year among physical anthropologists and archaeologists. It follows policy changes at the top levels of NSF, ultimately initiated by the Clinton and Bush administrations toward greater openness of publicly funded research data and protocols.

From the Physical Anthropology grant information page:

NSF is committed to the principle that research supported with public funds should be made widely available. Under NSF's data sharing policy, the Foundation expects investigators to share with other researchers, at no more than incremental cost and within a reasonable time, the data, samples, physical collections, and other supporting materials created or gathered in the course of the work. To implement that policy in ways appropriate to Physical Anthropology and Archaeology, beginning July 1, 2005 these Programs will require that all proposals include a one-page detailed description of the applicant's data access plan in the "Supplementary Documents" section. This page will be in addition to the standard 15-page project description. Applications lacking this statement will not be reviewed. The Programs realize that individual cases may differ widely and recognize that any absolute timeline or rigid set of rules is not possible. They also recognize that revision and adjustment may often be required as the work proceeds. The data access plan, however, will be considered an integral part of the project and therefore subject to reviewer and panel evaluation. Major departure from it will constitute a significant project change and require NSF approval. Successful applicants will be required to address this issue in every progress and final report. PIs on all awards made under these guidelines will be expected to discuss implementation of their plans in the "Results of Prior Research" section when they submit subsequent applications.

To me, this appears to be a good compromise between the different positions on data access. Some researchers would prefer to have casts, photographs, and measurements of specimens become publicly available (without restrictions) immediately after they are published. Others (including a subset of primary excavators) would prefer to limit access to photos and data until after a full monographic treatment of the specimens is published. There are good arguments on both sides.

In favor of limiting access, specimens are rare and fragile, and access to them should be carefully limited to preserve them. The skills required to prepare fossil specimens are rare, and they must be cultivated in long-term research projects. The only way that such projects can survive is if they can maximize the impact of their most important finds, and this means controlling the publication of pictures, limiting the creation and distribution of casts, and promoting students of the principal invesigators as groundbreakers making new and important discoveries. If such research projects had to make their data public immediately, there would be no incentive for them to continue their work.

But even those who are in favor of limiting access to fossil specimens must recognize that the situation in paleoanthropology today does not benefit them. There are very few publicly accessible datasets. Even pure electronic data for which analysis has been published and for which the cost of transmission is negligible, such as CT data, generally cannot be had. There are exceptions, who either provide data for sale or for free, and from the bottom of my heart I thank them for their choice to better the science. Their choice is all the more laudable, because the situation at present has created absolute disincentives to share data. At present, closing access is the only way to punish freeriders who fail to share data themselves. And commoditizing data and casts can be the only way to get valuable data out of other researchers.

And in my opinion, the issue of access to new fossil hominids has received an unwarranted share of the attention. Ann Gibbons' 2002 article, "Glasnost for hominids," is an excellent treatment, but it only scratches the periosteum of the problem. If the only problem with access to specimens was that only a few people could see something until ten years after it was unearthed, that would be bad, but still much better than the situation as it stands.

The real problem is that twenty to thirty years after many fossils are uncovered, there is no cast availability, little public data access, few financial accommodations to make such access possible. Specialists like me often find ways around these barriers. But I do not think it would be overstating the problem to suggest that perhaps half the people teaching human evolution in four-year universities have never touched a cast of a Hadar fossil. I would be delighted to be proved wrong, but I don't think I am. Our field is educating students into a world in which A. afarensis is unknown in the laboratory and poorly represented in our textbooks. I'm not talking about new specimens, here, I'm talking about fossils that were found in the mid-1970's and monographed in 1982. Nor is this problem limited to early hominids. What proportion of people teaching about the modern human origins problem do you suppose have seen a cast of any "early modern" fossil other than Skhul 5?

One may object that this kind of teaching effort really isn't the same thing as primary research, and one would be right. But I am one who thinks that teaching is essential to my research. And I see it the same way as my high school band teacher: you can't have a good high school band without a good junior high and grade school band program. We can't train competent professionals without a strong undergraduate training, and the undergraduate training of our professional paleoanthropologists is a lot more varied than the graduate programs. Unless we strengthen the broad base of the field, we have little hope of strengthening its research depth.

