Homo habilis

Today we finally get to learn about the exceptional discovery of four partial hominin skeletons from Malapa Cave, South Africa. Two of the fossil skeletons are described by Lee Berger and colleagues in the current issue of Science, descriptions of two more are still forthcoming.

A kind journalist sent me a copy of the research papers a few days ago, so my graduate students and I have had a chance to think about them a little bit and compare them with other material.

Berger and colleagues have named a new species to contain the fossils, Australopithecus sediba. For anybody who follows paleoanthropology, the new species won't be surprising -- if I found a fossil, I'd surely make up a new name for it, even if I thought it was my great-great-grandmother. In this case, the morphological reasons for naming a new species aren't trivial, but I'll begin by approaching them skeptically, especially in comparison with the large samples of South African fossils both earlier and later than Malapa. I'll conclude that a new species within Australopithecus was probably the right call, but not an easy one.

The press is running with a "new fossils provoke debate" storyline -- are they possible ancestors of Homo or not?

The simple answer to that question is that the Malapa skeletons are too late to be ancestors of Homo. After all, we have early Homo nearly a half-million years earlier.

A more complicated answer is that it depends what we mean by Homo. My feeling is that these skeletons don't comport with what most of us mean when we say "Homo". Most of us have in mind an adaptive shift from Australopithecus to Homo that included larger brain size as a significant element, and the MH1 skeleton has a small endocranial volume.

But if we accept that model of Homo, we have to accept its consequences, as the Malapa skeletons now make clear. One important consequence is that, if we assume that MH1 isn't Homo, we can no longer say have any skeletal evidence of Homo from before 1.95 million years ago. Because the Malapa specimens are more like Homo in their dental and mandibular features than are earlier specimens that have usually been called Homo.

And if we throw out all those earlier Homo specimens...well, then suddenly Malapa isn't too old to be an ancestor of Homo after all.

How old are they?

The fossils lay above a flowstone with a U-series and paleomagnetic date consistent with an age just around 2 million years ago. That's a maximum age for the fossils; they must be younger than that.

The hominins are in water-deposited sediments, which are inferred to represent ancient washes of subterranean water flows through the cave system. Two elements above the flowstone contain the hominin specimens, called facies D and E, and both have normal magnetic polarity. The most likely interpretation is that they belong to the Olduvai paleomagnetic subchron, which occurred between 1.95 and 1.78 million years ago. A specimen of the sabertooth cat Megantereon in one of these facies has a last appearance elsewhere in Africa at 1.5 million years ago. So it appears that 1.78 million years is a very likely minimum age for the fossils.

That's about as good as dating gets in South Africa, where we're used to seeing very wide age brackets on hominin-bearing localities. It means that the Malapa hominins lived at around the same time as KNM-ER 1470 in the Turkana basin, or OH 24 at Olduvai Gorge. Until today, I think we could justly claim that the only australopithecines still known to occur in this time interval were the robust species A. boisei and A. robustus -- although the first appearance of A. robustus might (might) be later than Malapa.

Why aren't they A. africanus?

To me, this is the hardest question to answer.

The Sterkfontein Member 4 sample of A. africanus is tremendously variable. The postcrania of both Malapa skeletons are tremendously informative, but fall within the range of variation at Sterkfontein for almost every feature that the authors reported. The few exceptions (such as humeral torsion and femur neck/shaft angle) are right at the edge of the Sterkfontein range.

In other words, it's my impression that the postcrania of the Malapa skeletons fit within A. africanus. The limits of my impression are that there are a whole lot of observations here, and the paper generally does not report metrics for the postcrania. Maybe the sequel will give us some more surprises.

I would have added a comparison with the Swartkrans A. robustus sample, which overlaps nearly totally in body size with Sterkfontein and contains elements that are in some cases more comparable to the Malapa skeletons. In particular, the os coxa of MH1 looks a lot like SK 3155, and the proximal femur looks like SK 82 to me, at least in the tiny picture provided with the paper. On the whole, I don't think that the Malapa hominins are particularly like A. robustus, I just think that if you put together a reasonably large sample of australopithecine postcrania, these two skeletons don't stand out.

I'll take up the discussion of proportions of the different elements below. My feeling is that the proportions aren't exceptional for Australopithecus, either, but we have to temper that against the observation that really only AL 288-1 (Lucy) is comparable, and it's more than a million years older.

