A. boisei

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.

After my Q and A with paleoanthropologist Mica Glantz, I got a lot of great response -- people really liked reading about work in the field from somebody other than me!

So, I'm going to make these interviews a regular feature. When I was in Michigan last week, I got a chance to talk with Adam Van Arsdale, who graciously agreed to answer some questions about his work.

UPDATE(11/29/2007): After posting, I heard from a reader who reminded me that I omitted Adam's affiliation and info! Adam is a lecturer in anthropology at the University of Michigan. You can find out more about his interests on his webpage.

Hawks: You were lucky enough to work at one of today's most exciting paleoanthropological sites, Dmanisi. What can you tell us about your experience there?

Van Arsdale: Dmanisi is a wonderful place and I can't say enough positive things about the site and all the people I have worked with through the project. To begin, the site itself is just a nirvana for anyone with an interest in history or prehistory. The primary excavation area is in the middle of a ruined medieval citadel complex which rose to prominence as a trading town along the silk road; down from the promontory are the tombs of Mongols who sacked the city in the 12th century; further down are early Christian burials, and along the river are the remains of bath houses for travelers along the Silk Road. It is a literally a place where time seeps out of the ground.

Leaving the setting aside, the people associated with the project have been wonderful to work with. The size of the excavation team would vary but there would be times when, at the end of a long excavation day, I would find myself sitting at a long dinner table surrounded by 40 people speaking more than half a dozen languages. In the years I worked there as a graduate student I think we had students and researchers from 15 different countries (and I'm probably missing a few). Everyone who works at the site, including the local residents of Patara Dmanisi, adds their own character to the project. As a graduate student, my summers at Dmanisi served as something of a Paleoanthropology bootcamp, with regular discussions and debates between all of us with very different training and different theoretical perspectives on the issues of human evolution.

And then on top of all of this there are, of course, a remarkable set of fossils and archaeological materials.

Hawks: Do you want to give a shout-out to anybody in Georgia?

Van Arsdale: There are too many to name, but certainly David Lordkipanidze, who first invited me to Dmanisi in 2001, deserves recognition. I'll also add Gocha Kiladze, Teona Shelia and Dato Zhvania, who began working at Dmanisi in 1991 as students and who continue to play a significant role in the operation of the site today. One of the great things about the site is that it has served as a tremendous springboard for Georgian students interested in paleoanthropology. I think it is a safe bet we will be hearing a lot from our Georgian colleagues in the years ahead.

Hawks: Your dissertation work focused on the Dmanisi mandibles. I know that you still have publications coming out on these, so feel free to keep quiet about anything you're saving for print. What can you tell us about the sample?

Van Arsdale: The Dmanisi mandibles are a remarkable sample. They show a huge amount of morphological variation in a set of fossils derived from a temporally and geographically constrained set of deposits. One of the mandibles is in many characters the largest mandible assigned to the genus Homo. Two of the others are quite small, with variably large and small teeth. And the fourth specimen is one of the earliest edentulous mandibles in the hominid record. Given the current season, it is perhaps appropriate to describe the sample as a real cornucopia of variation. And the location and date of the site itself is surprising. Dated to 1.8 million years and about 2000 miles from the outlet of the rift valley in northeast Africa, the site is a long way from the contemporaneous and well-known deposits from the Turkana Basin in Kenya and Olduvai Gorge in Tanzania.

So how do we account for all this variation? That was basically the question of my dissertation. I sought to answer this question by testing a series of hypotheses focused first on sources of intraspecific variation, particularly age and sexual dimorphism, then secondarily on hypotheses of interspecific differentiation (i.e. multiple species). I then evaluated the results of these quantitative tests in the context of the comparative anatomy of the Dmanisi sample. Sparing you all the details, I think there are strong reasons to consider the Dmanisi hominid sample as that of a single species, but one displaying considerable amount of variation associated with age and possibly elevated levels of sexual dimorphism relative to what we observe in contemporary and recent human populations.

Hawks: Of course, your work required a lot of comparisons with other samples, and mandibles are among the most common skeletal elements represented in the fossil record. How did you handle your comparative work?

Van Arsdale: Paleoanthropology is at its root a comparative discipline. It is difficult to interpret any set of fossils outside of some comparative model. My work is no different. In asking questions about variation associated with age and sex, my dissertation is really asking how strange (or not strange) does the variation in the Dmanisi sample look if we treat it like a mixed age and sex sample of humans? Of chimpanzees? Of gorillas? Each of these species possess somewhat differing patterns of variation so that our final understanding of the Dmanisi specimens is based on a combination of similarities and differences with these different comparative models.

