Homo rudolfensis

I keep seeing this story about Tim Bromage's "computer-simulated" reconstruction of KNM-ER 1470.

Bromage said his team's reconstruction includes biological principles not known at the time of the skull's discovery, which state that a mammal's eyes, ears and mouth must be in precise relationships relative to one another.

"It doesn't matter if you're a rat, a kangaroo, an elephant, a human or a dog -- their [facial features] are all organized to a very specific architectural plan," Bromage said.

Well, let's see. Here's the lateral view of the old and new reconstructions in the article:

The photo accompanying the article

Wow, that new reconstruction sure has a sloping face doesn't it? Oh wait! It's rotated at a different angle from the old reconstruction! Let's use Photoshop to fix that right up:

Same picture, with the reconstruction rotated to the Frankfort horizontal, like the old reconstruction.

Well, now, that's better. Now the vaults are at the same orientation. And the reconstruction does have a bit more sloping face. By about 5 degrees.

Now, a good cast of ER 1470 comes with the face and vault in separate pieces. There is only one join between them, at the nose, and it's not a very good one. Every graduate student in the world has probably taken these two pieces and rotated them back and forth to decide on the best angle. No doubt, there is five degrees of variation between them all. I'm perfectly willing to believe that the skull should have five more degrees of inclination to its face.

I wish that new reconstruction had the nasal bones pictured, though -- a greater slope makes the join there worse, which is probably why the original reconstruction was made with the more vertical orientation. Oh wait! It looks like the nasal bones have been crammed back under the frontal bone! That seems odd...considering the fronto-nasal suture is there underneath the small browridge. Hmm...

I think this particular issue is one for which more detail will be necessary. That seems like an unreasonable placement of the nasal bones, but we only have the one view to work with.

There is a lot of talk about the brain size of the specimen. I don't have any details of the presentation, and it is possible that Bromage was incorrectly quoted. Here is what the article says:

The new reconstruction suggests H. rudolfensis' jaw jutted out much farther than previously thought. The researchers say the cranial capacity of a hominid can be estimated based on the angle of the jaw's slope and they have downsized KNM-ER 1470's cranial capacity from 752 cubic centimeters to about 526 cc. (Humans have an average cranial capacity of about 1,300 cc.)

That, of course, is utter nonsense. Ralph Holloway produced an endocast, the joins between the fragments are good, and the volume of 752 cc was measured by water displacement. Why in the world would you estimate brain size from the face when you have a perfectly good vault? It has to be a misquote.

The article quotes Bob Martin as a skeptic:

"What they're claiming is you stick the face out, and because the face sticks out more the brain capacity has to be less. I don't follow that at all," said Martin, who is an expert on hominid skulls and who was not involved in the study.

"They haven't changed the skull at all; they've simply rotated the face outwards," Martin added.

He mentions that the 752 cc estimate is not a problem in comparison to other contemporary hominids. We can also mention the Dmanisi crania, the largest of which (D2280) has a brain size essentially the same as KNM-ER 1470, at 1.75 million years. KNM-ER 1470 is one of the most solid endocranial volume estimates in the fossil record. It's the face that's crummy!

The new Human Origins hall at the American Museum is the occasion for a big Newsweek story, with the tagline, "The New Science of Human Evolution". Author Sharon Begley isn't stingy with the prose:

Whether or not you believe the hand of God was guiding these changes, the discoveries are overturning longstanding ideas about how we became human.

Not that fossils are passé. New discoveries are pruning and reshaping humankind's family tree as radically as bonsai. The neat traditional model in which one species gave rise to another like Biblical "begats" has been replaced by a profusion of branches, representing species that lived at the same time as our direct ancestors but whose lines died out. It's like discovering that your great-great-grandfather was not an only child as you'd thought, but had a number of siblings who, for unknown reasons, left no descendants. New research also shows that "progress" and "human evolution" are only occasional partners. More than once in human prehistory, evolution created a modern trait such as a face without jutting, apelike brows and jaws, only to let it go extinct, before trying again a few million years later. Our species' travels through time proceeded in fits and starts, with long periods when "nothing much happened," punctuated by bursts of dizzying change, says paleontologist Ian Tattersall, co-curator of the American Museum's new hall.