What about the teeth? Generally speaking, the teeth of MH1 and MH2 are both at the small end of the A. africanus range. In a couple of cases (the lower canine of MH1, the lower second molar of MH2), the teeth are absolutely smaller than any Sterkfontein individual. The canines are within the range of A. robustus (remember that the robust australopithecines have small anterior teeth), but the premolars are nothing like the large, molarized Swarktrans sample of premolars.

They're a little small but within the range of those known for Homo habilis at Olduvai Gorge. For example, OH 7 -- the type specimen of Homo habilis has molars that are 1.5 mm larger than MH1 in both dimensions.

But then, Homo habilis really doesn't differ much in tooth size from Sterkfontein.

In size, the Malapa teeth are exactly what you would expect for Homo erectus. The first molars are smaller than those of Dmanisi D2700/D2735, for example. But unlike H. erectus dentitions, the molars of the Malapa hominins get bigger toward the back -- M3>M2>M1.

The Malapa mandibles are strikingly gracile. The MH1 mandible has a relatively vertical symphysis with a small cross-section. The long, parallel upper and lower corpus borders really strike me like a mandible of Homo erectus, something like KNM-ER 993 or OH 22 -- but this impression may be exaggerated considering the M₃ of MH1 has yet to erupt. Metrically, the corpus breadth and height are most like OH 13. There are small australopithecine specimens that compare to this, such as AL 277-1, and it is worth remembering that MH1 is a juvenile mandible. I can't compare the ramus heights with those of other samples because the authors don't report those measurements.

An interesting question: If these mandibles had been found in isolation, would we call them Australopithecus? The Olduvai H. habilis mandibles OH 7 and OH 13 have M3>M2>M1, while OH 16 has M2>M3>M1. The Malapa mandibles look much more like later Homo than do early Turkana basin mandibles like KNM-ER 1801, KNM-ER 1802, or KNM-ER 1482, all of which are much more robust and have larger, more molar-shaped premolars than MH1, and all of which have M3>M2>M1 except KNM-ER 1802 which lacks M₃. This is a quick comparison on my part, but I think the Malapa mandibles look more like Homo than does the existing hypodigm of Homo habilis. It's hard to imagine that the mandibles in isolation would have been referred to Australopithecus. More on that below.

Compared to the mandibles, the cranium of MH1 looks more like its counterparts from Sterkfontein. To be sure, it is an 11-13-year-old juvenile and more gracile in some respects than any of the Sterkfontein crania. But take a look at it next to Sts 71:

MH1 (left) next to Sts 71 (right)

They're not identical, naturally. Sts 71 has higher temporal lines, a slightly smaller vault, and more prominent cheeks. It also has more postorbital constriction compared to MH1, though that isn't obvious from this angle. MH1 has a true superorbital torus, Sts 71 has at best a shade of one. But you can see the similarities -- the angle of the zygomatic process of the maxilla, the narrow and concave interorbital region, the tall and narrow orbits. MH1 has no prominent anterior pillars (bony swellings on either side of the nasal aperture), but Sts 71 is not very different in this region. Sts 71 has bigger teeth.

Consider also Sts 52:

MH1 (left) next to Sts 52 (right)

Again, Sts 52 has anterior pillars and bigger teeth, but the shape of the face is very comparable between these two. The nasal bones in particular are similar in this pair, almost "pinched" at the midline, with a lateral expansion both superiorly and inferiorly.

We can do a similar exercise for most of the features of the MH1 cranium. What is exceptional, in the context of the Sterkfontein sample, is the overall gracility of the masticatory apparatus.

One important thing that is not in the least bit exceptional: Its brain. An endocranial volume estimate of 420 ml (from CT reconstruction) puts MH1 at the bottom of the range of variation at Sterkfontein -- the best-known skull from Sterkfontein, Sts 5, has a volume of 485 ml, while STW 505 has a volume larger than 550 ml. Before MH1, the smallest of the South African crania were estimated to have volumes of 428 ml. This one seems to be smaller mainly by being flatter -- a shape that it shares with early Homo, but I wouldn't say it was without parallel in Australopithecus.