You can also try to understand the sample from the perspective of other fossils. These comparisons are more challenging because we have less certainty regarding the things we think we know about fossils. For example, in my dissertation I also make a series of comparisons between the Dmanisi mandibles and a sample of Australopithecus boisei mandibles from East Africa. It is much more difficult to say for certain whether any given fossil specimen is male or female, and in the absence of well preserved teeth, young or old. That uncertainty limits the power of the hypothesis tests we can bring to the question by limiting the amount of information we have to work with.

One of the exciting aspects of Paleoanthropology's comparative perspective is that new fossils give us new ways of looking at old fossils. Possibly the most exciting aspect of the Dmanisi fossils is that they provide us a tremendous platform from which to look back at these large samples from East and Southern Africa that we have known about for a long time and reexamine questions which had either previously been unanswerable or whose accepted answers no longer seem so clear.

Hawks: Any stories you can share about your travels?

Van Arsdale: One of the more unique experiences from my travels occurred while I was tagging along with a graduate student from Yale on her project involving 4.5 million year old fossil exposures in the Tugen Hills of the Central Rift Valley, Kenya. I was off on my own one day, walking along one of the exposures when I came across what appeared to be part of a fossilized crocodile skull just barely sticking out of the ground. I sat down and began very carefully exposing its boundaries so that it could be properly prepared and taken out. After about 20 minutes of this, a young Tugen boy came out of the bushes and sat down next me and began watching me work. I tried to say a few words of greeting in my very rudimentary Kiswahili, but either my pronunciation was too terrible to be understand (quite likely) or he was too young to have yet learned Kiswahili (he looked like he was between 8 and 10). After a few more minutes the boy, who had been carrying a small bow and set of arrows, took out one of his arrows and began using its steel tip as a mini-trowel. I would have discouraged him out of fear he might damage the fossil or go on trying to dig up other fossils in the area, but as I watched him he was exceedingly careful and seemed completely enraptured by the work. It was just one of those moments where, while the event was going on, I recognized how amazingly unique it was. Here we were, a graduate student from the University of Michigan with twenty plus years of formal education and a young Tugen boy with at most a few years of schooling, sitting side by side on a hillside in the middle of Kenya carefully exposing a 4.5 million year old fossil. The only common language between us was the action of my Marshalltown trowel and his handmade arrow point and a basic curiosity in this fossil.

Hawks: It's a story you hear from students a lot: teeth and mandibles are "bor-ing". But of course, they're the best representatives of variation we have through much of human evolution -- if you want to study evolution, you'll be studying jaws and teeth. What keeps these questions exciting for you?

Van Arsdale: One of the reasons I enjoy looking at mandibles and teeth are that they can potentially provide a window into numerous aspects of human evolution. As you point out, they are the most abundant element in the fossil record and therefore provide a large set of data with which to address questions of evolutionary relationships and evolutionary change. They can also tell you something about the ecology and diet of the individual specimen. Finally, they tell us something about how an organism develops throughout life and ages.

This also means that questions regarding variation in jaws and teeth can be difficult to answer because many different processes might account for the observed variations. When testing hypotheses about mandibular variation it is important to keep this in mind. It is always striking to me how many hominid type specimens are or have served at some time as type specimens for a new species. This is in part a reflection of their relative abundance, but I think it also reflects how difficult it is to adequately address all the potential sources of variation in mandibles. If you accept the conclusions of my research, the Dmanisi mandibles serve as a cautionary tale in this regard.

Hawks: Some readers may know that you and I share the same graduate advisor, Milford Wolpoff, who has certainly been a strong influence on the way I approach evolutionary questions. But I also find myself going back to other people who influenced my training. Who/what really got you interested in the field, or shaped the way you think about evolution?

Van Arsdale: I initially entered anthropology by happy circumstance. Entering college (Emory University) I was interested in majoring in both English Literature and Evolutionary Biology. My first year two things happened; I realized Emory's biology department was primarily focused on microbiology and full of pre-med students (something I was not interested in) and I took my first Anthropology course to fulfill a distribution requirement. I was immediately hooked. Here I could have the best of both worlds... an integrative approach towards understanding what it means to be human and a careful examination of the evolutionary processes which have shaped the pattern of human evolution. I owe a huge part of my perspective to Milford and the other faculty and students I worked with as a graduate student, but I don't think I fully realized the influence my undergraduate teachers had on my perspective till the AAA meetings last year when I was able to attend a session honoring the graduate advisor (Jack Kelso) of my undergraduate advisor (George Armelagos). I listened to talks by people I had never met, but with whom I share some of my academic phylogeny, and what I heard were familiar themes on the interaction of human biological and cultural processes. This bio-cultural perspective is something I carry with me from Emory and is evident in the approach I take towards questions of Pleistocene human evolution, where changes in human skeletal form cannot be understood outside of the context of our ever-expanding brains and the increasingly complex ways in which we interact with the people and environments around us. Now that I am teaching, it is something I am aware of when I am in front of the undergraduates in my own classes.