And the fact is that primary paleoanthropological research is no longer the province of a few dozen professionals. The field is increasingly interdisciplinary, involving hundreds of people with no expertise in anatomy at all. The fossil record is an afterthought to many of these people, and it is our task to continue to show its relevance. We can't do this without the tools.

Righting the paleoanthropology ecosystem

In this sense, the current ecosystem in paleoanthropology is dysfunctional, and the problem of data access has had a negative impact on the quality of science in the field.

New specimens are a bottleneck in paleoanthropological research. The pace of research is positively limited by the rarity of fossils. This bottleneck has several consequences, including the complete absence of research on some topics that are poorly addressed by fossils, the high citation rates of initial announcements of fossil discoveries, and a funding structure that privileges field research leading to new discoveries. Because this bottleneck is so acute, a naive observer may confuse it for the entire field.

But except for this one part, paleoanthropology as a whole is a normal part of evolutionary biology. Like other parts of biology, ours is a comparative science in which all competent work depends on thorough procedural knowledge of evolutionary theory and factual knowledge of comparative samples, such as extant apes, humans, and other primates.

Even most paleoanthropologists do not themselves recognize the breadth of their field. There is a tendency to see the field as an unstable ecosystem, in which a very small number of primary producers (who find new sites and excavate and prepare fossils) support a huge number of consumers:

The classical ecological pyramid has a broad base of primary produces, with increasingly smaller numbers of secondary and tertiary consumers. Modern paleoanthropology, however, is like an inverted ecological pyramid. Armchair commentators abound. Actual producers of fossil data are increasingly rare. But boosting the number of producers is not feasible because so few professionals have the requisite specialized skills. Even fewer are qualified to teach them. The production of primary paleoanthropological data requires physical search, discovery, extraction, dating, contextualizing, preparation, photography, molding, analysis, writeup, and publication. The process now takes years of work by large coordinated teams (White 2000:289).

Tim White is one of the premier fieldworkers in the discipline, and it is not surprising that he should display a fossil-centric view of the field. But is it really true that we have nothing of value besides the fossils; that they are the only "product" we deal in? Are the rest of us really nothing more than jackals nipping at his heels?

I would propose an alternative model of our ecosystem. Rather than privileging the mere objects that fossils actually are, I would privilege the knowledge that we gain about human origins from them. Fossils are far from the only source of this knowledge. Indeed, all the knowledge that we obtain from fossils ultimately comes from comparing those ancient fragmentary remains with the more complete comparative samples of extant species, not to mention their rich genetic, behavioral, and soft-tissue morphological record. Even modelers and mathematicians, like myself, wring data out of fossils that ultimately do not inhere in the bones themselves but in their relationship with other specimens and species.

Left to itself, this work is steady and vegetative. We produce observations, comparisons, hypotheses, and ultimately evolutionary theory. We travel, we study specimens, we present our work to public audiences and to groups of our peers for scrutiny and comment. And this open process helps us to make our knowledge better. Without a single fossil, this body of theory would be left sorely wanting for accuracy, but it would exist nonetheless and would be nonetheless be the most valuable evidence for our evolution that we have. Just as Darwin's Descent of Man preceded all but the Neandertals, our work today precedes the next hundred years of fossil discoveries and awaits testing in light of them.

Those of us familiar with this kind of work tend to call it not "armchair commentating" but instead "critical thinking." We train our students in it, and work to make them knowledge producers as well. We socialize them that the best way to succeed in the real world is to share data and to play well with others. And we hope they won't get burned in their first encounter with a real predator.

Our field has its T. rex and the like. The activity of these top predators is spastic and episodic. When they roar, presenting us with a new precious relic, much of the field cowers and prays that we don't have to relearn everything from our graduate training that the new fossil makes obsolete. These carnivores devour comparative biology, for their fossils have little relevance outside its context. Newton called it "standing on the shoulders of giants," but sometimes it seems more like Spinal Tap dwarves trodding on a tiny Stonehenge.