But the smallest endocranial volume known for early Homo is KNM-ER 1813, at 510 ml. That specimen is extreme: the next smallest is 585.

The vault fits in A. africanus, most of the facial features have comparable specimens in the Sterkfontein sample, with some exceptions, and the postcranial skeleton is unexceptional. The teeth are mostly within the range at Sterkfontein with some exceptions. But the mandible -- like those few facial characters -- stands out.

Australopithecus sediba -- a new species within Australopithecus -- then seems like a fair diagnosis. The craniodental derived features are of the sort that would usually justify a new species. Heck, in the case of Kenyanthropus, even more minor differences in the face and size of teeth from contemporary A. afarensis caused Leakey and colleagues (2001) to name a new genus.

Is MH1 really a male?

Berger and colleagues (2010) infer that the MH1 skeleton (the one with the skull) is a male. It is large and more robust than the MH2 skeleton: Its teeth are bigger than the MH 2 skeleton, its mandible is more robust with a taller ramus, the articular ends of its limb bones are a bit larger. In addition, the greater sciatic notch on its preserved os coxa is narrower than other australopithecines like Lucy and Sts 14, and the pelvic inlet may (based on the anterior position of the auricular surface) have been smaller.

But the skeleton isn't really very big. Its endocranial volume is small, its long bones are not nearly so robust as some australopithecines. There are large male australopithecine skeletons -- STW 431, for example -- and MH1 doesn't seem so large as these. Again, it's hard to tell without postcranial measurements, but the sex of this specimen isn't a clear call either way.

The sex of the specimen is important to the way we interpret it, because the features that make it stand out from A. africanus concern masticatory gracility. If it's a female, it doesn't seem quite so different from A. africanus as if it's a male.

Are they Homo?

Let's start with the brain size, which at 420 ml seems to be the most obvious thing separating MH1 from our genus. Well, except for Liang Bua 1 -- with its endocranial volume of, um, 420 ml. Is brain size fundamental to Homo? Maybe. Maybe not.

Alan Boyle's report on the fossils ("Fossils shake up our family tree") has an excellent letter from Don Johanson, who makes the argument that the Malapa fossils should be assigned to Homo. Of course, Johanson and Bill Kimbel in 1996 described a 2.33-million-year-old fossil from Hadar as the earliest clear maxilla of Homo. That maxilla, AL 666-1, resembles Homo in having a more vertical subnasal profile, a parabolic dental arcade, molars that are long relative to their breadth, and a "squared-off" jaw that is relatively straight across the anterior dentition. In other words, basically the dental features seen in the MH1 maxilla.

We've got two choices. Maybe these are genuine shared derived features with these specimens and Homo -- in which case, we should probably name them Homo, as Kimbel and colleagues did for AL 666-1.

Or, there were several australopithecines after 2.5 million years ago with these dental and maxillary (and for the Malapa hominins, we can add mandibular) characters. In which case, they're not signs of Homo at all. They may reflect parallel dental reduction in several australopithecine lineages, all of which faced niche differentiation from the emerging robust australopithecines. One of those lineages may have given rise to Homo, but we don't know which. Maybe it was South African, but it need not have been. It could even have been Asian.

The question is just how important we think brain evolution was to the origin of our genus. If the brain was the key adaptation, then Malapa shows that the dental features are irrelevant to the brain -- because these skeletons have more dental reduction than most of the East African Homo habilis sample, but MH1 has a much smaller brain.

What about tool manufacture?

Part of the logic of pre-2-million-year-old Homo is the emergence of stone tool manufacture 2.6 million years ago. It stands to reason that this major shift in behavior and diet might have given rise to a new adaptive plateau for early hominins, and that would have been tied to the evolution of larger brains. The problem is that we don't have larger brains in any fossil remains until after 2 million years ago -- KNM-ER 1470 remains the earliest hominin with a brain larger than 600 ml. Up to now, people have conjectured that large-brained hominins may have existed earlier, even to the point of arguing about the brain size reflected by the otherwise-robust temporal bone from Chemeron. But it's worth pointing out that none of these pretenders to the Homo throne have smaller teeth than A. africanus. The diet shift that should have been made possible by a meat-eating stone tool economy didn't lead to smaller teeth until much later.

And now we know that at least one small-toothed hominin also was a small-brained one.

