A. africanus

I've followed the literature on early hominid diets from the beginning of the weblog. In 2005 I discussed Peter Ungar's analyses of dental occlusal morphology in A. afarensis versus Homo, concluding:

The contrast between Homo and A. afarensis is in the same direction as the contrast in occlusal morphology between primarily meat-eating carnivores like felids and canids as opposed to more omnivorous carnivores like bears. Another observation is that meat is a major food resource of chimpanzees, although this is hardly a fallback resource. Indeed, if meat eating was indeed an important component of the behavioral repertoire of early Homo, it probably is not fair to assert that the difference in diet between Homo and Australopithecus was primarily a difference in fallback resources. It may be true that australopithecines and early Homo overlapped in their food resources, particularly in plant species consumed. But considering the likely effectiveness of early humans as predators, I think it likely that the fallback foods of early humans--when hunting was ineffective--may well have been the preferred foods of australopithecines. And when australopithecines were forced to abandon their preferred foods by early humans, they were forced to fall back upon resources that either were common or were difficult for early Homo to exploit. The disappearance of early small-bodied Homo by around 1.6 million years ago, and the ultimate extinction of the robust australopithecines after a progressive increase in their molar sizes (Wood and Lieberman 2001) indicate that this fallback strategy could not be maintained in the face of increased hunting effectiveness by large-bodied Homo.

The concept of "fallback foods" has captured a large mindshare in explaining early hominid diets. The idea is that a species may depend on preferred, staple foods for most of the year, but adopt less preferred, "fallback" foods when their staple is not available -- for instance, during the dry season.

What can fallback foods explain about early hominids? For one thing, they could explain the difference between robust and non-robust australopithecines. We know from isotope data (reviewed in this 2005 post about Matt Sponheimer's work) that A. africanus and A. robustus had similar fractions of C₃ and C₄ plant source foods in their diets. Across the year, they may have eaten roughly the same mix of foods. A 2005 paper by Greg Laden and Richard Wrangham (discussed here) explored the idea of underground storage organs of plants, or tubers, as fallback foods for australopithecines. Later studies of isotope data using laser ablation of small segments of the enamel (discussed here) showed that diet proportions may have substantially varied across the time that teeth were developing -- possibly concordant with the idea of seasonal or longer-period fallback foods. An earlier analysis of dental microwear in the two hominids by Scott and colleagues (discussed here) came to a similar result: there was great variability in wear properties, especially within A. robustus, although the average in the two species showed a possibly greater fraction of brittle, hard foods consumed by the robust australopithecines.

So I've written about the topic a lot, and followed it closely.

Now, Peter Ungar, Frederick Grine and Mark Teaford have examined the wear properties of the molars of Australopithecus (Paranthropus) boisei. They find that -- unlike A. robustus -- none of the seven specimens showed any evidence of having eaten hard or brittle foods:

Comparisons with the extant baseline series suggest that none of the Paranthropus boisei individuals examined consumed extremely hard or extremely tough foods in the days before death. All of these specimens lacked the extremes of Asfc evinced by Lophocebus albigena and especially Cebus apella, both known to consume hard, brittle foods. Paranthropus boisei molars also lacked the extremes of epLsar seen in Trachypithecus cristata and Alouatta palliata, both known to consume tough leaves and stems. The P. boisei individuals examined evidently avoided such metabolically challenging foods, at least in the days before death. This is notably consistent with Walker's [23] early assertion that P. boisei microwear patterns resemble those of living frugivores, and differ from those of living grazers, leaf browsers, and bone feeders.

Comparisons with the South African hominins suggest that while Paranthropus boisei may have consumed foods with similar ranges of toughness as those eaten by Australopithecus africanus, the eastern African "robust" hominin did not eat harder and brittler foods than the South African "gracile" form. Further, the patterns for P. boisei and P. robustus are very different. Paranthropus robustus likely ate foods that were on average much harder and less tough than P. boisei. The differences in both central tendencies and ranges of variation suggest different feeding strategies, and by implication, that the two species of Paranthropus probably had markedly different diets or foraging strategies (Ungar et al. 2008, italics lost).

That is very interesting that A. robustus and A. boisei are so different in their microwear patterns. It makes me wonder whether there may have been substantial habitat variation in the use of hard foods -- maybe the extant A. robustus sample, mainly drawn from a small area of South Africa, had access to some food items that were rare or absent across the larger East African range of A. boisei. But if some A. boisei populations had also depended on such hard resources some of the time, you might expect that we would have found one, or at least a bit more variability. Yet the sampled specimens, drawn from a distance from Ethiopia to Tanzania and well over a half million years of time, are pretty uniform in their microwear, showing some variability in the anisotropy dimension (here, high values have mostly parallel striations, attributed to fibrous food consumption).

So we can return to the question: the major hominid competitor of A. boisei was Homo. Both lineages appeared in the period around 2.5 million years ago, and remained sympatric throughout the next million years. Some of the dynamics of that interaction must have involved diet (considering the different dietary adaptations of the two). We can speculate that A. boisei didn't get much meat, which would then be an important difference. But what else was A. boisei eating?

Meanwhile, the data are still consistent with the idea of fallback foods in A. robustus as a driver of dental morphology, but the story for A. boisei now seems less clear. With only seven specimens, there is almost certainly not enough data to test the hypothesis -- which after all predicts that the use of hard brittle foods may be rare. But that's not positive evidence either. Is there some other food that might explain the hyperrobust craniodental morphology?

References:

Ungar PS, Grine FE, Teaford MF (2008) Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE 3(4): e2044. doi:10.1371/journal.pone.0002044

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.

It's that time of year again -- the time when those boring ``Year in Review'' magazines are on newsstands, and when pundits make fools of themselves predicting what will happen in the next year.

Well, I'm not too proud to join the fools, as I've shown the last two years. In 2006, I got five predictions right out of ten. Not bad for my first outing, but you'll see that last year's predictions fared even better:

• 10. Sahelanthropus postcrania will be published. I'm frankly shocked that this didn't happen. I don't doubt the rumors, but I'm starting to wonder whether this story is more interesting than it looks....
• 9. Two words: Holocene evolution. OK, this was a little unfair, considering that my work was an important part of making this prediction come true. Still, Discover made ``recent human evolution'' one of its top 100 science stories of the year, even before our December paper came out -- mainly on the strength of the paper by Scott Williamson and colleagues from earlier this year. And "Human genetic variation" was Science's "Breakthrough of the Year" -- most of that variation representing recent evolution.
• 8. Despite (or because of) the success of the Neandertal genome project, there will be no genetics of any kind published on early modern skeletal material. Puzzling, isn't it? But then, considering the trouble with Neandertal contamination reported in August, maybe we're better off leaving the early Upper Paleolithic alone for a while.
• 7. The mitochondrial history of human dispersals will become more and more detailed, but no paper will test against other loci. D'oh! Reading this one a year later, it's pretty obvious that I should have included Y chromosome in this one, since those two get compared all the time! Proofread, Hawks!
• 6. Another (yes, another) paper about the chimpanzee-human divergence will peg it between 5 and 7 million years ago. Will they never tire of these? Hobolth et al. (2007, PLoS Genet 3:e7) pegged the divergence at 4.1 million years. That's too recent to fit my prediction. Instead, I have to turn to Ebersberger et al. (2007, Mol Biol Evol 24:2276), who placed the divergence at 5.7 million years ago. Both estimates are too recent for Sahelanthropus, which the geneticists have started to figure out....
• 5. Three papers with new Ethiopian fossils. The last few years, one annual Ethiopian find seemed to be predictable enough. So I figured, why not three? We got a not-nearly-noted-enough paper this summer by Gen Suwa and colleagues descringing the Konso Homo erectus remains. Then, Suwa brought us Chororapithecus -- hey, I didn't say "hominid!" That's two. But despite the long-ago announcement of the Woranso-Mille skeleton, its appearance in a meetings abstract and a mid-summer press release about further Mille fossils, all we got from the peer review system is a lousy faunal list. Well, the faunal list does include the hominids. Should it count as a "paper with new Ethiopian fossils?" I'll say yes -- hey, unlike Aramis, at least the Mille fossils are new!
• 4. Another early Upper Paleolithic specimen will emerge from a museum collection. The only bizarre thing about this one was the location: South Africa. Hoffmeyr may not be that convincing as a European early Upper Paleolithic skull, but it was sure sold that way. Weird.
• 3. A big year for Miocene apes, which will look increasingly important in the story of human brain evolution. No brains, but it sure was a big year for Miocene apes, with two significant East African discoveries claiming to push back the timeline of African ape divergence.
• 2. Maturation rate in early Homo becomes a dead issue, because of the variation in dental and skeletal maturation in living people. Wishful thinking. Still, did Tanya Smith (2007) breathe new life into perikymata? Let's just say that unresolved questions remain.
• 1. The year will end without a single new hominid species having been named. This one was like dodging a bullet, since new species riffle out of paleoanthropologists' minds all the time. From 2001 to 2006, there were six (six!). In 2007, none.
• BONUS: A dramatic development in the problem of pre-2.0-million-year-old Homo. Rats.

OK, that's seven out of ten. It's beyond belief that I did better in the top five than the bottom five -- I picked those because they were far out there. I mean, really -- a new Upper Paleolithic specimen from a museum collection? After Muierii, that's like calling lightning to strike twice. But there it is, and in January, no less.

I'm clearly going to have to pick stranger predictions this year. And I'll have to be careful about that "dramatic development" line -- I mean, it's appropriately Delphic, but what is it supposed to mean, really? I wonder whether "operatic development" might be better.

And do I dare call down my non-lightning strike for a third year? It's ruining my percentage! It's starting to reek of desperation -- I mean, it starts to look like the stopped watch effect even if it happens.

Oh, well. I mean, those are just the risks of predictions, right? Suppose in the preseason I had picked Kansas to win the Orange Bowl!

• 10. A dramatic development in the Sahelanthropus story.
• 9. Both major-party candidates for the 2008 U.S. Presidential election will accept evolution.
• 8. This year's featured piece of anatomy: the femur.
• 7. No new hobbits, at least, not from Flores.
• 6. An incisive example of introgression in East Asia.
• 5. A viral insertion in the human genome will tell us about a disease of the australopithecines.
• 4. Another language gene joins FoxP2. No word on whether Neandertals have the human version.
• 3. Homo habilis: an endangered species?
• 2. This year, something new from three A's: A. afarensis. A. africanus. Atapuerca.
• 1. Oh, and one more A. Ardipithecus.
• BONUS: A big, big year for Neandertals. I mean, besides the election.

There you have it. I'm not sure which of these is the riskiest, but I'm sure they're more out on a limb than last year!

Raymond Dart (p. 198 of Australopithecus africanus, the man-ape of South Africa, Nature 115:195-199, 1925), summing up why hominids might have lived in what seemed "harsh and forbidding" environments for a primate:

In anticipating the discovery of the true links between the apes and man in tropical countries, there has been a tendency to overlook the fact that in the luxuriant forests of the tropical belts, Nature was supplying with profligate and lavish hand an easy and sluggish solution, by adaptive specialization, of the problem of existence in creatures so well equipped mentally as living anthropoids are. For the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of intellect -- a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species. Darwin has said, "no country in the world abounds in a greater degree with dangerous beasts than Southern Africa," and, in my opinion, Southern Africa, by providing a vast open country with occasional wooded belts and a relative scarcity of water, together with a fierce and bitter mammalian competition, furnished a laboratory such as was essential to this penultimate phase of human evolution.

Just noticing, in this John Noble Wilford article:

A new report, to be published Thursday in Nature, will review more skeletal evidence of the transitional aspects of the Dmanisi specimens.

More later...

UPDATE(2007/09/18): Wilford doesn't directly state the article's theme but it clearly has one: Why the heck can't these people agree about these fossils that have been out of the ground for thirty years?

The first answer that everyone has given him is about the "million year gap" between 3 million and 2 million years ago. People can't agree about early Homo because they can't decide what its ancestors looked like. Without any ancestors, they don't know which of the traits of early Homo are derived.

For a good example, we can turn to a feature Wilford doesn't mention: limb proportions. Recently, a lot of ink has been spilled discussing the evolution of arm size in later australopithecines and early Homo. OH 62 (probably Homo habilis) and A. africanus have been argued to have large arms compared to their legs. A. afarensis and Nariokotome (KNM-WT 15000, probably Homo erectus) have relatively small arms compared to their legs. Did H. habilis and H. erectus have different ancestors? Did H. erectus evolve from H. habilis, reverting its limb proportions to earlier A. afarensis? Or are all these comparisons just batty, since only three specimens have arm and leg elements whose length can be compared? There's no clear answer; but one of the most important specimens in the question (with sort-of-intermediate limb proportions) is the Bouri skeleton, BOU-VP 12/1, which at 2.5 million years old is right in the middle of that "gap."

The more you look at the "gap," the less gap-like it looks. For one thing, we have a pretty good idea of what was going on behaviorally during that million year span. The first stone tools are 2.6 million years old. The technology of these toolmakers -- although simple -- included all the basic manufacturing methods used before 1.5 million years ago. The tools were used to butcher animals and break bones for marrow; so we know that the toolmakers were depending on meat.

Second, we actually have quite a lot of fossils from this time period. The entire South African A. africanus fossil record, with the exception of a few early specimens like STW 573, come from this "gap." A fairly extensive record of the appearance and evolution of early robust australopithecines comes from this time period in East Africa.

And, here and there, a few specimens look Homo-like. Wilford's article discusses AL 666-1. To this we can add the Uraha mandible, Omo 75-14, an additional series of teeth from Omo, and possibly the Bouri BOU-VP 35/1 skeleton.

Properly considered, the rarity of early Homo in these contexts is not a problem; it is information. Wilford quotes Philip Rightmire to this effect, and we can easily expand on the basic concept. Early toolmakers did not undergo an immediate geographic expansion upon their origin. They spread across a relatively narrow strip of East Africa and stayed there for more than a half-million years. They were initially rare. That means that their adaptation was not immediately a barnburner of a success -- the early toolmakers took a while to perfect the adaptation of later Homo.