Most of us recognize that new fossils are more than bludgeons to beat away the jackals. They are the only tests that many of our hypotheses can ever hope to have. And I don't see anything to be gained in classing part of our science as highly important and another part as irrelevant or worse. The fact is that all of us work with each other's data and conclusions. Some of us have established barriers to make that process more difficult. All of us deal with the same bottleneck of fossil evidence, but for many of us that bottleneck is a mere inconvenience, while for others it is the crook used to lever an entire career.

It is fitting and just that the acquisition of new fossils should be a high funding priority; if not the highest. This bottleneck prevents progress, and we should do anything in our power to alleviate it. But high funding for new field research does not imply that access should not be more open.

Closed access unnecessarily impedes progress in other areas that might otherwise be made. The present situation is unstable, and I see these critical problems:

1. The slow reporting of specimens and failure to share casts and data slows research on some important topics, limiting them to a small cadre of researchers. As an extreme example, no study of the energetics of the earliest bipeds is now possible, because many major specimens currently exist without having been reported, and none of the people working on them specialize in energetics. But more practically, only two years ago it was reported that only a single person had seen all of the then-extant evidence for Miocene hominids (Gibbons 2002). How can a field progress when so few people are in a position to review its data? If these people review each other's papers (because they are the only recognized experts), then how can any of us have confidence in their rigor?
2. Studies published on inaccessible fossils are not replicable. Suppose that someone publishes the energetics of the earliest bipeds, using measurements from new specimens. Certainly anyone reading this research can run the same measurements through the equations, but how can they be sure that the measurements are accurate or relevant, without examining the fossils or reconstructions themselves? This is the current situation with Sahelanthropus and its CT reconstruction, for instance: the publication exists, but is not replicable because access does not exist.
3. Students who can study inaccessible fossils can trade on this knowledge to promote themselves. Now, I don't think there's anything wrong with self-promotion; after all, jobs are scarce. But quality of access has increasingly become confused with quality of training. Ideally, a student will have both. Paleoanthropology is a comparative science, and extensive experience with comparative samples such as extant apes is needed for any competent research. To the extent that some students exploit the fossil bottleneck to leverage greater visibility, the quality of training expected of new hires is diminished.
4. Casts are generally inaccessible. Despite the current ubiquity of CT scanning of fossils and creation of stereolith casts, even these cannot be purchased. All of the problems above would be less pressing if there were some assurance that eventually all qualified researchers would have access to casts and scans. But when an initial description, peer-reviewed by only friendly colleagues, stands for decades without reanalysis because of the lack of access, a mistake that shouldn't occupy more than five pages in a dissertation ultimately bends the course of the discipline for years.
5. Most important, public support for our discipline depends on its perception in a country where a majority of people don't believe that humans evolved. Those arrayed against us argue that new fossils are hidden away and not studied by the scientific process of peer review. They argue that many human fossils are manufactured, and that there are no guarantees that they are not the product of a small group of scientists with an anti-creationist agenda. As long as we do not open access to the primary evidence of human evolution, these criticisms are not only damaging, as far as the nonspecialist public is concerned they are also valid. We do nothing but damage the profession when we fail to share the products of our research as freely as possible, not only with each other, but with humanity.

Will the policy work?

To the extent that new grants will make data more available, will encourage the spread of CT scans of fossils, and will help to spread photos and observations of new discoveries to the public, I think the data access policy will be helpful. I think there may be nothing to be done about the availability of casts, as long as museums control their reproduction. I respect and value the work of all museums who conserve fossil remains, but they are not set up for widespread public sale of fossil replicas. And a commercial solution will have little incentive to reproduce rare fossils that are not part of the central story of evolution. In my opinion, the most important aspect of data access is to increase the effectiveness of peer review and to guarantee replicability of research. For these goals, I think the new policy has a maximal chance of success.

Of course the real test of the value of the new policy is to see whether grants start to be declined on the basis of data access restrictions. As I read it, this new policy basically sets the clock at zero. There is no condition that specifies that previously funded work should be made public, and no effective means of pressure to create a situation favoring the sharing of old data and specimens. There are now specimens that have been out of the ground for thirty years that cannot be studied. There are hominid specimens that have been out of the ground for ten years or longer that remain undescribed. This situation will not change.