It's a little sad to see the article organized around a 15-year-old storyline. No More Unilineal Evolution! Hey, if it's a "new science", why do we keep hearing from the same old people?

Still, there are some brain evolution subplots, and a few genes mentioned. Aside from the flowery analogies, Begley is a good writer and can capture the essence of most of these stories in a few lines. As an exercise, let's try to take those few lines and change one crucial word to find the weakness of each hypothesis. For each quote, I'll strike out a word in the article and add the correct word in brackets.

You dirty louse

For example, let's start where the article does, with the "body lice = no fur" story:

That fork in the louse's family tree, [Mark Stoneking] and colleagues at Germany's Max Planck Institute for Evolutionary Anthropology concluded, occurred no more than 114,000 years ago. Since new kinds of creatures tend to appear when [correct word: after] a new habitat does, that's when human ancestors must have lost their body hair for good - and made up for it with clothing that, besides keeping them warm, provided a home for the newly evolved louse.

You see how easy that is? Yes, new species adapt to new niches, but there is no reason to think this happens immediately. For that matter, there is no reason to think that hominids lost their fur instantaneously.

And hey, if the theme of the article is that human evolution has lots of extinct branches, then why doesn't that apply to louse evolution? We just saw last week how complex the louse phylogeny has been in hominoids. Who says that the current body louse was the first to fill that niche?

Oh, savanna, don't you cry for me!

Here's a short one:

The apes that stayed in the forests hardly changed; they are the ancestors of today's chimps. Those that ventured into the newly formed habitat of dry grasslands [correct phrase: open woodlands] had taken the first steps toward becoming human.

None of the earliest hominid sites are open savanna. All of them come from sites that preserve other woodland creatures.

By the way, my favorite quote in the whole thing comes here:

Instead, evolution played Mr. Potato Head, putting different combinations of features on ancient hominids then letting them vanish until a later species evolved them.

I just love that analogy! Forget "mosaic evolution". I'm calling it "Mr. Potato Head evolution" from now on.

My what small teeth you have

This part is a little confused:

And it helps explain why Lucy's kind were the way they were. Afarensis women and men stood three to five feet tall and weighed 60 to 100 pounds. They had small [correct: big] teeth good for fruits and nuts, but not meat. (The available prey was [correct: competing predators were] enough to make one a confirmed vegetarian: hyenas the size of bears, saber-toothed cats and other mega-reptiles and raptors.) That suggests that early humans were more often prey than predators, says anthropologist Robert Sussman of Washington University, coauthor of the 2005 book "Man the Hunted." The evidence is as stark as the many [correct: two] fossil skulls containing holes made by big cats and [correct: one containing] talon marks from raptors.

Well, that's taphonomy for you. There is plenty of evidence for predation on ancient hominid bones, and a National Geographic News article from 2002 details work showing the contribution of felids. But only two skulls have holes that may have come from ancient cats (those would be SK 54 from Swartkrans and D2280 from Dmanisi). Only Taung has evidence of raptor damage.

Splitting straws on habiline brains

Dmanisi has left people pretty confused about what explains hominid dispersal from Africa. Some are groping for other hypotheses. Just check out this paragraph:

Erectus shows that brain size is too crude a measure of a species' talents. At Dmanisi, the brains range from 600 to 770 cubic centimeters, comparable to the more primitive habilis. But while erectus did not distinguish themselves in brain size, brain structure is more telling [correct: nor does its brain structure provide any clues]. They were [correct: They were not] the first of our ancestors to have an asymmetric brain, as modern humans do; Australopithecus species do not [correct: did]. Asymmetry is a mark of increasing specialization and therefore complex cognitive ability [correct: of questionable value, since apes and australopithecines have asymmetries to varying extents]. Erectus used it to, among other things, discover and tame fire [add: apparently much later]. What they did not use it for is technology. Tools found with the Dmanisi fossils include cutting flakes, rock "cores" from which flakes were made and a chopper, all primitive even for their time [correct: like those made in Africa]. "The old idea that you needed a master's degree in stone tools to leave Africa is crazy," says Bernard Wood.

Wow, how confusing. The Dmanisi crania had H. habilis-sized brains. They're like KNM-ER 1470. So brain size isn't the key characteristic that allowed hominids to disperse from Africa. Nor is body size, since the Dmanisi hominids were relatively small. That's a genuinely interesting problem.