Hawks:Some of your current research involves a lot of genetic modeling. How did you get into this area? Can you tell us about some of your thoughts?

Van Arsdale: My interest in genetic modeling first began as an undergraduate. In part it reflects my status as an admitted math nerd. I like numbers, I like using computationally intense models and simulations to address specific hypotheses, and I like understanding how evolutionary and cultural processes interact in dynamic ways. But when I was an undergrad my interest in genetic models stemmed out of my interest in modern human origins and the belief that any really good model should be able to simultaneously explain the pattern of fossil, archaeological, and genetic evidence. At the time there was quite a bit of discussion not just about how the increasing amount of genetic data related to previously held understandings of the fossil and archaeological record, but also how compatible data from different genetic systems were with each other. In particular, data from non-recombinant genetic systems (mtDNA and parts of the Y-chromosome) seemed to provide a different picture of human evolution than data from recombinant genetic systems. My attempt to understand these differences is what really drew me into aspects of genetic modeling.

Since that time my interest genetic modeling has really developed out of what I consider an anthropological approach towards understanding genetic systems. I like to quote one of the take-away messages from the dissertation defense of another Michigan graduate, Keith Hunley, who modeled genetic aspects of South American population structure in his dissertation. As Keith said in his defense, what people do matters. Most genetic models are dependent on a variety of demographic parameters (population size, structure, etc.), all those things that people do. And yet most geneticists do not, or simply cannot directly address these demographic parameters with the data available to them. As a paleoanthropologist, one role my research serves is to provide better understandings of what people did and the ways in which they interacted in the past so as to better inform such genetic models.

On a more theoretical level I am very much interested in exploring how the unique ways in which humans shape and interact with our evolutionary landscape serves to structure genetic variation and the evolutionary forces which shape it.

Hawks: What's the next step for you? Where do you go from here with your research?

Van Arsdale: Most of the questions I am working on now reflect my current thinking that the basic pattern which characterizes Pleistocene human evolution; the complex interaction between increasing cultural complexity, expanding ecological niches, and basic anatomical changes (encephalization, dental reduction); establishes itself early in the Pleistocene if not prior than that. Essentially, that sometime around 2-2.5 million years ago a group of hominids stopped acting like bipedal apes (the Australopithecines) and started acting human. This basic human pattern then continued to develop and characterize Pleistocene hominids until about 10-20,000 years ago when we stopped acting like humans and started acting like domesticated humans.

By understanding how this pattern manifests itself early in the Pleistocene, for example, by considering how, why and with what changes human populations expanded into places like Southern Georgia as early as 1.8 million years ago, you can develop broader understandings of the Pleistocene as a whole. I am just finishing up two projects related to this broad topic, one examining the Habiline-Erectine transition in the Lower Pleistocene and another attempting to characterize broad demographic changes within the Pleistocene.

I also want to continue my involvement in paleoanthropological field work and would like to continue examining Plio-Pleistocene deposits in Western and Central Asia. Dmanisi is an incredible site and has provided a great amount of detailed evidence to address questions of human evolution from this time period. But the detailed picture it provides encompasses only a narrow range of time and space...the more we can expand that window the better we can understand the broad patterns of change which characterize humans in the Plio-Pleistocene.

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

Rex Dalton reports in this week's Nature on permit problems in Olduvai Gorge:

For 18 years, the Olduvai Landscape Paleoanthropology Project (OLAPP) -- led by anthropologist Robert Blumenschine of Rutgers University in New Brunswick, New Jersey, archaeologist Fidelis Masao of the University of Dar es Salaam and Jackson Njau, principal curator at Tanzania's National Natural History Museum in Arusha -- has collected plant and animal specimens to learn how these early relatives of man lived in the region (R. J. Blumenschine et al. Science 299, 1217-1221; 2003).

Last summer, the OLAPP team was distressed to learn that Tanzanian officials had issued permits to a group led by Manuel Domínguez-Rodrigo, of Complutense University in Madrid, and Audauz Mabulla, of the University of Dar es Salaam, to dig within the OLAPP region. The OLAPP researchers then found the competing group a kilometre away from their campsite, probing trenches the OLAPP team had dug near the bed where Leakey uncovered 'Zinj', the original P. boisei skull.