If the new policy is to be a success, then the proof of it cannot wait for ten to thirty years. It needs teeth. It needs two or three high-profile grants to be declined because of data access issues. And it needs those cases to be made public, so that everyone can have confidence in the openness of the process. This doesn't mean that the names of the applicants and their alleged sharing violations should be dragged through the press. It does mean that NSF should publish the number of grants (and their proposed funding amounts) declined for failings in the data access plan.

But more importantly, it needs replication among other granting agencies. A large set of molecular anthropologists have just shown their willingness to completely forego public funding, in order to maintain certain kinds of controls (in this case ethical ones) over their research (See Genographic Project). Will paleoanthropologists do the same? It would be helpful if some of the important private foundations, such as the National Geographic Society, the Leakey Foundation, Wenner-Gren, and others would establish data access provisions also.

Another helpful idea would be for one of these foundations to establish a data bank. Notice what is missing in the NSF policy is any discussion of a data archive. Other areas of NSF and NIH have such archives and maintain policies of mandatory deposition of data. This is most prominent for genetics, with the GenBank archive and journal publication of most results conditional on mandatory submission of data to the archive. Thus, there is no logical impediment to the creation of such a resource by a federal agency. The fact that they chose not to implement such a policy, I find significant.

Among other considerations, this choice probably depended upon discussions with museums and governmental agencies in other countries, who are the conservators and permit-granters for most fossil research. There are good reasons for the U.S. government not to compromise the activities of international museums by making public images, casts, and CT data of their fossils. On the other hand, much money and effort could be saved if such an archive were available, and it would increase the quality of published science by increasing sample sizes, consistency of measurements and estimates. It would also help conserve the fossils by protecting them from the investigators themselves. Non-governmental agencies are probably the best sources for such a centralized archive because they may have more ability to work directly with multinational sources to broker a solution. In my opinion, such an archive would be more important and would have a more positive scientific effect than five years of ordinary research funding for such an organization.

Not so long ago, Wenner-Gren was the principal international sponsor of cast production. There is no logical reason why it or some other foundation could not be again.

Final thoughts

This turned into more of an essay than I really intended, but it is a subject that I think all of us are strongly invested in. The issues at stake are what kind of science we want to have, and how do we want to limit access to its findings. I believe that our research should be as public as possible. I think that openness leads to better science, and I think that restrictions to access only make us suffer at the hands of those who wish us ill. I hope that this new policy will lead to more conversations about the future of the field. I will be most pleased if I can play some role in moving those conversations forward.

References:

Gibbons A. 2002. Glasnost for hominids: seeking access to fossils. Science 297:1464-1468.

White TD. 2000. A view on the science: physical anthropology at the millennium. Am J Phys Anthropol 113:287-292.

I am at the AAPA meeting in Milwaukee this week, and so posting is by necessity very light. However, the news of the new Sahelanthropus remains and CT reconstruction have come out this week. I have been thinking about them since I got a hold of the proofs last week, so I can post some comments about them. There are some thoughts I'm holding on to for now, however, since I have a manuscript that covers some of them. It's bad enough to be scooped by other people; I surely don't want to scoop myself!

BBC News story, with artist rendition from Nature cover.

The lead story seems to be the reconstruction, probably because it was intended to sort out many of the problems with the distortion in the original fossil. To some extent it succeeds in simplifying the interpretation. For example, the reconstruction clearly places the foramen magnum in a more anterior position than the original. It is not clear to me how the anatomy of the original could conform to the reconstructed base, but doubtless working with a CT is better than working with photographs.

Actually, the article does not place a great emphasis on the anterioposterior position of the foramen magnum. This is sensible, because chimpanzees and australopithecines overlap considerably in this position compared to other basicranial landmarks like the bicarotid line. TM 266 is within the region of overlap, both in the original distorted version and in the reconstructed version.

Instead, Zollikofer and colleagues make two complementary arguments for why the skull is hominid. The first concerns the angulation of the foramen magnum (characterized by the basion-opisthion line) compared to a line tangent to the upper and lower orbital margins.