We don't know whether the Malapa hominins would have been toolmakers. The fact that they weren't found with tools isn't really evidence either way. Dirks and colleagues (2010) suggest that the skeletons were deposited by water washing them from an initial death trap into a secondary location. If true, it would be a miracle beyond belief for stone artifacts to have made the trip with them.

We do know that stone tools are present in Sterkfontein Member 5 and Swartkrans Member 1, and cutmarked fauna are in the latter. Both these may be roughly contemporaneous with the Malapa hominins, depending on their date. So toolmaking hominins were in the immediate area, around the time that the Malapa hominins lived.

SK 847 is from Member 1 of Swartkrans, and could be as old as the Malapa skeletons. Its endocranial volume isn't known, but facially it looks even more like Homo erectus than does MH1. It seems plausible that this skull represents the local toolmaking population, but even so, this skull does resemble MH1 in several respects, and again we don't know its volume. STW 53, probably a bit older than Sterkfontein Member 5, has also often been referred to Homo but it definitely doesn't have a substantially larger endocranial volume than MH1.

So again, we seem to be faced with two choices: Broaden the definition of Homo to include this very australopithecine-like sample, or restrict it to later large-brained hominins. In either case, brain size and tool manufacture do not necessarily go together.

What's the single most obvious thing that the paper doesn't describe?

Which brings me to the fingertip. MH2 has a distal phalanx. The paper doesn't describe it, even though this bone element has taken on such importance in the evolution of Homo compared to Australopithecus. Big fingertips are supposed to be adaptations to force transfer through the fingertip grip used in tool manufacture.

The picture of the thing is so tiny -- I mean, literally we're talking about two pixels of finger -- that I can't make anything out of it. Does it have a large apical tuft, like OH 7? Or is it like the Hadar distal phalanges, with narrow, apelike apical tufts?

If one was wondering about whether the thing was Homo or not, I would think this is one of the first things you would check....

What about those limb proportions?

For fifteen years, a bunch of otherwise sensible paleoanthropologists have been engaged in a debate about the limb proportions of A. africanus and H. habilis. The reason why this particular question may not be sensible is because the debate is about the length of the arm relative to the leg, but there's no specimen of A. africanus that preserves both the length of the arm and the length of the leg.

What there are: OH 62, a skeleton apparently of H. habilis that has a complete humerus and more than half the length of one femur, STW 431, which has an acetabulum and mostly complete humerus, and Sts 14, which has a partial femur, an acetabulum, and a piece of distal radius. On the basis of these fossils, we've seen some intense debate about the reconstruction of the OH 62 femur length, and a lot of discussion about whether the sizes of articular surfaces are relevant to the function of the limbs. Indirectly, it has appeared that A. africanus and H. habilis shared longer arms than were present in AL 288-1 (Lucy).

Well, now we have two fossil skeletons with both hindlimb and forelimb elements. The paper doesn't address the issue directly, nor does it provide raw measuremnets that would lead to a quick answer. But the humerus is short relative to the size of the femur head, compared to earlier hominins, while a bit long relative to Homo by the same comparison. So it looks like the Malapa skeletons may be somewhere in between.

The authors do argue that OH 62 is an odd skeleton in one respect: They consider the "diaphysial strength" of the humerus and femur. This is a cross-sectional measure of the area of cortical bone, and reflects the robusticity of both forelimb and hindlimb elements. In their estimation, OH 62 has a much stronger arm relative to its leg strength than the Malapa skeletons.

It's not obvious how to interpret this observation. Is OH 62 more apelike in its locomotor pattern than Malapa? Or does the strength ratio vary allometrically with body size, and OH 62 is just at the smallest end of the comparison? Hard to tell without the length measurements.

OK, what's the bottom line?

Here's the important thing. From today forward, there are a bevy of features of the face, teeth and jaw that are no longer "derived characters" of Homo.

Some people will want to fix this by broadening the definition to Homo to include the Malapa skeletons. Others will want to narrow the definition of Homo to include only large-brained specimens.

Every specimen attributed to Homo before 2 million years ago is now up for grabs. Maybe they're Homo, or maybe their resemblances to Homo are just masticatory parallelism. We already know that parallelism explains many of the derived locomotor and masticatory resemblances of great apes, and many strongly suspect that parallelism explains the derived masticatory resemblances of robust australopithecines. If the dental reduction that once was a marker of Homo joins this list, it will hardly be surprising.