But asymmetry doesn't solve it. KNM-ER 1470, either Homo habilis or Homo rudolfensis depending on your taste in hominids, has a well-defined Broca's area on the left hemisphere, which I would say is the main informative aspect of asymmetry in fossil endocasts. Chimpanzee brains are asymmetrical in some respects, so "asymmetry" itself is an irrelevant criterion without some specific anatomical feature in mind. The thing that people used to think might be important was petalial asymmetry -- one hemisphere of the cortex shifted forward compared to the other. Early Homo endocranial surfaces show fairly strong petalial asymmetries, including KNM-ER 2598 and KNM-WT 15000. But some Australopithecus endocasts share a similar pattern of asymmetry with later hominids (Holloway and De La Costelareymondie 1982). We don't know how to interpret petalial asymmetry in functional terms, by the way. There appears to be some correlation with handedness, but it's not clear that hand preferences and petalial asymmetries evolved at the same time or for the same reason.

Somebody could write a really interesting story just out of the material in this one paragraph. Just not this story!

Out of Africa

The bottleneck scenario always seems like a hard one for journalists to get right. This article is no better than usual:

Peter Underhill, a molecular anthropologist at Stanford University, tracked 160 such changes in the Y's of 1,062 men from 21 populations across the world. Applying the molecular-clock technique, he concludes that the most recent common ancestor of all men [correct: all Y chromosomes] alive today lived 89,000 years ago in Africa. The first modern humans-and therefore, unlike the earlier wave of Homo erectus into Asia a million years ago, the ancestors of everyone today-departed Africa about 66,000 years ago.

These pilgrims were strikingly few. From the amount of variation in Y chromosomes today, population geneticists infer how many individuals were in this "founder" population. The best estimate: 2,000 men. Assuming an equal number of women, only 4,000 brave souls ventured forth from Africa [correct: were isolated from other humans for thousands of years inside Africa]. We are their descendants.

Hard to get straight: genetic drift takes a long time to fix a gene. We don't necessarily know the number of founders of the out-of-Africa population; what we do know is how many individuals the ancient African population must have had under the hypothesis of genetic drift.

Other genes might well have more recent common ancestors, who would also have been more recent common ancestors of all men. This is especially true if any genes were under selection.

People who see my meetings talk will appreciate the irony of that last sentence...

References:

Holloway RL, De La Costelareymondie MC. 1982. Brain endocast asymmetry in pongids and hominids: some preliminary findings on the paleontology of cerebral dominance. Am J Phys Anthropol 58:101-110. doi:10.1002/ajpa.1330580111

In case you haven't been paying attention, the chronology of early African Homo has been completely turned upside-down this year. Well, "upside-down" isn't precisely right; "displaced younger by a quarter-million years" is better.

The redating has come from Frank Brown's group, which in a series of papers has defined and dated stratigraphic units between the major tuffs of the Koobi Fora formation, between the KBS Tuff at 1.87 Ma and the Chari tuff at around 1.38 Ma. Gathogo and Brown (2006) outline the consequences of this redating for fossils of early Homo. Their paper focuses on the fossils from area 123 at Koobi Fora, but discusses the likely consequences of redating on other localities.

Fossils of Homo now estimated to be 1.65 +/- 0.15 myr in age in the Koobi Fora region are currently assigned to at least two taxa on the basis of both crania and mandibles. Homo habilis is represented by specimens KNM-ER 1501, 1502, 1805, and 1813, and H. ergaster is represented by specimens KNM-ER 730, 1812, and 3733 (for attributions, see Wood, 1991, 1992; Wood and Richmond, 2000). The ages of specimens KNM-ER 1501, 1502, 1812, and 1813 have been discussed above, and although not the main focus of this paper, a few notes are offered below on the others.