The middle part of the article takes in another reason for disagreement: whether H. habilis and H. erectus were ancestor-descendant:

Several scientists, notably Dr. White of Berkeley, took issue with the interpretation seeming to imply that evidence for the two species overlapping in time and exhibiting variable sizes was new. That, he said, had been recognized for a couple of decades.

Dr. Kimbel, who was not involved in the new research, defended the authors, saying that they had not "meant to imply that habilis could not have been ancestral to erectus, presumably on the basis of their being contemporaneous at Turkana," the site in Kenya where the fossils were found.

Susan C. Anton, an anthropologist at New York University who was a member of the Spoor-Leakey team, said, "My money is still on habilis as the potential ancestor, but there is a lot of room for additional knowledge, given the dearth of fossils."

None of these statements really disagree with each other. If anything, this particular question may have gotten easier to resolve lately, not as a consequence of new fossils, but as a result of new dates for many of the old ones. Susan Anton is later quoted saying that anagenesis "is the only option that is no longer on the table," and it seems to me that this is the clearest statement most likely to invite some hypothesis testing. But it is fairly clear that this problem cannot be resolved in terms of earlier fossils: I don't think there's any compelling evidence of H. erectus before 1.6 million years ago.

There is one significant word that doesn't appear in the article -- an absence that is especially interesting considering the quoted scientists:

Kenyanthropus

Remember, the dominant theme is about complexity and bushiness. And yet, here's that forgotten branch of the family tree; the one that was supposed to clarify everything by providing a different ancestor for KNM-ER 1470 from other H. habilis specimens, the one that showed a distinct line leading to Homo originating in the Early Pliocene.

I think our bush may have been pruned.

In case you're following the debate about Homo habilis limb proportions, there's a new contribution by Martin Haeusler and Henry McHenry in the JHE holding pen. They examined the partial KNM-ER 3735 skeleton.

KNM-ER 3735 is often assigned to Homo habilis, but it's not exactly an easy diagnosis. There are a few pieces of the skull preserving anatomy, including the cheek, frontal and temporal. Here's what Bernard Wood (1991) had to say about the skeleton:

The form of the mandibular fossa and malar region virtually preclude this specimen from being attributed to A. boisei. Its general affinities are with Homo. Some features (e.g. vault thickness) ally it with a Homo erectus-like hominid, but in other areas (e.g. the frontal) it is more like crania such as KNM-ER 1813, a conclusion endorsed by Walker (1987) and by Leakey et al. (1989). Tobias (1989) includes KNM-ER 3735 within H. habilis.

Provisional taxonomic assessment: Homo sp. indet.

Well, that's not exactly a rousing endorsment. You can see the problem --- and it's a common tale for hominid fossils. It has a smaller brain than early H. erectus (that would be the "frontal looks like KNM-ER 1813 bit). But its cranial bones are thick. The most complete of the bones in the skeleton is a radius, but it's not complete. The best bone for estimating joint surface area is the sacrum; a femur shaft is there, but it falls short of the midshaft length.

And there's a problem: the radius seems pretty big, but the sacrum is little. If it were a human, the radius looks like it came from a body twice the size of the sacrum. There's something going on here. Previous work has assumed that the sacrum is more likely reflective of the size of the body, and the radius is therefore big compared to a small body mass. Maybe that means more climbing, leading to a greater role in weight support for the arms. Or maybe it means a retention of more apelike proportions.

This is a frustrating literature to follow, because pretty much every other early specimen except Lucy (AL 288-1) and the Nariokotome skeleton (KNM-WT 15000) present exactly the same problem. You can't estimate limb sizes very accurately from small pieces of bone. And you can't estimate proportions accurately at all without estimates of size. Plus, it's not clear that you can interpret limb proportions without a decent estimate of body mass. Two years ago, there was a huge go-around about the limb proportions of OH 62. Like KNM-ER 3735, it looks to have a relatively large arm compared to its body. Or maybe the legs are short. Or maybe the estimates are bad. You get the picture. So everybody has a different clever statistical transformation to try to make these fossils comparable to each other. I have no argument with any of the work; but it seems like the error involved in these assessments of proportions is pretty large relative to the information content of the bones.

Here's some of the conclusion from Haeusler and McHenry:

Our analyses suggest that the idea that KNM-ER 3735 had more primitive body proportions than A.L. 288-1 (e.g., Leakey et al., 1989) needs to be refined. We found a unique but distinct mosaic of modern and ape-like limb proportions in the two early hominid species. H. habilis shares a gracile humerus and radius and a small base of the hand phalanges with the earlier A.L. 288-1 and modern humans. In addition, other characteristics, including the relatively small size of the sacrum and a robust midshaft of the phalanges, are common to both early hominids and extant great apes. Surprisingly, however, those upper limb proportions that differ between the two fossil species, such as a robust scapula, a long radial neck, and a long forearm, are all more ape-like in H. habilis.

In KNM-ER 3735, the shoulder muscles that originate on the scapula (trapezius, deltoid, supraspinatus, and infraspinatus) as well as the biceps brachii were, therefore, probably not only more powerful than in modern humans, but also stronger than in A.L. 288-1. On the other hand, the extraordinarily short lever arm of A.L. 288-1's biceps muscle, the minute elbow size, and the small radial head may indicate a weaker arboreal component in its behavioral repertoire than in H. habilis. However, in the absence of modern correlates, caution is needed with respect to possible behavioral implications of the different forearm proportions in the two species.

They also note the Homo-like anatomy of the femur shaft, including a marked pilaster.

Seth Dobson (2005) claimed that that the sacrum of STW 431 (A. africanus) is also small -- it certainly yields a small mass estimate compared to other elements of the skeleton. Heck, all of the early hominid sacra yield small mass estimates. Well, you can see it's confusing.

References:

Haeusler M, McHenry HM. 2007. Evolutionary reversals of limb proportions in early hominids? Evidence from KNM-ER 3735. J Hum Evol (in press) doi:10.1016/j.jhevol.2007.06.001

Dobson SD. 2005. Are the differences between Stw 431 (Australopithecus africanus) and A.L. 288-1 (A. afarensis) significant? J Hum Evol 49:143-154. doi:10.1016/j.jhevol.2005.04.001

I'm about two-thirds of the way through Mike Morwood's new book, The Discovery of the Hobbit, and I'll be posting a review when I'm through. Generally, I have a positive opinion of the book so far.

Henry Gee has reviewed the book in this week's issue of Nature. I wanted to point out my generally positive attitude about the book, so that you'll know that my miserable opinion of Gee's review has little to do with the book's merits.

Consider how Gee starts his review:

The unicorn, wrote Jorge Luis Borges (in Kafka and His Precursors), is universally regarded as a supernatural being of good omen. But there's a problem: despite its folkloric familiarity, we wouldn't know how to recognize a unicorn if we met one in real life. It "does not figure among the domestic beasts, it is not always easy to find, it does not lend itself to classification," Borges continues. "It is not like the horse or the bull, the wolf or the deer. In such conditions, we could be face to face with a unicorn and not know for certain what it was."