I don't know anything about the details of this dispute, but the article seems to tilt toward the OLAPP point of view. It quotes Domínguez-Rodrigo, but doesn't really provide any detail that might support his team's point of view.

The article does provide some details intended to undercut the claims that others claimed they made, but they claim they didn't claim. You follow? Me neither. It's a short he said, he said kind of article that doesn't do anything but flag a "controversy." These kinds of articles always irritate me.

You know I like the lizard analogies for human evolution -- I wrote about limb length and predation last time around -- and now we have another paper from Jonathan Losos' group looking at ecological differentiation and sexual dimorphism:

Sexual dimorphism is widespread and substantial throughout the animal world (1, 2). It is surprising, then, that such a pervasive source of biological diversity has not been integrated into studies of adaptive radiation, despite extensive and growing attention to both phenomena (1, 3, 4, 5, 6, 7). Rather, most studies of adaptive radiation either group individuals without regard to sex or focus solely on one sex. Here we show that sexual differences contribute substantially to the ecomorphological diversity produced by the adaptive radiations of West Indian Anolis lizards: within anole species, males and females occupy mostly non-overlapping parts of morphological space; the overall extent of sexual variation is large relative to interspecific variation; and the degree of variation depends on ecological type. Thus, when sexual dimorphism in ecologically relevant traits is substantial, ignoring its contribution may significantly underestimate the adaptive component of evolutionary radiation. Conversely, if sexual dimorphism and interspecific divergence are alternative means of ecological diversification, then the degree of sexual dimorphism may be negatively related to the extent of adaptive radiation.

These anoles have evolved into four different ecomorphs repeatedly on different islands of the Greater Antilles, and the sexes differentiate not only in their morphology but also their habitat use and diet.

Primates are generally group foragers, and because they forage together, males and females eat the same foods a lot of the time. The major components of sexual dimorphism across primates have mostly been considered in relation to body size and canine dimorphism, both of which have a strong social import, but less obvious ecological import. That is an apparent contrast to the anoles, whose dimorphism allows males and females to specialize to slightly different niches.

But even though body size and canine size are the main elements of dimorphism that can be compared across all primates, both these features and others may take on ecological importance within primate species. For one thing, sexual dimorphism leads to ecological differentiation even within foraging groups -- not necessarily because different sized individuals can exploit different foods, but because large individuals have preferential access. This has clear dietary and behavioral import -- for example, hunting is a social activity in chimpanzees; males hunt and females don't, and if a female did hunt (with males around), the males would probably take away the kill. That's not entirely because males are larger, but sexual dimorphism helps to determine the social ecology.

What about hominids? In the Plio-Pleistocene, there were at least three sympatric species of hominids in East Africa (and possibly more) and at least two in South Africa (and possibly more). These species were differentiated by body size, relative brain size, and masticatory adaptations. In other words, they occupied different ecologies involving different foods, and natural selection reinforced their ecological differences (even if the average diet involved much overlap, as I reviewed earlier).

The robust species in East Africa (A. boisei) appears to have had substantial body size dimorphism. The habiline species (H. habilis) was either substantially dimorphic, or was actually composed of two species. The large-bodied Homo may have had reduced dimorphism comparable to that in recent humans. Yet, many people have suggested that this least dimorphic species should have been the one where males and females had the greatest ecological differentiation. This is based on analogy with recent hunter-gatherers, assuming that the introduction of meat in substantial quantities requires a sexual division of labor.

Male and female lions have substantial body size dimorphism, and they are ecologically differentiated by prey size. Just thinking out loud...

References:

Butler MA, Sawyer SA, Losos JB. 2007. Sexual dimorphism and adaptive radiation in Anolis lizards. Nature 447:202-205. doi:10.1038/nature05774

In a 2002 paper on cranial remains from Sterkfontein, Lockwood and Tobias write the following in a section called "Are there multiple hominin species from Sterkfontein Member 4?":

The Group C specimens (Stw 183 and Stw 255) arguably represent a phenon, as they deviate from the A. africanus sample in the same direction. Each exhibits some derived characters of A. aethiopicus, A. robustus, and/or A. boisei. Moreover, based on dental size and morphology, Stw 252 probably belongs in this group (Clarke, 1988).

Stw 183 is the strongest craniofacial evidence for a second species of Member 4, though it is not by itself definitive (Lockwood, 1997). Stw 183 is an immature individual, and a full interpretation of this specimen relies on a comparative framework of early hominin ontogeny that is at present incomplete. Despite its youth, it possesses characters typically found in A. robustus, such as an incipient maxillary trigon and a rounded lateral portion of the inferior orbital margin (not present in any other specimen from Sterkfontein Member 4).