Specimen KNM-ER 730 derives from a level 5 m below the Koobi Fora Tuff Complex in Area 103 (Feibel et al., 1989), and is thus ca. 1.6 myr old. Feibel et al. (1989) gave an age of 1.85 myr for KNM-ER 1805, but this specimen lies "just below the base of the Okote Tuff" in Area 130 (Leakey et al., 1978), and is more likely closer in age to that of the base of the Okote Tuff Complex (ca. 1.6 myr) than it is to that of the KBS Tuff (1.87 myr). On the basis of mollusc-packed sandstones and algal horizons correlated from Area 102 to Area 104, Feibel et al. (1989) estimated that KNM-ER 3733 was 1.78 myr in age. Although the age of KNM-ER 3733 cannot be confirmed without additional fieldwork, the White Tuff, with an estimated age of 1.63 myr (Brown et al., 2006), is the nearest unequivocally identified unit in the local section in Area 104. This tuff is exposed <300 m from the location of KNM-ER 3733, and Tindall (1985) records only 8 m of section below the White Tuff nearby. Therefore KNM-ER 3733 should be approximately the same age as KNM-ER 1813. Indeed, all specimens from Koobi Fora assigned to H. aff. H. erectus by Wood (1991), many of which are now referred to H. ergaster (Wood and Richmond, 2000), are now estimated to be 1.45 to 1.65 myr old with the exception of KNM-ER 2598. The latter specimen, which is a partial occipital bone from Area 15, was placed 4 m below the KBS Tuff by Feibel et al. (1989) and estimated to be about 1.9 myr old. This age estimate is reasonable because strata do not extend more than 7 m above or below the KBS Tuff at the recorded location of KNM-ER 2598 (Gathogo and Brown 2006:7-8, emphasis added).

This raises a question: Just how much evidence is left for large-bodied H. erectus-like hominids earlier than 1.65 Ma?

Wood (1991) didn't diagnose postcrania, and Gathogo and Brown (2006) don't comment on them. At least KNM-ER 1808 would seem to fall under this umbrella, since Wood (1991) did diagnose that. But more important in bracketing the evolution of large body size is KNM-ER 3228, a hip bone previously dated to 1.95 Ma. It's pretty big for a human, let alone an australopithecine. On the other hand, McHenry and Coffing (2000) suggested that KNM-ER 3228 might belong to H. rudolfensis. To my eyes, this would make it a pretty big specimen compared with femora like KNM-ER 1472 and KNM-ER 1481, but who knows?

Another uncomforable fit in an H. rudolfensis would be KNM-ER 2598. It sure looks like a large-brained, thick-boned specimen. It doesn't look much like KNM-ER 1470. But then, maybe 1470 is the unusual specimen...

Gathogo and Brown (2006) take on directly the issue of KNM-ER 1470 and KNM-ER 1813. The two were formerly considered contemporaries at around 1.89 Ma, but now KNM-ER 1813 is only 1.65 Ma.

KNM-ER 1813, lateral view

The real offshoot of this is that there are no longer any early small-skulled habilines. The question of whether KNM-ER 1470 and KNM-ER 1813 were too different to belong to a single species has drawn a lot of ink, but it was always a non sequitur, because the two weren't the only crania to consider. The more interesting observation had been that Olduvai Gorge preserved only small-skulled habilines, while Koobi Fora had both small and large ones. This was not only a geographic problem but also a temporal one, since the Olduvai habilines were all relatively late (less than around 1.8 Ma) and the Turkana habilines were mostly earlier.

Now the situation has changed. The small Turkana habiline, KNM-ER 1813, is now contemporary with the Olduvai sample. There are no longer any small-skulled early Turkana habilines. KNM-ER 1805 makes sense as a male of the later, small-skulled sample because it is relatively small-brained but robustly built (e.g., with a sagittal crest). That leaves KNM-ER 1470, KNM-ER 1590, KNM-ER 3732, and KNM-ER 3735 as plausible habilines before 1.85 Ma.

This seems like a nice sample as a possible ancestor for both later large-bodied Homo and later habilines. Heck, Wood (1991) even wrote this in his description of KNM-ER 3735:

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 (Wood 1991:134-135).

What more could you ask of a common ancestor? But then if some of this ancestral population would be expected to resemble later H. erectus-like specimens, then why not KNM-ER 2598?

And what, exactly, would make such a population -- with its mixture of H. erectus-like and habiline-like features -- different from Dmanisi? The answer, of course, is KNM-ER 1470. It's still the odd one in this lineup. But then, it does have the largest brain in this set, which might help to explain the rounded occiput.