Is Gee smoking crack? What kind of blather is this?

First of all, I know I'm being terribly literal, but a unicorn is a horse with a horn. One horn. Not so hard to recognize! Maybe my 3-year-old daughters could help edit at Nature.

Let's see, where have I seen one of those that Gee might recognize? Oh, yeah:

Photo credit: Simon Stratford (via stock.xchng)

There it is, sound as a pound.

Next, Gee spends several paragraphs expositing on his own role in the publication of the Homo floresiensis announcement. We learn some interesting little facts, like how the authors wanted to name the species "Sundanthropus floresianus" until a reviewer pointed out that future students would confuse the name with a flowery butt.

I kid you not. Nature has a layer of reviewers to take tushie references out of taxonomy. Somehow they can't tell a left femur from a right, but they're on the watch for sphincter-species!

The review is entirely self-serving -- there are only three paragraphs that include any reference to the book! In the midst of this babbling about unicorns and hobbits, Gee tells us that skepticism at new hominid discoveries should be dismissed as the predictable result of "mindsets" of the skeptics:

Such reaction is common in the wake of new hominid discoveries, which are routinely dismissed either as pathological humans (Homo neanderthalensis) or apes (Australopithecus africanus and Sahelanthropus tchadensis). Such reactions say less about the facts than the mindsets of commentators, who might be unwilling to have their comfortable views of the world so forcibly changed. Confronted with what might be a genuine unicorn, many would prefer to see a pantomime horse with a spike glued to its head.

Ooooh! Since I'm one who has been notably skeptical of Sahelanthropus and have approached H. floresiensis skeptically, I'm obviously a prime target for this paragraph. It is so comfortable to stay in my view of the world where hominids interbreed with each other. Clearly, a bestiary that includes small-brained island bipeds must shake me out of my comfort zone.

How could I have been so wrong! When every species ever proposed has faced the same resistance? Sure, Tim White says that Kenyanthropus is a glued-together matrix-filled A. afarensis, but that's just his mindset. Or how about Eoanthropus? Sure, Franz Weidenreich thought that it was just a concoction by "English authors," but couldn't he tell that it was more than just a pantomime skull with an orangutan jaw? Why couldn't I see that these petty minds were just holding back the important work of taxonomy!

No, no, no. You see, if we approach things skeptically, we won't dare to dream about the unicorns:

The unicorn remains as it always did, frustratingly elusive. This year, the researchers will return to Liang Bua to see if they can discover more. But stories such as this demand a mythological beast altogether less serene. It is as if the researchers had set out to discover some new form of fossil mouse, only to find that they had grabbed a dragon by the tail instead. And as any devotee of Harry Potter will remind you: Draco dormiens nunquam titillandus.

The theme of the review is perhaps to be expected from Gee, otherwise known as the author of The Science of Middle-Earth. But I find his mixture of fantasy and science to be especially malaprop in the context of the Flores fossils, since with every fantasy word he detracts from the credibility of the journal's review process!

Some of you will have seen the episode of The Simpsons, titled "Lisa the Skeptic," where Lisa excavates an "angel" from the ground. Here's part of the synopsis from Wikipedia:

As Homer attempts to get a motor boat, a new shopping mall in Springfield is being built on an area where a large number of fossils were found. Lisa condemns and protests the building of the mall. Thanks to her protest, it prompts the school to conduct an archaeological dig. When Lisa is digging, it reveals a human skeleton with wings. Springfield's residents are convinced it is an angel, and Homer cashes in by moving the skeleton into the family's garage; however, Lisa is skeptical, believing it may not actually be an angel, and even has Stephen Jay Gould test a sample of the skeleton. The next day, Dr Gould runs to the Simpson house and said the tests came out inconclusive and after Lisa on television compares belief in angels to belief in unicorns and leprechauns, Springfield's religious zealots riot and destroy all of the scientific institutions.

Later, we find out that the "angel" is a publicity stunt for the new mall; Guest voice Gould confesses that he never really performed any tests on the "angel". This is one of my favorite episodes: it's a rare one where Lisa's preachy skepticism is entirely justified, and the "expert" doesn't care enough to do anything at all.

Now I know, that the episode was missing a scientific editor to encourage Lisa to forget about her doubts, and just to accept the "angel" for what it is. After all, every new discovery has its skeptics.

Well, there is a lesson to take away from all the unicorn talk. If you are in Cardiff and find the skeleton of a giant, be sure to send your report to Nature, where you'll find a receptive editor. Despite what they may say, there's not one of those born every minute.

UPDATE (4/26/2007): A reader e-mails, "Remember that Borges was blind." True. Perhaps we can extend this analogy further?

Another reader: "Well, at least we can expect a fair set of reviews on the Sahelanthropus postcrania...D'oh!"

References:

Gee H. 2007. In a hole in the ground.... Nature 446:979-980. doi:10.1038/446979a

Sponheimer and colleagues (2006, link) zapped some Swartkrans teeth with lasers to measure their ¹³C content. I wrote quite a bit here last year about australopithecine diets, including a long review of isotopic evidence for australopithecine diets.

With respect to dietary differences between A. africanus and A. robustus (the two species with any substantial isotopic sampling), there are four essential observations:

1. The apparent C_{4 dietary content of the two species is basically the same, and fairly high.}
2. High C₄ foods are not so easy to come by, they include some grasses and sedges and the animals who eat them.
3. The Sr/Ca ratios of the two species are fairly different.
4. The postcanine teeth of A. robustus seem to be adapted to crushing and grinding, moreso than A. africanus.

One hypothesis for the difference in Sr/Ca ratios is exploitation of underground tubers (warthogs and mole rats have elevated Sr/Ca similar to A. africanus). A mix of C₄ foods has been proposed to solve the grass-eating problem, including seeds, rhizomes, insects, lizards, and herbivore meat. But these don't really solve the postcanine tooth conundrum, and while they may both be true; neither is really testable.

OK, so does the new laser ablation study solve any problems? First, let's read a bit about what exactly it is, and why it might be useful. Ann Gibbons has written a ScienceNOW article:

[A] team of American and British researchers studied the teeth of four individuals of Paranthropus robustus (also known as Australopithecus robustus) from the Swartkrans Cave in South Africa. The team scanned the teeth with a sensitive laser, which did not destroy the teeth but etched them lightly enough to free carbon gases long trapped in the enamel. Because different plants absorb atmospheric carbon dioxide differently, the researchers were able to see what types of vegetation the hominids ate based on the ratio of carbon isotopes in their teeth.

An accompanying perspective by Stanley Ambrose explains:

In contrast to conventional methods, the laser ablation technique used by Sponheimer et al. barely penetrates the enamel surface of an area of less than 0.5 mm² and is thus nearly nondestructive (2). Laser ablation also avoids the problem of time averaging in large drilled grooves. Moreover, perikymata can be counted, providing a good estimate of the minimum time interval sampled and of the duration of tooth formation.