The temporal bones catalogued at Stw 255 (including Stw 266a) resemble A. africanus in essentially one autapomorphic character: the prominent eustachian process. Otherwise, this individual shows traits corresponding to A. boisei, especially in the relationship of the tympanic to the postglenoid and mastoid processes. On the whole, Stw 255 suggests the appearance of the temporal bone in KNM-WT 17000 of A. aethiopicus. Moreover, Spoor (1993) showed that Stw 255 has a combination of features regarding the orientation of the posterior petrosal surface that may correspond to the external anatomy of KNM-WT 17000: an unflexed cranial base combined with a petrous axis that is relatively coronally oriented in the transverse plane. Fossils catalogued as Stw 255 may also be associated with the various specimens that make up Stw 252, but this is uncertain, as a second individual (Stw 265) of similar preservation was found in close proximity to Stw 252 (Lockwood and Tobias 2002:446-447, citations in original).

They go on to say they do not think that these specimens are sufficient evidence that another species was present, and they note details of a few other fragments that are different from the sample as a whole. They differ from Clarke (e.g. 1988), who would have divided the relatively complete cranial specimens into two samples.

Some other workers have suggested that individual specimens from Sterkfontein Member 4 might represent other species besides A. africanus. Kimbel and Rak (1993) proposed that Sts 19 probably represents Homo, and that the inclusion of the specimen into A. africanus inflates the variation within that species. This proposition was tested by Ahern (1998), who found the Sterkfontein Member 4 specimens to be quite variable without Sts 19 also. In other words, this site preserves a variable sample -- especially considering nonmetric traits observed on individual specimens.

The traits of Stw 183 and Stw 255 may fall in that category of variation, and Lockwood and Tobias (2002) suggest that possibility. I think the specimens are interesting because of whose traits they share. Studying A. africanus not as a branch of a cladogram, but as a real species with possible ancestors and descendants, the occasional presence of characters of earlier and later species is to be expected. The question is whether these characters document an ancestor-descendant relation for A. africanus and the robust taxa, or whether they might be shared by collateral taxa by virtue of common ancestry alone. The half-million years preceding the origins of Homo may have been just as interesting for the study of populations as the last half-million years.

References:

Ahern JCM. 1998. Underestimating intraspecific variation: the problem with excluding Sts 19 from Australopithecus africanus. Am J Phys Anthropol 105:461-480.

Clarke RJ. 1988. A new Australopithecus cranium from Sterkfontein and its bearing on the ancestry of Paranthropus. In (Grine FE, ed) Evolutionary history of the robust australopithecines. Aldine de Gruyter, New York. p. 285-292.

Kimbel WH and Rak Y. 1993. The importance of species taxa in paleoanthropology and an argument for the phylogenetic concept of the species category. In (Kimbel WH and Martin LB, eds) Species, species concepts, and primate evolution. Plenum Press, New York. p. 461-484.

Lockwood CA and Tobias PV. 2005. Morphology and affinities of new hominin cranial remains from Member 4 of the Sterkfontein Formation, Gauteng Province, South Africa. J Hum Evol 42:389-450.

The April issue of Discover has a feature article on PhyloCode, focusing on the roles of Jacques Gauthier and Kevin de Queiroz in trying to revise the code of biological nomenclature. It is an interesting introduction to the issues, but is a little short on specifics, so I went to some additional resources to examine the impact of the whole PhyloCode debate on human phylogenetics.

Proliferating ranks

PhyloCode is an attempt to address two simple problems with the Linnaean system. The first is the problem of ranks. The Linnaean system provides seven ranked positions for species and higher-order taxa. These are the levels familiar to anyone who can remember King Phillip's soup, or his Peter's German origin, or any of the other mnemonics. These seven levels (kingdom, phylum, class, order, family, genus, species) have been supplemented over the years with in-between levels at almost every rank, such as suborders and infraclasses. For example, the most basic division among living primates is into superfamilies, which is the rank occupied by hominoids (great apes and humans), cercopithecoids (Old World monkeys) and ceboids (New World monkeys). The grouping of all three of these superfamilies, Anthropoidea, is a suborder, while the grouping of Old World monkeys and hominoids is the infraorder Catarrhini.