If we follow the logic that connects tool use to dental reduction, however slowly and indirectly, then I think we have to conclude that A. sediba was likely a toolmaker and meat-eater. This hypothesis is testable, both by bone chemistry and dental morphology and wear.

Malapa suggests the hypothesis that brain evolution followed the appearance of stone tool manufacture by a considerable delay. If so, I wonder what exactly caused the brain to expand. Did the diet shift to higher-quality foods unfold over a long time, allowing brains to expand only after 3/4 million year delay? Or was brain evolution caused mostly by non-dietary factors, such as social dynamics or climate instability?

Or did the evolution of our genus happen somewhere else, far from the places where we currently have fossil samples? The Rift Valley and South African cave systems may have been wonderful for preserving fossils, but who's to say they weren't relative backwaters when it came to the evolution of Homo?

Well, I'm running out of gas for this installment. More later....

References:

Berger LR, de Ruiter DJ, Churchill SE, Schmid P, Carlson KJ, Dirks PHGM, Kibii JM. 2010. Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science 328:195. doi:10.1126/science.1184944

Dirks PHGM, Kibii JM, Kuhn BF, Steininger C, Churchill SE, Kramers JD, Pickering R, Farber DL, Mériaux A-S, Herries AIR, King GCP, Berger LR. 2010. Geological Setting and Age of Australopithecus sediba from Southern Africa. Science 328:205. doi:10.1126/science.1184950

OK, I'm going to live-blog this show. I've been looking forward to it for a while -- I loved the old NOVA series with Don Johanson and have often showed it in classes but I had to stop several years ago because it's getting out of date. These are great overview-type programs, unlike the more special-purpose one-topic shows.

The producers gave me the opportunity to review the program's script a few months ago (that's explains the acknowledgement at the end), so I'm not expecting any unpleasant surprises.

The pre-credits opening: Naked people smiling. Naked chimps grooming...

7:01: "What set us on the path to humanity? The questions are huge, but at last, there are answers..."

"For millions of years, many human-like species coexisted on our planet, until one day, there was only us."

7:03: "Apes that had walked on four legs stood up and walked on two." We see apish CGI hominins. Then, to the Sahara to see Toumaï. Michel Brunet is describing the skull.

"We, Homo sapiens, are the first ever to be alone."

7:06: To the Afar, explaining the Rift Valley and its erosive contexts. The Insta-Zoom effect across the desert is actually kind of cool. We see Zeresenay Alemseged driving an SUV, then walking in badlands with scattered bones. Nice photographs of the Dikika skull in context.

7:09: Zooming backward into a timeline, as if the years are sucking us back, the program explains the timespan of human evolution as a series of doublings backward in time.

7:10: Alemseged is in the National Museum of Ethiopia, preparing the skull. It's a nice video treatment, shoing the slow preparing with dental drill. The long shots of the postcranial elements are very illustrative -- this is a good demonstration of how the anatomy informs us about the developmental schedule and lifeways.

7:13: Don Johanson is explaining how he found AL 129-1. Then, he explains the difference between the chimpanzee and human pelvis. Too bad they couldn't have included Ardipithecus; it would be interesting.... I'm really liking the fact that you have people interacting with actual casts instead of lots of CGI images. You have a much better impression of the scale

7:15: Now the scene moves to Kenya, this is going to be about paleoenvironments. Yannic Garcin and Daniel Melnick are describing how the now-desert landscape was once much wetter. We go back to the Afar, with Alemseged explaining the fauna that's just eroding up out of the ground (wonder how set up that scene was...).

7:18: Bipedalism. It's like Saturday Night Fevur. Brian Richmond appears to explain theories about why bipedality was adaptive. This is all accompanied by contemporary dancers wiggling around. Chimpanzee-like ancestors are illustrated with video of actual chimpanzees (wonder what Lovejoy is thinking...). Dan Lieberman is talking about energy budgets. People and chimps on treadmills hooked up to oxygen meters.

7:22: Mark Stoneking explains the molecular clock. "The dates that one almost always gets are 5 to 7 million years ago for when humans and chimpanzees shared a common ancestor."