The Paranthropus teeth studied by Sponheimer et al. show interesting patterns of seasonal variation in diet and climate. All have the isotopic composition of mixed feeders, and two show at least ca. 40% variation in the proportions of C3- and C4-based resources over 1 year. One individual had a predominantly C3-based diet and foraged in a cooler, more humid environment; it may have formed its tooth in a very wet year. The others ate more C4-based foods in a warmer, drier environment. Their average carbon-isotope ratios are similar to those of adaptively versatile savanna baboons (2). Analyses of seasonal variation in teeth of modern and fossil baboons and of other hominin species are necessary to evaluate dietary specialization in Paranthropus and niche overlap with other hominin species.

Back to me. There are two possibilities. First, the differences between ¹³C values for different samples might be sampling the actual dietary variability of single A. robustus individuals over the course of their tooth development (in this paper, sampled over a course of a couple hundred days).

Or second, they may just be sampling noise.

The paper presents comparative data to suggest that this is actual variability in diet and not isotopic noise. They sampled some steenbok teeth from Swartkrans with the same technique. Steenbok are consistent C₃ browsers; their diet doesn't vary much in its ¹³C proportion over time. And the samples from the steenbok teeth didn't show very much variation across different sampling zones from the same tooth. Hence, it looks like the samples from different perikymata actually may give a consistent picture of dietary ¹³C composition over time.

Compared to the steenbok, the A. robustus samples show great heterogeneity in ¹³C content. This heterogeneity is manifested when looking at multiple samples from the same tooth, and it is also manifested when looking at different individuals. So far, that would seem to indicate dietary heterogeneity -- the A. robustus individuals ate a different mix of foods over time, and different individuals ate different foods.

On the basis of the magnitude of difference (particularly within the single specimen SKX 5939), Sponheimer et al. propose that some individuals must have gone from a diet predominantly composed of C₃ foods to one predominantly C₄ within the span of two years (estimated 644 days).

Here's how their paper concludes:

A dental microwear study of the earlier (3.0 to 3.7 Ma) hominin Australopithecus afarensis found no evidence that its diet changed over time or in different habitats (20). In contrast, stable carbon isotope (3, 4) and dental microwear texture analyses (1) of the slightly younger (3.0 to 2.4 Ma) hominin A. africanus demonstrated that its diet was far more variable. This suggests the possibility that a major increase in hominin dietary breadth was broadly coincident with the onset of increasing African continental aridity and seasonality after 3 Ma (21, 22) and only shortly antedated the first probable members of the genera Homo and Paranthropus (23-25) and the earliest stone tools (26). The undoubted toolmaker Homo is thought to have been a dietary generalist that consumed novel foods such as large ungulate meat and tubers that are abundant in savanna environments (27-30). Paranthropus, in contrast, with its extremely large and flat cheek teeth, thick enamel, robust mandible, and heavily buttressed facial architecture, is often portrayed as a dietary specialist (27-29). Further, it has been argued that this specialization contributed to its extinction when confronted with increasingly dry and seasonal environments later in the Pleistocene, whereas Homo's generalist adaptation was crucial for its success (28, 29). Our results suggest that Paranthropus had an extremely flexible diet, which may indicate that its derived masticatory morphology signals an increase, rather than a decrease, in its potential foods. Thus, other biological, social, or cultural differences may be needed to explain the different fates of Homo and Paranthropus (31).

We have lots of other reasons to believe that robust australopithecines were not dietary specialists, as pointed out by Wood and Strait (2004). Robust australopithecines had broad geographic ranges, were able to disperse over long distances, and persisted despite substantial climatic and environmental changes. The evidence for dietary differences across the lifespan is certainly consistent with this.

It does, however, make for an interesting conundrum: if australopithecines were selected on the basis of their ability to find different foods over the course of years, that suggests a strong role for social learning of more food types and broader geographic ranges. But if this was the path taken by robust australopithecines, what was the path taken by Homo?

References:

Ambrose SH. 2006. A tool for all seasons. Science 314:930-931. DOI link

Gibbons A. 2006. Not just nuts and berries for these hominids. ScienceNOW 9 Nov. Full text

Sponheimer M, Passey BH, de Ruiter DJ, Guatelli-Steinberg D, Cerling TE, Lee-Thorp JA. 2006. Isotopic evidence for dietary variability in the early hominin Paranthropus robustus. Science 314:980-982. DOI link

Wood B, Strait D. 2004. Patterns of resource use in early Homo and Paranthropus. J Hum Evol 46:119-162. DOI link

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

Tim White and colleagues (2006) report on new fossils from Aramis and a new site, Asa Issie, with estimated dates between 4.1 and 4.2 million years ago.

In addition to the paper, there are articles in the New York Times (by John Noble Wilford), the Associated Press (by Seth Borenstein), and BBC (by Paul Rincon).

The story is being played as another "missing link" -- this one between Ardipithecus and Australopithecus. From the Times:

Tim D. White, a paleontologist at the University of California, Berkeley, who was a leader of the team, and his colleagues said the 4.1-million-year-old fossils were anatomically intermediate between the earlier species Ardipithecus ramidus and the later species Australopithecus afarensis, the Lucy family. The newfound bones and teeth are the earliest remains of the most primitive Australopithecus, known as anamensis.

"This new discovery closes the gap between the fully blown australopithecines and earlier forms we call Ardipithecus," Dr. White said in a statement. "We now know where Australopithecus came from before four million years ago."

The fossil specimens are a partial maxilla from Aramis, ARA-VP-14/1; two partial maxillary dentitions from Asa Issie numbered ASI-VP-2/2 and ASI-VP-2/334; and a large femur shaft fragment, ASI-VP-5/154. There are also several postcranial bones -- phalanges, vertebrae, a metatarsal -- that are pictured in some of the press accounts and briefly discussed but not pictured or numbered in the paper. The postcanine teeth in the maxillary specimens are larger than the known sample of Ardipithecus, but the canines are larger and more mesiodistally elongated than in Australopithecus afarensis. The best anatomical match for these features is with the Kanapoi and Allia Bay samples assigned to Australopithecus anamensis, and White and colleagues assign the new fossils to that species.

So why are these fossils important? On the surface, there isn't very much to them. Three piecemeal upper dentitions don't tell much. They have big molars and big canines, both within the range of Au. anamensis. Neither they nor the femur shaft extend the known range of variation in early hominids.

Remembering that every fossil fragment is a precious relic of a bygone age, the main importance of these is that they may address hypotheses about the biogeography of Early Pliocene hominids. The maxillae show that a large-molared hominid existed in the same geographic location at a later time than the small-molared Ardipithecus. That could be interesting, and it is the hook for the news stories and the team's press statement.