But when it gets to the level of infraorders and superfamilies, the phylogenetic pattern of relationships is already stretching the Linnaean classification to its limits. This degree of differentiation is more or less well suited to primates, but many other groups of organisms have even more complicated phylogenies with many more branches. This leads to some big confusion:

As part of their work, [Gauthier and de Queiroz] created a lizard family tree, but when they began to assign names to the important branching points on the tree, they realized there were more groups to name than there were ranks in the traditional system. "I started using these exotic ranks like parvorder, cohort, and microorder, and all that kind of crap," Gauthier says. "Then we'd learn more about the tree, and all the names would have to change. I thought, 'That sucks. All these ranks, they're a problem.'" (Foer 2005:48-49)

This is a problem I've thought about for a while also, ever since I was learning Mesozoic mammals and encountered exotic taxonomic ranks like "tribe" and "domain." Unlike suborder and infraorder, many of these give no indication at all about where they belong in the phylogenetic hierarchy. If this complication actually helped organize species, that would be forgivable. But even the extension to thirty or more ranks is not enough to encompass all the possible groupings in some phylogenies, especially where extinct species must be placed in a hierarchy including living species and their ancestors.

And of course the probability of disagreement among authorities on names increases combinatorially with more taxonomic ranks. Even within the hominoids there is at present substantial disagreement on the names of groups at almost every taxonomic level, despite the fact that almost everyone agrees about the phylogeny of the living species of apes and humans. Some of this disagreement is purely nomenclatural, while the rest comes from genuine disagreements about the phylogeny of extinct apes. It seems especially problematic that disputes about the relationships of extinct and fragmentary fossils could substantially alter our judgment about the nomenclature to apply to living species, but that is exactly where we stand.

Hominids and hominins

This leads to the second major problem of the Linnaean system, the problem that the names of groups themselves are formulated in a way that cannot be divorced from their taxonomic level. What this means is that if our hypothesis of phylogeny changes, the names of taxa must also change. The problem with this is that it subverts the goal of communication:

In the zoological code, family names must end with the four letters idae, for example, and subfamily names must end in inae. If taxonomists decide that a group once considered a family should instead be ranked as a subfamily, the group must, under the rules of the current system, get a new name. This frustrates the PhyloCoders to no end. "It's still the same tree," Gauthier says. "Nothing has changed, except how we spell the names. In a day when all this information is going onto the Internet, this is a bad idea. It's a constant change of PIN numbers." Some taxa have gone through a number of different names over the course of just a decade. Several years ago, for instance, it was decided that the great-ape family Pongidae couldn't exist at the same rank as the human family Hominidae because humans are a subset of the great apes. To fix the problem, researchers proposed that humans and their great-ape relatives be combined into a single family, Hominidae, and members fo the family Pongidae became the subfamily Ponginae. This can make literature searches a real pain, Gauthier says: "To a computer, there is a world of difference between iguanidae and iguaninae" (Foer 2005:50).

In my mind, computers are the least of the problem. Replace "to a computer" with "to an undergraduate" and you are closer. Really, even this understates the problem. If we could ensure that a new taxonomy established by universal consensus today would not change in the future, then it would be well worth changing all the names. But we can be pretty sure that things will change in the future, repeatedly. It just isn't worth having a system where the names have to be changed all the time, because such changes render all past research at best confusing, or at worst nonsensical.

The Hominidae-Homininae problem is not the only one in paleoanthropology, but it is a convenient example. Foer's description of the problem is one possible reformulation, but not the most popular one. We all recognize that African apes and humans are more closely related than either is to orangutans, and chimpanzees and humans closer than either is to gorillas. Many people would apply Hominidae to all the great apes, ponginae to orangutans, and homininae to the African apes and humans. This leaves the human lineage (including australopithecines) in the "tribe" Hominini (The tribe Panini would therefore include tasty Italian bushmeat sandwiches). Thus, orangutans would be hominids, gorillas would be hominines, and australopithecines would be hominins.

Consider the problems with this arrangement. First, it isn't comprehensive. There is no name for the human-chimpanzee clade, for example. The taxonomic level for that clade would properly depend on the details of the evolutionary divergence among gorillas, chimpanzees, and humans. If, for instance, there was a substantial adaptive radiation between the gorilla divergence and the human-chimpanzee divergence, then these fossil lineages might be placed with chimpanzees and humans within an infrafamily, with the chimpanzee-human clade placed as a supertribe. Likewise, the branch points leading to the dryopithecines depend on their relationships with the later African apes, or even to the Asian apes. In other words, the taxonomy still hangs on currently unknown phylogenetic branchings, and the choice of taxonomic level is entirely arbitrary.

The arbitrariness of the naming system is highlighted by some other alternatives for the hominoids. For many years, molecular researchers like Morris Goodman have suggested that the genetic similarities between chimpanzees and humans are consistent with those within genera of most mammals, and the time of origin of these lineages is also consistent with the antiquity of mammalian genera. So Goodman et al. (1998) took the logical step of including both humans and chimpanzees in Homo. The great apes in this scheme are all hominins (tribe Hominini) and the living hominoids are all hominines (subfamily Homininae).

