Flores

In another post I write about the Martin-Falk exchange on the microcephaly issue.

Here, I review the Paleoanthropology Meetings summary by Elizabeth Culotta in Science.

Along with some background, the article basically covers two talks: one by Susan Larson concerning the humerus, and one by Bill Jungers about the pelvis.

Larson concluded that the upper arm and shoulder were oriented slightly differently in H. floresiensis than in living people. The shoulder blade was shrugged slightly forward, changing its articulation with the humerus and allowing the small humans to bend their elbows and work with their hands as we do. This slightly hunched posture would not have hampered the little people, except when it came to making long overhand throws: They would have been bad baseball pitchers, says Larson.

When Larson looked at other human fossils for comparison, she found another surprise: The only H. erectus skeleton known, the 1.55-million-year-old "Nariokotome boy" from Kenya, also has a relatively untwisted humerus, a feature not previously noted. Larson concluded that the evolution of the modern shoulder was a two-stage process and that H. erectus and H. floresiensis preserved the first step.

Humeral torsion was the feature that previously led to the (wrong) idea that the hobbits were quadrupeds, but that wrong idea just shows how odd the feature is in the context of hominids.

In that context, you might wonder how it could be missed in a well-known specimen like KNM-WT 15000. Well, the picture helps explain:

LB1 (top), human (middle), KNM-WT 15000 (bottom) humeri. Sized to approximately equal lengths, not to scale.

Hmm.... I know I wouldn't want to be in that position -- it's a tough assessment to make on that specimen. The head might have been slightly posteriorly oriented there, but the torsion in LB1 is pretty low (low being strange in this comparison) -- so I would be really hesitant to match them up. The problem is that identifying the feature really takes a fairly complete humerus, and there aren't very many either from Australopithecus or early Homo. As reconstructed, AL 288-1 (Lucy) doesn't have low torsion, and I wouldn't have assessed Sts 7 as having it either, although there is some possible distortion there. So it wasn't an ancestral feature in hominids, so far as we can tell. Without an epiphysis, KNM-WT 15000 lacks some of the most relevant anatomy, but there might be enough....

I'll really look forward to seeing more detail about this argument -- it should be interesting to see the anatomical comparisons!

The other talk was Bill Jungers', which focused on the pelvis:

In a separate talk, Jungers reported more unexpected findings. He was able to reconstruct the pelvis, which had been broken when the bones were moved to a competing lab in Indonesia (Science, 25 March 2005, p. 1848). Although previous publications had described the pelvis as similar to those of the much more primitive australopithecines, Jungers found that the orientation of the pelvic blades is modern. The observation adds weight to the notion that hobbits had H. erectus, rather than australopithecine, ancestry.

This should be interesting too -- there has been a lot of disagreement about how flat australopithecine ilia actually were. Many of the most well-known fossils are very flattened, but there is also a lot of distortion. Hopefully any distortion introduced by the breakage (pictures in earlier post) hasn't affected these comparisons.

The spin on these stories is that they make it more likely that the Flores hominids are descendants of early Homo rather than Australopithecus. Personally, I would guess that the extent of dwarfing necessary to explain the size of the specimen pretty much throws its postcranial affinities up for grabs. For example, it's never been clear how many of the Australopithecus-Homo anatomical differences are merely consequences of body size, and how many reflect different adaptations. Since so many of the apparent differences are at least responsive to allometric constraints, it's a problem that always has to be faced.

Let's put it this way: if it weren't a problem distinguishing these genera from postcrania, there would be a lot less confusion assigning the unassociated Koobi Fora postcrania. For LB1, it helps that the bones are complete -- but, hey, we can't even tell yet that it's not a pathological modern human!

No story yet, in the history of the weblog, has had so many people writing to ask me if I am alive. Even my parents are asking me if it's real.

I am alive, and I have to say it is somewhat heartening that so many people care whether I'm watching. It's been sort of like H.G. Wells' Time Traveller (no Morlock jokes, please), just watching a couple weeks of this story's progress makes a few things about paleoanthropology and the press much clearer.

It is not a hoax. It is a BBC publicity blitz. And to me, it seems like a stunningly cynical PR blitz. In other words, we're being played.

First, the facts, which can be drawn both from a working paper by psychologist Nicholas Humphrey, John Skoyles and Roger Keynes, as well as the published paper in International Journal of Neuroscience by Uner Tan, and the genetic paper by Turkmen et al. (2005) in Journal of Medical Genetics :