The strongest part of this story is the geographic -- finding them in the Middle Awash instead of Kenya -- and the paleoenvironmental. There is some suggestion in the paper that there may be a paleoenvironmental difference at the sites that currently have evidence of Au. anamensis:

Palaeoenvironmental circumstances surrounding Au. anamensis ~1,000 km to the south in Kenya have been described for Allia Bay as a mixed assemblage sampling aquatic, forest, grassland and bushland. Nearby Kanapoi conspecifics were found in another mix of environments described as dry, possibly open, wooded, or bushland conditions with a wide gallery forest in the vicinity. Habitat preferences in such mixed assemblages are difficult to ascertain despite the assertion that the hominids "favored mosaic settings". In contrast, the Ethiopian occurrence of Au. anamensis described here allows its tight spatial and temporal placement in a vertebrate assemblage with taphonomic integrity. Its relative abundance suggests that it was a regular occupant of a wooded biome that appears to have persisted in this part of the Afar during the 200,000-yr interval subsequent to Ar. ramidus at Aramis (White et al. 2006:887-888).

This points to two salient facts about the Australopithecus lineage: they were able to disperse effectively across relatively long distances, and occupy at least those habitats where wooded cover and resources were available.

On the other hand, the fossils don't really "fill a gap" between Ardipithecus and Australopithecus, because they are pretty firmly within the time range of known Au. anamensis, being around the same age as the Au. anamensis sample from the Lake Turkana area -- the oldest Kanapoi hominids may be between 4.1 and 4.2 million years old also. The paper points out the other East African examples of Australopithecus at or above 4 million years ago; but it omits the Sterkfontein Member 2 remains, which are also conceivably in the age range of Au. anamensis. Or, for that matter, the Lothagam mandible, which might be the earliest australopithecine even if its date weren't as high as the >5 Ma estimate.

The paper attempts to close off -- for the moment -- the idea that there were allopatric species of early (ca. 4 Ma) australopithecines with differing dietary adaptations. But the paper cannot reject this hypothesis without caveats:

Two phylogenetic hypotheses concerning the origin of Australopithecus can be offered to account for the available data. The first hypothesis derives Au. anamensis phyletically from Ar. ramidus within a 200,000-yr interval [i.e., between 4.4 and 4.2 Ma]. The second involves cladogenesis of Au. anamensis from an ancestor (presumably Ardipithecus or some close relative) even deeper in the Pliocene or Late Miocene. Under the latter hypothesis, Ar. ramidus would represent a relict species in an ecological refugium (White et al. 2006:888).

This latter alternative is the only "bushy" interpretation -- the idea that known species of Ardipithecus can't really be the direct ancestors of Australopithecus, but that there must be some as-yet-undiscovered hominid (or better yet, hominids) that are the common ancestors, cousins, and other bushy relatives of the known species. White and colleagues cannot reject it, but they clearly do not favor it.

In its place, they suggest Ardipithecus ramidus as a lineal, possibly anagenetic ancestor of Au. anamensis, and Au. anamensis as the anagenetic ancestor of Au. afarensis. It's a ladder from primitive to derived, small-molared to big-molared, big-canined to small-canined.

I tend to think this is the null hypothesis -- we have sampled adaptations that differ because of evolution in what is essentially a single lineage of successive species. I say "essentially" because there was not necessarily a wholesale transformation of one species to another across its entire range. Instead, dispersals of new adaptive packages by population movements were probably important biogeographic aspects of evolution in these early hominids. But I think it important to recognize that one species can indeed be the ancestor of a later species.

People who like their phylogenies bushy and their speciations punctuated can take solace in that 200,000-year gap. The finding of Au. anamensis within the already-known time range of Au. anamensis means that the new fossils haven't really added much to the question of phylogenetic diversity in early hominids.

As a postscript, I have a nomination for "most significant sentence" in the paper:

At Aramis, the lone hominoid and largest primate was Ar. ramidus (109 of 6,156 identified specimens so far) (White et al. 2006:888, emphasis added).

References:

White TD and 21 others. 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature 440:883-889. DOI link

The paper by Guatelli-Steinberg et al. (2005), earlier referred to here, is now available online from PNAS.

The results are basically as reported by National Geographic, finding that Neandertal anterior teeth have perikymata counts within the range of living human populations. Perikymata are microscopic ridges on the enamel surface of teeth; they mark the incremental growth of the teeth over small periods of time. The idea has been that these ridges work a bit like tree rings; they mark the amount of time that the tooth took to grow. However, as this study indicates, the formation of perikymata is not quite so simple as the addition of tree rings, and human populations actually vary substantially in the number of perikymata on their teeth.

What makes this different from earlier work (like Ramirez Rossi and Bermudez de Castro 2004) is the inclusion of an African sample. The very low perikymata count of the recent Africans significantly extends the range, which had previously been assessed in Europeans only. Thus, the conclusion here is that there is no evidence from perikymata to indicate that Neandertal development was any different from that within living human populations.

Now we can wait for the other shoe to drop:

The finding from the African population sampled here shows that some developmentally normal humans have much lower perikymata counts than others. This varies by tooth (since they don't all develop for the same time): the lower canines have the highest counts, with a mean over 150; the lower incisors have the lowest counts with a mean down near 100. Remember that these values are means; individuals in the sample must have scored lower, although the range of the sample is not reported in the paper.

With this sample, the human range encompasses the Neandertals. It encompasses all the earlier European hominids (chiefly from Atapuerca) sampled by Ramirez Rossi and Bermudez de Castro (2004), because these hominids had counts higher than Neandertals.

Let's take a look at Dean et al. (2001:628), who give values for earlier hominids. Here's a table including some earlier hominids along with the South African values from Guatelli-Steinberg et al. (2005). The current paper does not include the numbers, so I am reading estimates off the figure, but considering they are means and the important aspect is the total range, the numbers aren't critical. Lower numbers are less like the recent Europeans that were the standard before the new comparative work.

From these numbers, Sangiran 4 and SK 27 are within the range of modern human population means. So are three of the teeth of Australopithecus (i.e. A. africanus), and the remaining three teeth are pretty close, so that it seems likely the A. africanus dentition wasn't very different in its perikymata number from the range of living Africans.

The standouts are KNM-WT 15000 and Paranthropus (i.e. A. robustus). A. robustus is easy to explain: its anterior teeth are a lot smaller than ours. A lot smaller. If enamel formation rates were similar, then they ought to have taken less time to form, regardless of other aspects of somatic development.

The puzzle is KNM-WT 15000, the famous Nariokotome skeleton. Is this skeleton a normal representative of early human populations? Is it at the extremely low end of a normal range including others like KNM-ER 1590 (also a bit smaller than the mean, although probably not outside the range of living Africans)? Is it pathological?

The other shoe is the research paper that will cover all these questions.

Now it could be that these numbers really aren't comparable for some reason; I don't do perikymata, but I can tell that the counts depend on estimates of crown height and packing density, so it's not obvious that they were derived in the same way (although the papers do share one author).

But the Neandertals are far from the most interesting part of this perikymata problem. Can we tell a human from an australopithecine from these data? If so, why do some of the earliest humans have the lowest (i.e. sub-australopithecine) counts?

I think we can disregard the idea that their somatic development rates were "highly derived" in a non-human-like direction. It's not like they're Neandertals, after all.

References:

Guatelli-Steinberg D, Reid DJ, Bishop TA, Larsen CS. 2005. Anterior tooth growth periods in Neandertals were comparable to those of modern humans. Proc Nat Acad Sci USA 102:14197-14202. Abstract

Dean C, Leakey MG, Reid D, Schrenk F, Schwartz GT, Stringer C, Walker A. 2001. Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins. Nature 414:628-631.