7:24: We go to Chad. Brunet explaining why they needed to recover fossils from somewhere other than East Africa. "Everyone said 'no', there just aren't any [human-like] fossils there."

7:26: "There were no bones apart from the skull..." Er...

7:27: The skull is reconstructed with a CT scanner and then cast. Oops...the rest of the shots of casts are all taken directly from the skull, not the 3-d scan version. Nice artist's rendering of Toumaï here.

7:30: I'd hate to be one of the dancers walking by on the screen with the voiceover, "Walking upright didn't mean that they had big brains."

7:33: Brain growth in Selam. Hints of a longer childhood -- of course, at 330 cc, it's almost the size of a full-grown chimpanzee. Todd Preuss is discussing the evolution of the brain, showing us actual pickled brains of human and chimpanzees. Lunate sulcus -- was Selam like a human or a chimpanzee?

7:35: Ralph Holloway is describing the brain reorganization -- great shot of him with his collection of endocasts. The conclusion is that the lunate sulcus was human-like.

7:37: Now we have stone tools appearing, Brian Richmond explains how we recognize tools. Unlikely they were made by Australopithecus, because they didn't make them earlier. Skip forward to KNM-ER 1470, "the dawn of a new era, beginning around 2 million years ago." Tools were used for meat processing. Homo habilis was small in body size, but had a much bigger brain than Australopithecus.

7:41: Viktor Deak is reconstructing Homo habilis. I like it, more apish than the usual rendering.

7:43: "Africa's gradual drying trend was punctuated by bursts of rapid climate fluctuation." We see Rick Potts explaining the stratigraphy of a lake alternating with desert and volcanic layers over time. The idea of "variability selection" is explained.

7:45: Analyzing diatoms in layers of rock -- the species tell the alternation of shallow and deep lake levels. It's a record of strong fluctuations. We see rapid clips of three different scientists (Potts, John Kingston, and Mark Maslin) talking about water fluxes. It's a good way of explaining the climate instability -- although they could have gone a bit further: when they mention "Lake Victoria-sized lakes appearing and disappearing", for example, they might have pointed out that Lake Victoria itself has appeared recently.

7:48: Dust from ocean cores. Once again, it comes down to tiny sea creatures whose anatomy correlates with date.

7:50: We get a rapid montage reviewing the climate instability idea. Hmmm...I have to say that the very fast cutting of clips and louder music doesn't really add to the credibility of the idea -- it seems like something is being left out.

7:51: Rick Potts restates the variability selection argument. "Simple but revolutionary idea -- human evolution is nature's experiment with versatility...we are creatures of climate change."

That's the end. I think the paleoenvironment story was well done. The shots of how this science is done were very illustrative -- from the field to the lab, the program showed the fine layers of sediment and careful study of microscopic creatures.

On the other hand, the show may have gone a little too far in the "climate made everything happen" direction. I don't think the "variability selection" idea explains the origin of Homo, and while the program did briefly list alternative views about the adaptive value of bipedality, it left no doubt that African desiccation and loss of forest was the ultimate cause.

I think everything with actual fossils, dirt, or rocks was well done. In particular, we got a good view of most of the Selam skeleton, with the notable exception of the hyoid bone. These are the best available images of the specimen to date. Holloway's descriptions of endocast evolution were well done, placed in the middle of a big table of fossil casts. I like the solidity with which the program showed the fossil record. Hopefully the next two segments will also follow this technique -- much preferred over the CGI-reconstruction technique.

I will be out of the country for the next two parts of the trilogy, so I'll have to see if I can get them online. The NOVA Evolution website has the first episode online now, so there's some hope.

In last week's Nature, Russell Ciochon has a remarkable essay:

For many years, I used Longgupo to promote this pre-erectus origin for H. erectus finds in Asia. But now, in light of new evidence from across southeast Asia and after a decade of my own field research in Java, I have changed my mind. Not everyone may agree; such classifications are always open to interpretation. But I am now convinced that the Longgupo fossil and others like it do not represent a pre-erectus human, but rather one or more mystery apes indigenous to southeast Asia's Pleistocene primal forest. In contrast, H. erectus arrived in Asia about 1.6 million years ago, but steered clear of the forest in pursuit of grassland game. There was no pre-erectus species in southeast Asia after all.