Looking at what is left in the early part of the sequence is certainly interesting, but just as interesting is how all the H. erectus-like specimens are all bunched together between 1.65 and 1.45 Ma. This is the time interval that already held KNM-WT 15000, KNM-ER 3883, and KNM-ER 42700, and is just older than OH 9. Now we can add KNM-ER 3733, KNM-ER 730, KNM-ER 1808, and KNM-ER 1821. Isn't this an interesting sample? Don't you wish we knew about the other postcrania?

It seems to me that the hypothesis that H. erectus-like hominids first appeared in Africa around 1.65 Ma has interesting archaeological consequences. This isn't long before the appearance of the earliest Acheulean, and it plausibly makes the Developed Oldowan-Acheulean sequence a correlate of this evolution.

It is markedly not coincident with the earliest such evidence in Asia. But that raises the Dmanisi question again, doesn't it?

References:

Brown FH, Haileab B, McDougall I. 2006. Sequence of tuffs between the KBS Tuff and the Chari Tuff in the Turkana Basin, Kenya and Ethiopia. J Geol Soc 163:185-204.

Gathogo PN, Brown FH. 2006. Revised stratigraphy of Area 123, Koobi Fora, Kenya, and new age estiamtes of its fossil mammals, including hominins. J Hum Evol (in press) DOI link

McDougall I, Brown FH. 2006. Precise ⁴⁰Ar/³⁹Ar geochronology for the upper Koobi Fora Formation, northern Kenya. J Geol Soc 163:205-220.

Wood B. 1991. Koobi Fora Research Project, Volume 4, Hominid Cranial Remains. Clarendon Press, Oxford.

The article in the April 2005 National Geographic about Dmanisi has some interesting details that have not been made public before. The feature of the article is the D3444 skull, which I discuss in this post. But there are other points of interest.

The most significant is the short mention of the estimated stature of the Dmanisi partial skeletons. National Geographic says these are four feet seven inches, or 140 cm. To put this in perspective, the estimated adult height of KNM-WT 15000 is 180 cm or taller; the estimated height of AL 288-1 (Lucy) is 105 cm. The estimated stature for the KNM-ER 1472 femur, which is often assigned to H. rudolfensis and assumed to relate to the same population as the KNM-ER 1470 skull, is around 160 cm. So the Dmanisi hominids not only had habiline-sized brains; they also had habiline-sized bodies. Which makes them much more australopithecine-like than almost everyone had expected early humans to be.

There will be a lot of rewriting when these facts become official. Since the discovery of the Nariokotome specimen (KNM-WT 15000), there has been an emerging narrative of the evolution of early Homo. In this story, several anatomical and behavioral changes were confluent at a central speciation event that led to large-bodied Homo. These range from simple anatomical correlates, such as an increase in brain size, to far-flung behavioral inferences, such as the advent of menopause. All of these interpretations rest on the assumption that several behavioral and anatomical changes were coincident with the evolution of large body size. All of them now are thrown into question.

The other interesting feature is the John Gurche reconstructions of the Dmanisi crania. There is a multimedia presentation of the reconstruction of this and the other Dmanisi skulls at the National Geographic website. Like his other work, these are the best anatomical reconstructions of early Homo I've seen. I do wonder about the noses, though.

Dean and colleagues (2001) present a study of perikymata counts of anterior teeth (incisors and canines) in early humans and australopithecines, compared to extant apes and humans. The basic microanatomy of teeth and their process of development is briefly described in this write-up about Afropithecus.

The operating assumption that makes this study interesting is that "brain size, age at first reproduction, lifespan and other life-history traits correlate tightly with dental development" (628). Of course, this is true not only of dental development but all these other traits with each other and with others (most notably, body mass), since these kinds of "correlations" are really interspecies allometries of one kind or another. But the interesting thing is the apparent deviation of dental development rates in humans from other hominoid species. We take longer to grow our dental enamel, and this difference is likely to reflect a difference in the rate of growth or maturation:

Modern human enamel develops along a slower trajectory because the earliest-formed enamel, closest to the enamel dentine junction, is secreted in smaller increments for a longer time period. None of the trajectories of enamel growth in apes, australopiths of fossils attributed to Homo habilis, Homo rudolfensis or Homo erectus falls within that of the sample from modern humans (628-629).