Ramirez Rossi FV, Bermudez de Castro JM. 2004. Surprisingly rapid growth in Neanderthals. Nature 428:936-939. Full text (subscription)

From a new paper by Greg Laden and Richard Wrangham:

We propose that a key change in the evolution of hominids from the last common ancestor shared with chimpanzees was the substitution of plant underground storage organs (USOs) for herbaceous vegetation as fallback foods. Four kinds of evidence support this hypothesis: (1) dental and masticatory adaptations of hominids in comparison with the African apes; (2) changes in australopith dentition in the fossil record; (3) paleoecological evidence for the expansion of USO-rich habitats in the late Miocene; and (4) the co-occurrence of hominid fossils with root-eating rodents. We suggest that some of the patterning in the early hominid fossil record, such as the existence of gracile and robust australopiths, may be understood in reference to this adaptive shift in the use of fallback foods. Our hypothesis implicates fallback foods as a critical limiting factor with far-reaching evolutionary effects. This complements the more common focus on adaptations to preferred foods, such as fruit and meat, in hominid evolution.

Tubers are not the only kinds of USOs; there are also corms, bulbs, and rhizomes. I tend to use "tuber" as an easier-to-type version of USO, though. I was practically dared to review the paper here (nota bene: I do respond to dares, albeit more carefully and slowly than for most things), and Carl Zimmer has also written a short item on the idea. The mole rats are the lede, but there is much more to it than them, and in many respects they are the least problematic part.

So here is my semi-rambling take.

Take one

In 1999, Wrangham and Laden, along with David Pilbeam, James Holland Jones, and NancyLou Conklin Brittain, suggested that tuber cooking was central to the adaptation of early Homo. The evidence for that suggestion was and remains essentially absent. As Henry Bunn put it in his comment to the paper:

Why is there abundant evidence of hunting and some form of scavenging, carcass transport, butchery, and sharing and consumption of meat and fat in the behavioral and dietary adaptations of early Pleistocene Homo (e.g., Oliver, Sikes, and Stewart 1994 and references therin)? Why are the earliest stone tool kits of the Oldowan dominated by sharp-edged cutting tools? Why is there intensive meat polish on the edges of stone flake knives studied for microwear (Keeley and Toth 1981)? Why is there not microwear evidence of grit or sediment damaged on the teeth of supposedly tuber-feeding hominids themselves, including the robust australopithecines (Kay and Grine 1988)? (Bunn 1999:580)

Additionally there is the problem of the complete lack of evidence for cooking and the weakness of evidence for early control of fire, compared to the strong and substantial evidence for both much later in the Pleistocene.

So early Homo just doesn't show any signs of having been a serious tuber-eater. Not to say it is impossible; just that there isn't any particular evidence for the idea.

Take two

Now, Australopithecus, that's another story. Robust australopithecine teeth in particular have a lot of pits and scratches on them, as if they were eating some hard, gritty foods. Underground storage organs fit that bill. Eating a lot of dirt along with them might well explain the high rate of dental wear that robust australopithecines clearly had -- many had their first molars worn almost completely flat before the third molars came into occlusion.

In this context the fallback food idea seems like an especially good one. The tooth anatomy and microwear evidence indicate that robust and nonrobust australopithecines probably did not differ in most of their dietary spectra, but instead in the accentuation of different food sources that were shared by both. If food shortages were important in the evolution of these hominids, one way that the difference between them might have been sustained was an ecological difference in fallback food utilization. Hominids like A. afarensis and A. africanus undeniably had teeth adapted to heavy grinding, fracturing off brittle foods, and intensive attrition compared to any other living or fossil primate. So it makes no sense to propose that the difference between these "gracile" australopithecines and later robust australopithecines was that the "gracile" ones lacked the high-chewing element. Rather, it makes considerably more sense to suppose that both kinds of hominids were eating the high-chewing foods, with the robust ones making a more intensive use of them, and possibly lacking some of the tough pliable foods eaten by earlier nonrobust species. A difference in fallback strategies might comprise exactly this kind of dietary prediction.

To me, the coolest thing about the hypothesis is that it explains the postcanine adaptations of australopithecines without reference to the now-well-known carbon isotope data. Indeed, the question of C₄ versus C₃ foods is entirely irrelevant. I discussed the carbon and other stable isotope data in an earlier post; the short story is that all kinds of australopithecines appear to have included around a 25 to 30 percent component of C₄ foods, which include grasses, some sedges, and the animals who ate them.

Peters and Vogel (2005) proposed that the C₄ component of the early hominid diet could be explained as a sum of several different plant and animal sources, including around 5 percent each of seeds, roots and pith, insects, small mammals and vertebrates, and large mammal meat. That does a good job of describing a diversified hominid diet without reference to tubers.

But the thing about USOs is that relatively few of them are C₄ plants. If hominids did eat tubers, in other words, they still wouldn't account for the C₄ fraction of the overall diet.

However, they might account for the postcanine dental adaptations of later hominids, under the assumption that they represent a substantial part of the C₃ fraction. And the replacement of C₃ fruits by C₃ tubers would explain why robust and nonrobust hominids both have approximately the same C₄ fraction, while differing so greatly in their dental adaptations and dental microwear.

As far as I can tell, nobody has mentioned this implication, but it should be the next thing to test.

The evidence

But although I think Laden and Wrangham's study has some interesting possibilities, I think the data is a bit short of where it needs to be. What about the four lines of evidence used by Laden and Wrangham? Are they to be believed?

The first thing to point out is that a reading of the paper finds little detail to go along with two of the lines of evidence. It is true that australopithecine teeth are not like ape teeth, and that robust australopithecines were different from nonrobust ones. The innovative suggestion here, although brief, is that an enlarged oral cavity in australopithecines, particularly robust ones, may be an adaptation to increase the exposure of masticated tuber to salivary digestion.

But the dental discussion appears less as two independent lines of evidence converging to one conclusion, and more as throwing up whatever seems relevant to see what will stick. A review of early hominid dental evidence also reveals plenty that is less consistent with the hypothesis that USOs were an important food for most early hominids.

For one, the comparative dental evidence is questionable. As Laden and Wrangham review the issue, Hatley and Kappelman originated the argument that the early hominid dentition was adapted to tuber-eating:

In 1980, Hatley and Kappelman pointed out parallels in dental morphology that suggested that bears, pigs, and hominids are all adapted to eating significant amounts of plant underground storage organs (USOs). They summarized their argument as follows: "We believe that postcanine similarities evident among ursids, suids, and hominids are in part an adaptation for processing this tough, fibrous, and gritty plant part. Bears, pigs, and humans are adapted to exploiting plant roots and tubers, although their methods of food gathering are functionally rather than morphologically analogous. Convergence upon the resource of belowground plant storage parts appears to make the responses of nonretractable claws, cartilaginous snout, and digging stick equivalent" (Hatley and Kappelman 1980:380, quoted in Laden and Wrangham 2005:1).