I think it's interesting how much speculation Nature is willing to publish about hominid evolution in Asia. The 2005 review article by Robin Dennell and Wil Roebroeks, "An Asian perspective on early human dispersal from Africa," speculated that the origin and early evolution of Homo may have been in Asia, not Africa. And of course, several papers on the hobbits have included speculations about the pattern of early Homo in Asia, in pursuit of ways to derive Homo floresiensis from early hominids not yet found in Asia.

Ciochon's essay is part of this new tradition, but it bucks the trend. Instead of arguing that Asia was the home to an undiscovered diversity of hominids, he instead argues that the hominids have been overestimated (in part by himself) and that some fossils represent an undiscovered diversity of apes.

Ancient orangutans (Pongo) and Gigantopithecus are already known from China. Ciochon proposes a third lineage of great ape, one that would be similar to the earlier Lufengpithecus from China and Thailand:

Later, we had to field a serious proposal that Longgupo belonged to Lufengpithecus (4, 5). Although the age disparity remained troubling, the dental similarities could not be denied. I began to imagine a mystery ape as a possible solution to the problem.

The "age disparity" Ciochon refers to is that Lufengpithecus is known from the Late Miocene and very earliest Pliocene, but not the Late Pliocene. Still, if the teeth look like Lufengpithecus, it seems probable that the "mystery ape" actually is a late-surviving Lufengpithecus, or at least a close relative. Reference 5 is a paper by Dennis Etler, Tracy Crummett and Milford Wolpoff, which is available (PDF) from Etler's excellent website. Wolpoff refers to this in his 1999 book, Paleoanthropology:

The Longgupo mandible is actually a fossil ape that is related to Lufengpithecus, the missing P₃ was sectorial in shape.  However the prestigious British Journal Nature hastily published it as a hominid, with a picture of the specimen on its cover, and subsequently refused to accept papers establishing its identity.  The misidentifications actually started decades ago, when G.H.R. von Koenigswald identified an ancient australopithecine-like hominid from South China based on worn, isolated teeth, which he named “Hemianthropus.” These turned out to be worn postcanine teeth of a medium-sized Pongo species. The resemblances of the other materials to Australopithecus species were real enough, but they were not unique resemblances. A. Kramer and Zhang Yinyun have each shown there are no synapomorphies that support the hypothesis of Asian australopithecines.

So it's not a new idea that Longgupo represents an ape, or that the ape was different in size and morphology from Pongo or Gigantopithecus. It is probably natural that early paleontologists might associate these ape teeth with the hominids -- until 40 years ago, most paleontologists thought that hominids went back far into the Miocene. They were wrong, but a mistake like "Hemianthropus" was a natural one. The opposite mistake -- "Meganthropus" as an australopithecine-like hominid -- was also a natural consequence of the assumption that an unrecognized hominid diversity existed in Asia. That assumption has outlived Meganthropus, as we've seen.

Ciochon adds the idea that the ape may also be represented at other contemporary or later sites, and is apparently unwilling to attribute them to Lufengpithecus, at least not yet. He does not mention the isolated upper incisor from Longgupo, but he does appear to accept the claim that the two stone artifacts from the site are intrusive elements that are not contemporary with the jaw. The same is probably true of the incisor, which Etler and colleagues found morphologically most like living East Asians.

Ciochon suggests that some of the Hemianthropus collection may be his mystery ape:

Von Koenigswald viewed Hemanthropus as a distant relative of African Australopithecus. Later research revealed that these were worn or atypical orangutan teeth and Hemanthropus was quickly abandoned. But, had von Koenigswald actually discovered evidence of the mystery ape? In October 2005, I examined the original Hemanthropus collection. Among the many worn orangutan teeth I found several small ape teeth that very closely resembled the mystery ape teeth from Mohui. Perhaps von Koenigswald was the first to lay hands on the mystery ape.

It's not an easy task to sort through large samples of teeth trying to sort them into sets. Particularly not with these teeth -- sure, Gigantopithecus falls right out, but worn orangutan teeth aren't very easy to tell from hominids, much less "mystery apes." Ciochon ends his essay with a plan to revisit the existing samples of teeth, trying to document the variation in the mystery lineage. Sounds like a good topic for a TV show. There's a historical angle, lots of museums, a personal hook, reversal of fortune, the whole "mystery ape" thing....