By discarding past consensus, arbitrary changes impose a cost on any researcher or student, in discarding past consensus. The past fifty years or more of paleoanthropological research have shared a clear meaning for the term "hominid." Of course, one may read that literature today while remembering the past meaning of "hominid," just as we remember what "pithecanthropine" used to mean. But it is a cost that should come at some benefit. For "pithecanthropine," the loss of the genus Pithecanthropus combined with the discarding of the idea of a "pithecanthropine stage" of human evolution means that we no longer have any call to use the term. The benefit of the change is simplification and the recognition that an incorrect hypothesis of evolution has been refuted.

Many would argue that the replacement of hominid with hominin has similar benefits. After all, the use of "hominid" in the past was partly conditional on the acceptance of the family Pongidae to hold the great apes. Now that we know that humans and African apes are sister taxa, we should construe Hominidae differently. It is clear that the human lineage did not have a long independent evolution during the Miocene, that its origin is comparatively recent compared to other mammalian families, and that the gross genetic distinctiveness of humans is relatively low. Doesn't it therefore clarify our understanding of hominoid evolution to demote the human lineage from a family-level taxon to a lower taxonomic level?

The clade formerly known as Hominidae

The problem with this line of logic is that it is a purely aesthetic choice. There is no reason to suppose that a family-level taxon should have a particular date of origin or duration. One may argue that extant mammalian families have a distribution of ages, or even of genetic variation, and that this should inform our taxonomic choices. But the logical endpoint of this argument is not that the human lineage is a tribe-level or infrafamily-level taxon, but instead the endpoint is the conclusion of Goodman et al. (1998), that the human lineage is a subgenus-level entity and chimpanzees should be placed in Homo. The fact that this solution is viewed as "too extreme" is good evidence that this is at its core an aesthetic concern rather than a scientific one.

In fact, there is no scientific reason why a particular phylogeny should correspond to a particular range of phylogenetic ranks. Many extant families of organisms include hundreds of species, others include only one. Some extant vertebrate families originated in the Paleozoic, others in the Pliocene. And viewing only the variation of extant species is especially misleading on this issue. When we consider the relationships of extinct organisms, we find family-level groups originating across the history of the earth. The family rank has been applied to short-lived groups with uncertain affinities, to extinct collaterals of living orders or classes, and to single fossils. When it has been applied, it has usually been according to considerations of morphological adaptive pattern. On this basis, there is a good argument for the idea that the human lineage should be at the family rank, regardless of its antiquity. The adaptation to an obligate pattern of bipedalism along with the dental specializations of the australopithecines (shared with humans) set them apart from other apes to a greater extent than any great ape. These features probably mark the human lineage as substantially different from great apes in adaptive terms as the great apes are from hylobatids.

So what aesthetic considerations prevent us from simply continuing to calll the human lineage Hominidae? That usage requires that something be done to avoid a paraphyletic taxon including orangutans, chimpanzees, and gorillas. We seemingly have a choice: accept Gorillaidae, Panidae, and Pongidae alongside Hominidae, or demote all these taxa. The demotion also helps with (although does not solve, see above) the problem of assigning taxonomic ranks to the African-European ape clades. A lower-level human clade leaves more ranks below superfamily to apply to the great ape clade, its possible progenitors among the Afropithecinae or Proconsulidae, the possible ancestors of the African ape clade among the Dryopithecinae, and the possible ancestors of the human-chimpanzee clade. Each of these clades may need a rank, and there aren't enough ranks to go around.

I have no problem with aesthetic changes in nomenclature per se. After all, I wholeheartedly support replacing "Neanderthal" with "Neandertal." And in fact, I don't find "hominin" that objectionable. It may take me a while to get used to the sound of it, but it is very clear in its now-current application. Since it merely replaces the old use of "hominid," it is a simple replacement of one unambiguous term for another. It seems to me much better than relegating the human lineage to a subgenus, which would leave no taxonomic names at all to talk about the origins of the human lineage (notice how much more awkward this becomes when we can't say "hominid origins").

What I don't like is the confusion that comes from changing the meaning of "hominid." "Hominin" means nothing special to anyone now, so it has a low conceptual cost. In contrast, "hominid" until recently meant something entirely different from its proposed meaning, inclusive of all great apes. "Hominid" is how countless interested followers of paleoanthropology recognize our ancestors, and it is how many of us have presented our science publicly throughout our careers. It is bad enough that we have to get our students to understand that "hominoids" are not "humanoids," and "hominids" do not include all "hominoids." Now we have to get them to differentiate "hominins" from the rest.