1. There is a family of people in Turkey with 19 children (many adult), 6 of whom have a congenital form of cerebellar ataxia -- a condition in which the cerebellum does not develop to a normal size, with effects on muscle coordination and cognition.
2. Five of these six affected individuals walk on all fours, with their palms to the ground. One walks bipedally.
3. The cognitive deficits in the affected individuals include some linguistic limitations, although they communicate in Kurdish with a limited vocabulary and some can speak Turkish as well.
4. This family was studied by Turkish neuroscientist Uner Tan. Uner Tan is a real, live person (some have speculated that the name might be an anagram for "nature", indicative of a hoax). He has had a long career in neuroscience, and has a substantial publication record extending through the last several years.
5. Tan claimed in his paper that the connection between cognitive deficits (particularly emphasizing the linguistic) and the quadrupedal gait might indicate a link between these characteristics in the evolution of humans. Tan explicitly raises the idea of one or more macromutations causing bipedal locomotion to emerge suddenly from a quadrupedal ancestor, in association with human cognitive abilities. In his paper, the syndrome is described as a live model for human evolution.
6. A British group of researchers, including Nicholas Humphrey, were invited to investigate the family. A documentary film production became involved at this point.
7. A German team of geneticists took samples from the family, and found a region on chromosome 17 that appears to be associated with the ataxia (they term hypoplasia). The condition appears consistent with a recessive Mendelian inheritance; the parents are first cousins (once removed), which raises the likelihood of a recessive disorder being expressed. The German team claims that the gene involved may shed light on the evolution of bipedality.
8. Humphrey, Skoyles and Keynes deny that the disorder gives any indication of a coevolution of bipedality and language. Indeed, they are skeptical that the genetic basis of the trait says anything about the evolution of bipedality at all. However, they believe that the affected individuals have a unique form of quadrupedal gait, which they call "wrist-walking" because weight is borne on the heel of the palm. They argue that this form of locomotion may be a better model for the ancestral quadrupedalism of hominids than knuckle-walking.
9. The documentary is scheduled to run on BBC2 Friday, March 17 at 9:00 pm.

What about the science?

Of course, the most important question is whether any of this actually contributes to understanding human origins. Generally, I think the answer is no -- not just an itsy-bitsy no, but a great big honking no.

But I will qualify that for a couple of aspects of the story, since there is the potential of finding some interesting facts. So I will cover the different hypotheses in turn:

Is Uner Tan right? Is the condition really evidence of a rapid coevolution of bipedality and language? No. Human bipedality and human cognition are both highly complex traits involving anatomical, developmental, and behavioral specializations. Each of them involved hundreds, and for cognition I would say thousands, of different genetic changes. There was no small set of macromutations that caused these traits to arise.

Furthermore, the pathology manifested in this family does not indicate any kind of earlier evolutionary stage. There was no time when people with modern human limb proportions walked on all fours. It is doubtful that australopithecines, who were largely apelike in their cognition, ever walked on all fours. A developmental disability that causes mental retardation and inhibits the development of normal bipedal walking does not indicate any kind of earlier form of hominid.

Tan's paper has but a single reference in its bibliography -- to himself. And he's an editor of the journal, International Journal of Neuroscience. It happens every year or so that some paleoanthropology story breaks about a paper in some non-anthropological journal. These don't benefit from peer review by paleoanthropologists.

Is the gene underlying the condition important to the evolution of bipedality? This hypothesis is almost impossible to test, because "important" could mean almost anything. If "important" means that the gene underwent at least one substitution as a result of its relationship to bipedality, then the answer may well be yes.

But that doesn't really show that the gene is "critical" or "crucial" or "significant" or any other random signifier. If what we mean by "important" is that bipedality would not be possible without a particular genetic change in that gene, well, I think that is pretty difficult to test. You would have to show a link between gene expression profiles and the development of bipedality in normal people, not pathological ones. You would have to demonstrate a history of substitutions in the gene that match the timing of bipedality. And you would have to show that other functions of the gene either did not interfere with its evolutionary role in bipedality or were deemphasized because of that role.

I think it's really unlikely that a gene that causes cerebellar ataxia was a critical bipedality gene. It may be necessary to walking normally, but it probably (indeed, evidently from the nature of the disorder) is very important to a lot of other things as well. A gene that breaks early brain development is no more likely than other genes to have a specific function role in the development or practice of bipedalism in humans. More on the genetics of broken development below.

Is the specific form of quadrupedalism -- the "wrist-walking" -- of these people a clue about the nature of pre-bipedal hominids? This is the most likely to be scientifically interesting. And it raises the possibility of a test, if you could find bony correlates of the wrist-walking that could be distinguished from knuckle-walking or other forms of locomotion. And the question is well within the tradition of paleoanthropology -- I mean, we even have people studying knuckle-walking anteaters for goodness' sake.

On the other hand, there are several reasons to be skeptical. The working paper by Humphrey, Skoyles and Keynes only includes a couple of paragraphs on the evolutionary question, so it isn't fair to suppose they have presented a summary of all the pluses and minuses of the hypothesis (that early hominids or their ancestors were wrist-walkers in this way). What it has going for it is that early hominids were not chimpanzees or gorillas. Their hands had finger and thumb proportions much like ours, unlike the very short thumbs and long fingers of great apes. In other words, they were not adapted to suspension below branches in the way that apes are. So it is credible to think that if they walked quadrupedally, or evolved from a non-suspensory ancestor, that they might have different quadrupedal gait from living apes.