Despite considerable variation in external tooth morphology, early fossil hominins share a common and fundamental enamel growth trajectory with the African ape clade that is derived in modern humans. (629).

It is now generally held that a prolonged life-history schedule reflects a reduction in the mortality rate of adults, triggered perhaps by behavioural, dietary or other changes. An increase in brain size (and cognitive ability) is associated with this reduction, but does not necessarily drive it, even in the human lineage. The size of key brain components associated with learning and cognition correlates with the timing of dental development in primates as the cost in time needed to grow and learn to use a larger brain increases. In this context a slower trajectory of enamel growth in permanent teeth, one of the basic determinants of tooth formation time, can be regarded as a life-history attribute associated with the extended, or prolonged, growth period of modern humans. The first evidence for a shift in enamel growth rates in the hominin fossil record seems to be with the origin of the larger-brained Neanderthals (at least by 100 kyr ago; see Tabun specimen C1 in Fig. 1) and modern humans (629-630).

Dean and colleagues also posit that the dental development data for specimens attributed to Homo habilis and/or Homo rudolfensis are consistent with an assignment to Australopithecus. Certainly the data do not contradict such an assignment, since there is no apparent difference between australopithecines, early humans, or any of the habilines in the study. Of course, the data also do not contradict the assignment of KNM-WT 15000 to Australopithecus -- they are just uninformative about taxonomic assignment. The key here is that the enamel formation times of the teeth show that early humans were essentially australopithecine-like in their dental development; and that both kinds of hominids were basically apelike.

The authors promote an interesting linkage between dental development times and brain size expansions:

If correct, these estimates of molar emergence times have shifted a little, in step with brain size, from those known for African great apes and australopiths. Nevertheless, it now seems increasingly likely that a period of development truly like that of modern humans arose after the appearance of H. erectus, when both brain size and body size were well within the ranges known for modern humans (630).

This is not to say that dental development times determine brain sizes or vice versa, but that the processes that promote delayed maturation connect both of these features. I would suggest that this connection might involve a genetic correlation between dental maturation and brain size that altered both characters with selection on overall maturation rate. If a prolongation of juvenile growth was a product of selection acting on social learning, then both slower growth and larger brains may have emerged in concert with each other.

The interesting part of this potential linkage is that growth may not have reached its modern rate 2 million years ago, but instead may have continued to change as brain sizes increased during the Middle and Late Pleistocene. That scenario is consistent with the dental development patterns in this study. In that perspective, this study can be chalked up in a list of ways early Homo was not very much like modern humans in its adaptive pattern.

References:

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 414:628Ð631. Nature online

Peter Ungar (2004) investigated the dietary adaptations of A. afarensis and early Homo by looking at the three-dimensional topography of their teeth. the shapes of the teeth are expected to reflect diet because the teeth themselves are adaptations for processing food. Among mammals there are some regular relationships between the morphology of an organism's teeth and the type of food it prefers. Looking specifically at primates, high-crowned teeth that interlock between the upper and lower jaws are adapted to shearing foods, in a manner analogous to the pinking shears of a seamstress. Shearing is necessary for cutting and puncturing through food items, which are the predominant actions necessary to process leaves and insects, respectively. Primates that specialize on other foods, notably fruits or harder objects like seeds, tends to have flatter teeth without high crests. These teeth useful for crushing or grinding foods. So and examination of the anatomy of the teeth is expected to give insights about the diets of ancient primates.

But direct examination of fossil tooth anatomy is complicated by the fact that the teeth wear during the course of an animal's life. The effective chewing characteristics of a tooth therefore change over time, as a result of progressive attrition on its occlusal surface. This means that it's difficult to measure the anatomy of teeth, as in the lengths of different crests or the area of shearing surfaces, because these characteristics are altered in individuals of different ages. This is a special problem among early hominids, whose teeth are generally heavily worn even among relatively young individuals. It also means that the chewing characteristics of younger individuals and older individuals may actually have been different, possibly reflecting ontogenetic differences in diet. The details of the anatomy of unworn teeth may have been largely irrelevant to the pattern of mastication throughout most of the individual's life.