Despite substantial differences in neck orientation, humans and non-human primates tend to locomote with their orbital planes (the line joining the superior and inferior margins of the orbits) approximately perpendicular to the ground. In addition, primates orient the upper cervical vertebrae approximately perpendicular to the plane of the foramen magnum, and with only a limited range (about 10 degrees) of flexion and extension possible at the cranio-cervical joint. The combined effect of these angular constraints is that the angle between the foramen magnum and the orbital plane is nearly perpendicular in Homo sapiens (103.2 +- 6.9 degrees, n = 23) but more acutely angled in Pan troglodytes (63.7 +- 6.2 degrees, n = 20), and other species with more pronograde postures. The foramen magnum angle relative to the orbital plane in the TM 266 reconstruction is 95 degrees, similar to that in humans and later bipedal hominids such as Australopithecus afarensis (AL 444-2) and A. africanus (Sts 5). TM 266-01-060-1 as a quadruped would requier an unusually extended angle of the neck relative to the plane of the foramen magnum (Zollikofer et al. 2005:757).

A weakness in this argument is that this angle is exquisitely sensitive to the reconstruction. That is, a small difference in the vertical position of either basion or opisthion (the front and rear points on the foramen magnum border, respectively) will have a large effect on the angle of the line passing through these points. But assuming the reconstruction is correct, it is fairly compelling evidence that the habitual posture of the head in Sahelanthropus was not like chimpanzees or gorillas.

The second argument concerns the downward lip of the nuchal crest, which they argue indicates the directionality of the nuchal muscles. It is true that some other hominids have a downward lip on this crest, but I would like to go through a large ape sample to see the range of variation in this trait. In any event, this feature cannot be isolated from the exceedingly unique nuchal morphology in this specimen; the orientation and function of the nuchal musculature cannot be assumed to be like that of other apes whether it had vertical posture or not.

So was it a biped? From the reconstruction alone it may not be possible to confirm or deny the hypothesis. A more vertical habitual posture might or might not imply facultative bipedality. One possibility that would not imply bipedality is that Sahelanthropus had long arms, on the scale of Dryopithecus or longer. In this case, a quadrupedal stance would involve a more vertical trunk position. The distinction between this adaptation and that of gibbons or dryopithecines would be the larger body size and consequent greater degree of terrestriality. This hypothesis might also explain other Miocene hominoids that have been suggested to be like bipeds in certain characters, including Ouranopithecus and Oreopithecus. A test of the relationship of trunk position, limb length, and cranial base morphology might be informative.

Setting aside the question of whether it was a biped, was it a hominid? These are different questions if we assume that the advent of hominid bipedalism followed after some significant time the divergence of hominids from chimpanzees. Aside from Sahelanthropus, the earliest comparably complete hominid cranial remains are less than half its age. The closest is the as-yet-undescribed StW 573 skull. Then is KNM-WT 40000, followed by the cranial remains from Hadar, including the AL 444-2 specimen. A. afarensis and later A. africanus both have extensive adaptations to masticatory force. The extensive nuchal plane of TM 266 is long, narrow, and flat, and it is unlike any early hominid. The browridges are larger (especially in proportion to its relatively small overall cranial size) than in any australopithecine. Thus, it is a challenge to explain exactly what this skull represents in adaptive terms. I think an explanation of its anatomy is in order before it is accepted as being phylogenetically close to the australopithecines.

The paper by Brunet et al. (2005) presents new mandibular and dental remains of Sahelanthropus, including a lower canine with apical wear.

The new material presented here is important for several reasons. . . . The S. tchadensis hypodigm now includes a minimum of six individuals (a maximum of nine) from three sites in a small area of the Anthracotheriid Unit. Second, these new fossils now present a more complete and reliable understanding of this earliest known hominid taxon. S. tchadensis shares major derived features with other recognized hominids that are consistent with its position in the hominid clade, close to the last common ancestor of chimpanzees and humans. In the dentition these anatomical characters are a non-honing C/P₃ complex; no diastema between C and P₃; a vertical symphysis with weak transverse tori; canines with a small crown and long root; a lower canine crown with a large distal tubercle, both shoulders being very low; an upper P³ with a steeply sloping buccal surface; postcanine teeth with maximum radial enamel thickness intermediate between chimpanzees and australopithecines; and bulbous, slightly crenulated postcanine occlusal morphology. All the hominid mandubular premolar specimens from Toros-Menalla have the same root pattern, with two roots and three separate pulp canales in each premolar (one mesial and two distal) retaining the presumed primitive condition for the Pan/Homo clade (Brunet et al. 2005: 754).