An argument is that "hominin" is qualitatively more valuable than "hominid," because it conveys a more correct view of the human phylogenetic rank in comparison to other groups of mammals. This would be the "Copernican" analogy -- noting that the sun is the center of the universe "puts humans in their place," and noting that our taxonomic level is at the tribe rather than the family likewise shows how our place is less special among the species of the natural world. Or at least, it does not distort our view of ourselves by giving us a higher taxonomic rank than we deserve.

But of course, if it is our goal to have every name indicate its exact rank relative to other organisms, then we must also make mammalian groups consistent with insect groups, mollusc groups, and plants, for that matter. For this purpose, it might be as well to include a number after every taxonomic name, to represent the genetic variation encompassed by the group, or age of the group in millions of years, for example.

And more to the point, the next time someone decides that the hominoids subsume too small a segment of the mammalian phylogeny, it will seem necessary to some revolutionaries to change the taxonomy yet again. When we revise terms to give a "correct" understanding of their status, there is no end to "corrections" in pursuit of this goal.

So there are good reasons to resist the shift to "hominin." It renders "hominid" inconsistent with its historical usage in the literature. It unnecessarily confuses the public, especially those who follow our science at a distance. And most important, there is no guarantee that this change will be the last.

How does PhyloCode help?

This is not a full summary of the rules of the PhyloCode. These are available online.

PhyloCode is a system for naming clades. Under this system, each clade in the phylogenetic tree of life is eligible for a unique name. These names are not ranked, so that although clades are necessarily hierarchical, their names are not systematized in a hierarchical way. There are two basic reasons for the use of rankless names:

1. The number of clades on some phylogenies is so extensive that a rank-based classificaiton devolves into confusion.
2. Under a rank-based classification, any change in the rank of a single clade name requires concomitant changes to many other clade names, although neither their content nor their hierarchical placement has changed.

Thus, the PhyloCode "holds clades innocent" of changes in other clades, by retaining a single, unique, unchanging name for them.

Clades are may be defined in a number of ways, including by apomorphies, by descendants of a single ancestor, or by the inclusion of all species joined by a single node. This last, node-based clade definition is probably the most common. For example, the living African apes and humans belong to a clade that we might call "Clade Homo sapiens and Gorilla gorilla", while humans and australopithecines may be joined in "Clade Homo sapiens not Pan troglodytes."

Part of the appeal of this kind of scheme is that it approximates what we do much of the time anyway. The human-chimpanzee clade has no taxonomic name, at least not that most people would know, and when we talk about it, we use the term "human-chimpanzee clade." It is understood that this clade also includes Pan paniscus, and that bonobos are nevertheless not part of the name, although "human-bonobo clade" would be no less correct. For larger taxonomic groupings, this trends toward a kind of shorthand. "Human-gorilla" clade necessarily includes chimpanzees and bonobos, and it shorter than "the clade containing extant African apes and humans." PhyloCode effectively codifies this shorthand.

But at the same time it provides a procedure for giving each of these clades a name. Remembering that these clade names carry no rank information, it is possible to give every one of these clades a name that is at once unique and resistant to change with changes in our understanding of phylogeny within and outside of the hominoids.

Phylocode and hominoids

Considering all this, one may wonder what the PhyloCode proposal would say about our current taxonomic problems in paleoanthropology. In the central instance, does PhyloCode provide a way out of the hominid-hominin problem?

According to the current draft (June 2004) of the PhyloCode, "phylogenetic definitions for many widely used clade names" (Cantino and de Queiroz 2004:4) will be presented in a volume resulting from the first meeting of the International Society for Phylogenetic Nomenclature, in Paris, July 2004. That volume is not yet available, but the abstracts of the meeting have been compiled and are available in a PDF online.

Representing primate systematics at the meeting was a contribution from Kaila Folinsbee and David Begun. The part pertaining to Hominidae reads as follows:

We propose to redefine Hominidae Gray 1821 (converted clade name) as the most inclusive clade containing Homo sapiens and Pongo pygmaeus. We redefine Homininae Gray 1825 (converted clade name) as the most inclusive clade containing Homo sapiens and Gorilla gorilla not Pongo pygmaeus. Hominini Gray 1825 (converted clade name) includes Homo sapiens but not Pan troglodytes. The Ponginae has traditionally been paraphyletic, separating Pongo pygmaeus, Gorilla gorilla and Pan troglodytes to the exclusion of Homo sapiens. Ponginae Elliot 1913 (converted clade name) is defined as Pongo pygmaeus but not Homo sapiens. These converted clade names preserve the established endings of the older system in order of most to least inclusive. (Folinsbee and Begun 2004:39)