Ungar (2004) applied three-dimensional modeling to assess the shape characteristics of teeth at different stages of wear. In this way he was able to quantify the surface characteristics of the teeth--that is, how flat or jagged they were, how high the cusps were, and any angulation or deviation from a horizontal occlusal surface. He applied the technique to a series of second mandibular molars (M two) of Australopithecus afarensis and early Homo (including specimens attributed to H. erectus, H. rudolfensis, and H. habilis). As a comparison, Unger also examined a series of gorillas and chimpanzees, not because their diets were probably similar, but because their molar cusp patterns are similar, yielding comparable topographic results. The topographic values derived from each of the specimens included the "average slope," which was a measure of the average vertical displacement between adjacent points on the tooth, and the "occlusal relief," which was the measurement of the three dimensional surface area scaled to the two dimensional occlusal surface area of the tooth. In principle, more jagged teeth with higher cusps will have a higher average slope and a higher occlusal relief.

The results indicated that the early Homo specimens had relatively high average slope, except in the most worn molars. These specimens had average slope near that of gorillas, which were the highest among the samples measured. The occlusal relief of early Homo molars was not as great as that of guerrillas, but was substantially larger than chimpanzees, except again among the most worn specimens.

In contrast, A. afarensis had by far the lowest average slope and occlusal relief among any of the samples. Chimpanzees were lower than early Homo, but the A. afarensis sample was still significantly flatter in its dental morphology. These results held true across a range of wear categories, degrading slightly among the most worn sample of teeth in all four species. But as teeth wore down in all four species, they tended to become flatter. His results perhaps isn't surprising but does inform about the chewing characteristics of the teeth of older individuals.

Ungar (2004) includes a discussion that addresses why the differences in surface characteristics may have come to characterize these samples. He begins with a discussion of the differences between chimpanzees and gorillas in their diets:

The differences in occlusal morphology between chimpanzees and gorillas evidently relate to differences in the material properties of the foods they eat, particularly their fallback foods. Central African common chimpanzees are primarily frugivorous, with soft fruits reported to constitute 70-80% of their diets (Kuroda, 1992; Tutin et al., 1997). Fruit is also commonly consumed by western lowland gorillas, making up about half of the food species found in their fecal remains (Williamson et al., 1990; Nishihara, 1992; Remis, 1997; Doran et al., 2002). Differences and similarities in food preferences are most obvious were these taxa are sympatric and have access to the same resources. At Lope, Gabon, for example, the dietary overlap is substantial, with gorillas reported to consume 73% of the food species eaten by chimpanzees (Tutin and Fernandez, 1985). Differences between the two taxa are notable at times of fruit scarcity though, when gorillas fall back on tougher, more fibrous foods (such as leaves and stems) than those eaten by chimpanzees (Tutin et al., 1991; Remis, 1997) (615).

It should be reiterated that differences in occlusal morphology between P. t. troglodytes and G. g. gorilla evidently reflect differences in fallback resources rather than preferred foods. While both tax evidently prefer soft fruits when available, differences in occlusal morphology apparently allow the gorillas to take advantage of fallback foods that are less accessible to the chimpanzees (615)

For the case of A. afarensis, Ungar concludes that the differences in occlusal morphology from chimpanzees are not as great as the differences between chimpanzees and gorillas, although in the opposite direction. In his view this implies that A. afarensis included more brittle, less deformable foods, possibly as a fallback strategy in contrast to chimpanzees.

For the case of early Homo, the conclusion is that the molars were intermediate between chimpanzees and gorillas in their occlusal characteristics. This means that, compared to chimpanzees and compared to A. afarensis, early Homo was better adapted to chewing "tough, pliant foods" (616). This has important implications for dietary reconstruction in early humans:

[T]ubers, especially larger ones (Baritelle and Hyde, 1999) are often fairly brittle, whereas mammalian soft tissues tend to be tough and elastic (Lucas and Peters, 2000). Thus, meat seems more likely to have been a key tough-food resource for early Homo then would have USOs. It has also been noted that USOs those are of limited nutritional value (Schoeninger et al., 2001), and so would not have made very good keystone resources (616-617)

Ungar does not mention a number of other arguments that might also favor this interpretation. 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.

References:

Ungar P. 2004. Dental topography and diets of Australopithecus afarensis and early Homo. J Hum Evol 46:605-622.

Other references therein.