This is a bit of a confused list, since very few of these characters are actually both derived and shared with later hominids. For example, a character that retains "the presumed primitive condition for the Pan/Homo clade" clearly is not a "major derived feature" shared with "other recognized hominids."

The most persuasive similarity with hominids is the reduced canine. But to my eyes, the Sahelanthropus lower canine is distinct from later hominids, especially considering the prominent ridge, or shoulder, around the base of the crown. This feature is found among dryopithecines, and it may simply be a primitive feature retained in an otherwise reduced canine. So the idea that this is intermediate between a larger, ape-like canine and the canines of later hominids is possible, but not demonstrated.

So in my view, the hypothesis that Sahelanthropus is in fact an early hominid has not been strongly substantiated. In many of its features it is basically plesiomorphic, and shares the morphology of a number of Miocene apes. In a few features, it shares a derived (or partially derived) morphology with australopithecines. It also has cranial features such as its long flat nuchal torus and hulking browridge that are derived, not shared with later hominids, and would therefore tend to indicate a separate evolution for this taxon. In my opinion, we probably have entered a time period early enough that the relationships of early hominids, early chimpanzees, gorillas and their ancestors may not be readily resolved with morphological comparisons.

References:

Brunet M et al. 2005. New material of the earliest hominid from the Upper Miocene of Chad. Nature 434:752-755.

Zollikofer CPE et al. 2005. Virtual cranial reconstruction of Sahelanthropus tchadensis. Nature 434:755-759.

A recent spate of articles has carried on a debate about the age of the Sterkfontein hominids. Sterkfontein is a complicated site, including several distinct caverns and deposition layers, called members. The dating of these layers is a serious problem because of their complex stratigraphy and the lack of volcanics that could be subjected to radiometric dating. Until recently the only insights into the age of the fossils came from uranium-series dating and paleomagnetic analysis of calcite deposits in the caves.

The Sterkfontein deposits are divided into six members, and hominid have been recovered from Member 5, Member 4, and Member 2. Most of the hominid remains assigned to Australopithecus africanus come from Member 4, which was long thought to date to between 2.8 million and 2.6 million years. Before Member 5 was deposited, there was erosion on the top of Member 4, and the two are separated by an unknown period of time. This deposit is generally thought to be less than 2 million years in age, perhaps extending as recently as 1.4 million years (Kuman and Clarke 2000). In recent years, excavations lower in the deposit, including the Jacovec cavern and the Silberberg grotto, have produced hominid fossils attributable to Member 2. These were initially believed to be around 3.5 million years old.

The most important fossils from Member 2 belong to the specimen StW 573. The foot bones of this skeleton were initially found in a dump of breccia outside the cave (Clarke and Tobias 1995). The origin of the bones was traced to Member 2, and they were matched to the broken end of a tibia still in situ in the Silberberg grotto. The skeleton is now known to be largely complete, including a skull and mandible, forelimb and hindlimb elements, and much else. It appears to be considerably more complete than the "Lucy" skeleton from Hadar, AL 288-1, or any other australopithecine, but it is not yet fully excavated from the overlying breccia and flowstone. The idea that this skeleton might date to 3.5 million years was potentially very important. At this date, it would be a contemporary of A. afarensis from Laetoli and Maka (it would be earlier than the Hadar deposits). It is not clear yet whether StW 573 anatomically resembles A. afarensis or is more similar to later South African hominids, but this would certainly be an important question to answer from the respect of early hominid phylogeny.

Making Sterkfontein later

McKee (1996) suggested that Member 2 was likely immediately earlier than Member 4. His argument was that the fauna of Member 2 were all found in Member 4, but several species were absent from Makapansgat Member 3 and 4, which date to between 3.2 and 2.9 million years. He proposed that this could be explained by the chance lack of these species at Makapansgat, but viewed that possibility as less likely than the hypothesis that the species appeared after Makapansgat Member 4, to be found in the later Sterkfontein deposits.