This isn't obviously true. For one thing, Pliocene pigs appear to have been mainly grazers (Harris and Cerling 2002 -- not cited by Laden and Wrangham 2005). They increased in molar size and complexity in several different lineages, as a reflection of their increased reliance on C₄ vegetation. The diet of current-day suids in particular seems to share little in common with early hominids, at least as far as their stable isotope ratios are concerned. Nor are large and flat early hominid molars particularly analogous to those of most bears -- perhaps the closest are pandas, which are far from dedicated tuber-eaters.

Then there is the problem with the earliest hominids. These, like the later ones, are found alongside mole rats, at sites like Aramis and Lukeino. But they don't have the postcanine adaptations of later hominids. The essential problem with the earliest hominids is not postcanine specialization, but instead the changing role of the canine-premolar complex, and the reduction of the canines. There is no reason (at least that I can think of) to suppose that small canines are adaptive to tuber-eating (and a search of the paper finds no occurrences of the word "canine").

One way to avoid this problem is to suppose that the USO-eating adaptation was simply a feature of later hominids --- say, A. anamensis and later. Perhaps it's true, but if so, the hypothesis loses some of its punch, and possibly one of the converging lines of evidence, since the expansion of USO-rich savanna central to Laden and Wrangham's paper starts in the Miocene.

And the paper would prefer to displace the importance of tubers earlier rather than later in time:

There is growing evidence that middle to late Miocene hominoids, mainly in Europe, exploited relatively open habitats, and may have exhibited dietary adaptations (Teaford and Ungar, 2000, Smith et al., 2003 and Smith et al., 2004) that we claim here to be related to USO consumption. This lends support to our assertions that a USO niche may have emerged during the Miocene, that this niche may have been important for non-fossorial mammals, and that certain features, such as thick enamel and large teeth, can arise in response to this niche. However, we do not wish to make claims beyond the hominid taxon at this time, other than to note that this may be a fertile area of future research (Laden and Wrangham 2005:13).

If you are a student looking for a thesis topic, don't pick this one.

The most original suggestion is that hominid and mole rat remains are significantly coassociated. On the surface, this looks like fairly convincing evidence that the hominids lived in USO-rich environments, which is precisely what Laden and Wrangham conclude. And indeed, the number of sites either possessing both kinds of animals or lacking both (27) is higher than expected considering the small number that have one kind but lack the other (11).

But wait a minute. Neither "mole rats" nor "hominids" are species, they are groups composed of several species. Let's consider the same kind of comparison for other kinds of animals. How many hominid sites lack bovids? Or suids? Or crocodilians? Keep in mind that some groups are rare at early hominid sites because they hadn't diversified yet, like papionins, or hadn't yet appeared in Africa, like equids. But these groups are found at many later hominid sites. And of course, for many sites the total species list may reflect less intensity of sampling rather than the paleohabitat.

In other words, the mole rats may show that hominids had the opportunity to eat USOs -- at least, if they could compete effectively with the mole rats for them. But they don't show that the hominids actually ate USOs. At least not if we aren't equally willing to believe that the presence of crocodiles at hominid sites meant that hominids swam in rivers and ate migrating wildebeest.

The weaknesses NOT mentioned

I see two significant weaknesses in the hypothesis. The first is the simpler of the two: digging up tubers is a lot of work.

For groups like the Hadza who eat a lot of them, this work takes many hours (at least by some group members). That kind of work seems unlikely for australopithecines, even hungry ones. Especially considering the full scenario: australopithecines digging intensively for savanna-living tubers for hours at a stretch would have been highly exposed to predation and heat stress for hours at a stretch.

Might they have done it if they had nothing else to eat? Sure. But could they have done so efficiently enough to get a net return on their effort? There's a question worth answering.

Might they have banded together into large defensive groups? Maybe, but that would seem likely to decrease foraging efficiency -- how many tubers are there in any small patch of ground? However, there is slight evidence for large multimale groups (chiefly AL 333), as well as pretty good evidence that predation was high and survivorship into adulthood low. Another question worth answering.

There may be a solution for this problem: perhaps the plants themselves have evolved under intensive hominid predation. Maybe today they put their roots further underground, or maybe the plants with tougher and more fibrous roots have predominated since the Pliocene. If so, australopithecines might have had an easier time of digging them up.

The other problem is more vexing. How can we demonstrate that an extinct species was adapted to eat a food that it did not eat very often? Bone chemistry must predominantly reflect the foods that make up the majority of the diet, not those that are consumed only intermittently. Microwear also ought to reflect the majority foodstuffs, although perhaps more weakly -- especially if mortality occurs mostly during periods of dietary stress, when animals are eating more of their fallback foods than usual. This is perhaps worth looking into.

Maybe the most promising test would be variability in tooth wear. Presumably the need to rely on fallback foods would vary in accordance with climatic conditions, on a multigenerational timescale. If so, then some individuals might exhibit relatively great amounts of attrition due to their reliance on fallback foods during long periods of resource stress, while other individuals might have lived in times of relative abundance, and therefore not have experienced significant amounts of wear. This kind of heterogeneity would itself have created differences in selection on tooth size, enamel thickness, and occlusal anatomy over time: perhaps in ways that could be differentiated from alternative strategies. But even so, that kind of comparison is relatively far from the direct evidence, and may be impossible with the fossil record we have available.

Summary

Looking back at the post, I've written a balance of critical comments and supportive ones. I guess my opinion overall is that the USO hypothesis is certainly worth presenting, but it has a ways to go before it is really testable. I think there is a balance of good ideas here and evidentiary weaknesses, and it is certainly worth talking about them, perhaps with a bit more skepticism and documentation than has yet been done.

And if you are serious about tubers, as Wrangham clearly has shown himself to be, then you are going to have to choose a time when they were important. With this paper, I have now read that tubers were the key adaptation for Miocene apes, the earliest hominids, australopithecines, robust australopithecines, early Homo, and recent humans.

It can't be all of these. If it were, they would all look the same. And there wouldn't have been any reason for one to change into anything else! So you have to pick.

And making a choice means more than saying, "well, Miocene apes tasted tubers, early hominids needed them when the fruit ran out, for australopithecines they were a fallback food, robust australopithecines ate them all the time, early Homo cooked them, and recent humans pickled them with vinegar and caraway seeds. As yet, the many tuber hypotheses have been just-so-storytelling at its most self-contradictory.

If I were picking, I would put the best odds on Laden and Wrangham's current argument: USOs were important fallback foods for nonrobust australopithecines like A. afarensis and A. africanus, and equally or more important for robust australopithecines. In contrast, early Homo was adapted to meat eating, and the earliest hominids -- who lack the postcanine specializations of later hominids -- remain as yet a mystery, although a fundamentally apelike diet is a good first guess.

This post doesn't account for all the details of early hominid diets, but some previous posts review other sources of evidence, including:

Stable isotope analyses

Dental microwear

Occlusal anatomy

References:

Hatley T, Kappelman J. 1980. Bears, pigs, and Plio-Pleistocene hominids: a case for the exploitation of belowground food resources. Hum Ecol 8:371Ð387.

Laden G and Wrangham R. 2005. The