Meanwhile, the introductory paragraph at the top of the post raised two issues, not one. The first is the focus of the rest of the essay: Longgupo represents a third ape in Pleistocene China; smaller than both Gigantopithecus and Pongo. The second idea is covered briefly near the end of the essay, but I think it deserves more consideration. Is it true that humans reached China 1.6 million years ago, and then "steered clear of the forest"?

Here's what Ciochon writes:

Homo erectus, it seems from this perspective, hunted grazing mammals on open grasslands, and did not or could not penetrate the dense subtropical forest. In fact, there is no record of early hominins living in tropical or subtropical forested environments in Africa or Asia.

In resolving the mystery, two other Asian sites come to mind: Jianshi (Hubei province, China) and Tham Khuyen (Lang Son province, Vietnam). At both sites, teeth labelled variously as Australopithecus, H. erectus and Meganthropus are most likely to be the mystery ape instead. Others have come to similar conclusions; a 2009 paper identifies a tooth from Sanhe Cave (Chongzuo, Guangxi province, China) as belonging to an unidentified ape.

The map accompanying the article is mysteriously depauperate of actual early hominid sites in China. Considering their locations relative to the proposed distribution of subtropical forest in Pleistocene China, I don't see an immediate objection to the hypothesis. The earliest Chinese archaeological sites, from the Nihewan basin near Beijing (Majuangou and Xiaochangliang) and also from around the Yellow River (Gongwangling and Xihoudu), are north of the Stegodon--Ailuropoda fauna. Yuanmou may have been forested at this time, but the hominid teeth there appear to be later (Hyodo et al. 2002), when the Ciochon's forest-plains biogeographic proposal may no longer hold. Josette Sarel and colleagues (2009) report on stone tools from Baerya Cave, which does preserve the Stegodon--Ailuropoda fauna, but these are so far undated and the stratigraphy has not been worked out. For all we know, the association is no clearer than at Longgupo, but that may change.

The other early Chinese sites with hominid teeth, Ciochon suggests are not hominids -- Mohui and Sanhe. Since he has examined the Mohui teeth (Wang et al. 2007), this isn't an idle speculation, and it would be odd for humans to drop their teeth around these sites without dropping a single stone tool. If he's right, that would make the earliest clear evidence of human occupation of South China into the Middle Pleistocene in age.

So, it's an interesting generalization. It remains to be seen how true it may be -- was early Homo really limited to a biogeographic strip of plains and savanna as it left Africa, or were the early humans more broadly adapted -- or adaptable?

References:

Ciochon RL. 2009. The mystery ape of Pleistocene Asia. Nature 459:910:911. doi:10.1038/459910a

Ciochon R, Long VT, Larick R, González L, Grün R, de Vos J, Yonge C, Taylor L, Yoshida H, Reagan M. 1996. Dated co-occurrence of Homo erectus and Gigantopithecus from Tham Khuyen Cave, Vietnam. Proc Nat Acad Sci 93:3016-3020.

Etler DA, Crummett TL, Wolpoff MH. 2001. Longgupo: Early Homo colonizer or Late Pliocene Lufengpithecus survivor in South China? Hum Evol 16:1-12.

Hyodo M, Nakaya H, Urabe A, Sagua H, Xue S, Yin J, Ji X. 2002. Paleomagnetic dates of hominid remains from Yuanmou, China, and other Asian sites. J Hum Evol 43:27-41. doi:10.1006/jhev.2002.0555

Sarel J, Zhang P, Weng Z. 2009. Recent discoveries in Baerya Cave (Bijie District, Northern Province of Guizhou, China). Antiquity 83 (online).

Wang W, Potts R, Yuan B, Huang W, Cheng H, Edwards RL, Ditchfield P. 2007. Sequence of mammalian fossils, including hominoid teeth, from the Bubing Basin caves, South China. J Hum Evol 52: 370-379. doi:10.1016/j.jhevol.2006.10.003

Zhu RX, Potts R, Xie F, Hoffman KA, Deng CL, Shi CD, Pan YX, Wang HQ, Shi RP, Wang YC, Shi GH, Wu NQ. (2004). New evidence of the earliest human presence at high northern latitudes in Northeast Asia. Nature 431:559-562.