Clarke and Tobias (1996) responded to this argument by noting the long stratigraphy of Member 3 between the Member 2 and 4 sequences, with several flowstones that must have taken a long time to deposit. They note that although Makapansgat does not preserve all the Member 2 fauna, the species that are absent are known from other African sites prior to 3.5 million years, and therefore are not of use in dating the deposits. The exception is one baboon species, Papio izodi, which is known only from Member 4 and Taung, and may therefore be rare enough to be absent from other sites.

Berger and colleagues (2002) argued that the entire Sterkfontein sequence is substantially later than had previously been thought. They base their argument on biostratigraphic and paleomagnetic considerations. They have a number of reasons for this:

1. The presence of Equus in the deposit, which is not radiometrically dated in Africa earlier than 2.36 million years ago.
2. In addition to Equus, several other taxa are found in Member 4 that do not have secure radiometric dates above 2.5 million years anywhere in Africa.
3. A later date for Member 4 would suggest that the sequence of magnetic samples from the site should be displaced earlier by a reversal cycle. This would place the top of Member 2 within the Olduvai subchron, and the StW 573 hominid would then date to between 2.15 and 3.04 million years ago. If this is displaced by another cycle more recently, StW 573 would date to as recently as 1.07 to 1.95 million years.

As far as Equus, Kuman and Clarke (2000) are at pains to show that it actually may not occur in Member 4. According to them, only one equine tooth has been excavated from Member 4 in situ, with the remaining bones taken from fill that may derive from Member 5. They argue that the one tooth is insufficient evidence of the presence of the genus, considering the possibility of erosion from later deposits.

Making Sterkfontein earlier

Partridge and colleagues (2003) dated the Sterkfontein Member 2 deposits by using the radioactive decay of cosmogenic isotopes. These are created when cosmic rays from outer space interact with the elements in quartz grains near the earth's surface. In particular, aluminum-26 and beryllium-10 accumulate in quartz grains at a predictable ratio. These two isotopes have different half-lifes (²⁶Al = 1.02 million years, ¹⁰Be = 1.93 million years), which means that once the quartz grain is buried and no longer exposed to cosmic rays, the ratio of the two isotopes changes.

Sediments near the StW 573 specimen gave a date estimate of 4.17 million years, while the orange breccia in the Jacovec Cavern gave an estimate of around 4.02 million years. These date estimates are substantially earlier than were previously estimated for these localities at the site.

It was not possible to date Member 4 in this way, because it is shallow enough that cosmic rays can still affect the quartz grains used for dating.

Partridge et al. (2003) do not present a response to Berger et al. (2002), except to note that their earlier dating "is unsustainable on stratigraphic and faunal as well as on paleomagnetic grounds" (612, note 12). In any event, there seems to be no strong biostratigraphic reason to place Member 2 at either an earlier or later date; the preserved fauna is not specific as to age.

Member 5 stratigraphy

Kuman and Clarke (2000) review the stratigraphy of Member 5. The most important hominid specimen that has been attributed to Member 5 is StW 53, a nearly complete skull that has been variably attributed to A. africanus or Homo habilis. Kuman and Clarke (2000) show that the skull derives from an area that likely is intermediate in age between Members 4 and 5 proper. They call this area the "StW 53 Infill." No artifacts derive from this area. The authors argue that the infill is likely more recent than Member 4 because of the presence in the deposit of Theropithecus oswaldi, a species found in the later Swartkrans Members 1--3, and associated with drier open grassland habitats. On this basis, they place the StW 53 Infill between 2 million years ago and 2.4 million years, which marks the earliest appearance of T. oswaldi in East Africa.

According to Kuman and Clarke (2000), Member 5 can be divided by the presence of two distinct tool industries. The Oldowan Infill dates to between around 2 million and 1.7 million years ago, and preserves 3245 excavated artifacts (Field 1999). The paleoenvironment seems to indicate a grassland. The later phase is referred to the Acheulean because of the presence of bifaces, and is placed between 1.7 and 1.4 million years ago. Like the earlier Oldowan infill, the Acheulean infill represents a predominantly grassland fauna, similar to Swartkrans.

Kuman and Clarke (2000) provide a list of hominid fossils with their probable associations in the stratigraphy. They also discuss the taxonomy of the fossils and their resemblances with elements of the earlier Member 4 and Swartkrans remains.

More on Sterkfontein

More on Makapansgat

References:

Berger LR, Lacruz R, de Ruiter DJ. 2002. Brief communication: Revised age estimates of Australopithecus-bearing deposits at Sterkfontein, South Africa. Am J Phys Anthropol 119:192-197.

Clarke RJ, Tobias PV. 1995. Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269:521-524.

Clarke RJ, Tobias PV. 1996. Faunal evidence and Sterkfontein Member 2 foot bones of early hominid. Science 271:1301-1302.

Field AS. 1999. An analytic and comparative study of the Earlier Stone Age archaeology of the Sterkfontein Valley. MasterÕs thesis, University of the Witswatersrand.

Kuman K, Clarke RJ. 2000. Stratigraphy, artefact industries and hominid associations for Sterkfontein Member 5. J Hum Evol 38:827-847.

McKee JK. 1996. Faunal evidence and Sterkfontein Member 2 foot bones of early hominid. Science 271:1301.

Partridge TC, Granger DE, Caffee MW, Clarke RJ. 2003. Lower Pliocene hominid remains from Sterkfontein. Science 300:607-612.

The Journal of Human Evolution early access section has a paper by J. Michael Plavcan and colleagues that critically examines the case for low sexual dimorphism in A. afarensis.

To catch you up briefly with the story, here is a synopsis to date. Before 2004, the consensus about A. afarensis was that the samples from Laetoli, Maka, Hadar, and other smaller samples belonged to a single species with substantial sexual dimorphism (gorilla-like or orangutan-like in extent) and considerable temporal change from the early to late end of the sequence. A minority view was that there were actually multiple species in the sample. Some thought this because they thought that some of the important specimens (especially AL 288-1, Lucy) had been misidentified as to sex. Others were either unconvinced by the morphological similarities among the samples, or were rightly skeptical about the weakness of the test for sexual dimorphism within species. That is to say, the variation would have to exceed that found in gorillas (with males double the mass of females) before the single-species hypothesis would be rejected.

In 2004, Reno and colleagues added another perspective. They applied a resampling technique to estimate the sexual dimorphism in the A. afarensis sample, transforming the sizes of different skeletal elements to a single scale in order to increase the effective sample size. Their conclusion was that the variability in the A. afarensis sample was most consistent with a low level of sexual dimorphism, similar to humans. They used this observation to suggest that the social behavior of early hominids may have included a more humanlike mating system, consistent with Lovejoy's (1981) account of the origin of bipedalism.

Plavcan and colleagues (2005) present several arguments as to why the conclusions of Reno et al. (2004) may be flawed.

1. They suggest that the AL 333 sample, upon which Reno and colleagues hinge some of their conclusions, is likely to be biased, In particular, they suggest that there are probably more males than females, and probably many of the elements included as separate individuals by Reno et al. (2004) actually belong to a smaller number of individuals. Plavcan and colleagues suggest that the MNI for the postcranial elements alone at this locality is three adults and one subadult. Their conclusion from this is that estimates based on AL 333 are likely underestimates. However, in my view, this probably does not detract substantially from the results of Reno and colleagues' analysis. Consider that the overall A. afarensis sample was statistically similar to the AL 333 sample alone, and Reno and colleagues attempted to assess the possible effects of sample bias by simulating samples in which one sex was highly overrepresented.
2. Plavcan and colleagues argue that the non-AL-333 elements of the A. afarensis sample actually show high variation. This observation is not present in Reno and colleagues (2004), who instead present these remains together with the AL 333 locality as a "Combined Afar" sample. This is more of a problem. Reno and colleagues suggest that the "Combined Afar" sample should be more variable than AL 333 because the combined sample includes specimens across a broad time interval, but as Plavcan et al. note, there is actually little variation over time noted (as yet) for this time span. And Plavcan and colleagues provide an illuminating figure that shows that temporal variation in overall size (without dif