But although they didn't have the hands of apes, early hominids still had powerful wrist flexor muscles, as indicated by their curved hand bones and large carpal tunnels. The length and strength of these flexors -- necessary for suspension in great apes -- limit the extension of the wrist. This is one of the reasons why chimpanzees and gorillas are knuckle-walkers and orangutans are fist-walkers: their hands do not extend as easily to a palm-down position for weight-bearing. So although monkeys and other primates are palmigrade, apes are not. Humans can extend our wrists more readily, and walk palm-down when we want to, which is usually limited to the first year or two of life. But for early hominids, and presumably their ancestors, this may have been more difficult.

Now the most interesting part about the wrist-walking idea is that it might itself have caused greater hominid wrist mobility, which became more important to hand control and flexibility for toolmaking. But I think that more likely places the effect before the cause: if wrist flexibility and hand morphology evolved for the purpose of toolmaking, then there is no need to invoke an earlier wrist-walking stage. So while the speculation is worthwhile, I think it is unlikely to prove important.

Pathology and evolution

The fundamental question is how much a pathology can tell us about an evolutionary change.

Pathological individuals are by definition to some extent unique: they do not exhibit the characteristic pattern of development or behavior that for most people is normal. Their phenotype did not have to survive and reproduce in the past -- and in the present it generally fails to do one or the other. They may be helpful in uncovering the course of development -- if a critical stage does not happen, what effects does it have?

An analogy mentioned more than once in this story is the alleged relationship between the FoxP2 gene and the evolution of language. But a clear consideration of this link yields some insight about the difficulties linking pathology and evolutionary changes.

FoxP2 was singled out because of a mutant allele that causes specific language impairment in a well-studied family. The disorder has some other effects, but it targets language fairly narrowly. That argues that the normal form of the gene is necessary for normal language development. Moreover, the gene underwent a recent substitution in humans, with a new allele arising and becoming universal within the last 200,000 years. This change suggests the hypothesis that the rise of language required this substitution.

But as yet, that is only a hypothesis. And is quite a bit of evidence that detracts from it. For one thing, only two amino acid substitutions separate the human FoxP2 gene from the chimpanzee form. If this gene was critical for the appearance of language, that criticality did not involve repeated changes. Considering that language is a complex trait, that means that many other genes are probably also involved. Which are the critical ones? And does FoxP2 count as critical with only two possible changes? And how can we establish that the most recent sweep in FoxP2 was related to language at all? After all, evolutionary changes in other genes could have left language dependent on FoxP2 function, without necessitating any changes in FoxP2 function. So the link between FoxP2 and language is very suggestive, but to say that the gene is necessarily critical in the evolution of language is just not yet demonstrated.

The main point is this: the fact that a gene breaks something doesn't mean that it was the key gene necessary to create something. Suppose that you want to figure out how a car works. So you look at cars that aren't working right, and you see what is broken. Now, you will notice that cars run sort of poorly with flat tires, they run with depleted batteries but won't start, they will run for a bit without motor oil, but then seize up, and so on.

Cerebellar ataxia breaks a whole lot of things. It's like breaking the crankshaft -- the engine might run, but it is going to make a whole lot of noise, and the car isn't going to move. We may conclude that the crankshaft is necessary for the wheels to move. But does that mean that the crankshaft is the key component of the wheel? Clearly not.

The analogy between cars and organismal development is useful because both systems depend on hierarchical functions. Early things must all work right for later things to develop. When an upstream gene (or part) breaks, it doesn't mean that downstream things affected by the broken gene were caused by the broken gene.

But in biological systems these hierarchical functions sometimes involve the same genes -- regulation can make early-acting things also have later effects. So we may not prove out a critical relationship between a gene and a function on the basis of pathology, but we may find it equally difficult to rule it out. Here it is clearly useful to keep in mind the null hypothesis -- the gene isn't the unique, specific, or important cause until proven otherwise.

In this context, it surprises me that nobody has commented on the other prominent connection between Nicholas Humphrey, pathology, and human evolution. One of his best-known essays compares the cave art of France and Spain to the figure drawings of autistic children. The point is that understanding certain pathologies in the present may give hints about the evolution of humans in the past. In the case of autism, the idea is that ancient human minds may not have had a very different ontogeny as language and the ability to use and understand symbols emerged.

The paintings and engravings must surely strike anyone as wondrous. Still, I draw attention here to evidence that suggests that the miracle they represent may not be at all of the kind most people think. Indeed this evidence suggests the very opposite: that the makers of these works of art may actually have had distinctly pre-modern minds, have been little given to symbolic thought, have had no great interest in communication and have been essentially self-taught and untrained. Cave art, so far from being the sign of a new order of mentality, may perhaps better be thought the swan-song of the old.

It is a more sophisticated idea than that autism is an atavistic trait -- a throwback to an earlier stage. It is an analogy -- the cognitive traits that autism affects may not have been present in some earlier human. I don't believe that story either -- although it may be worthwhile to explore what it would imply for certain human traits to be absent, the fact remains that ancient people had to live somehow. They were social primates, and traits underlying their (and our) sociality are deeply embedded in our development. Autism may affect some of those, for genetic or nongenetic reasons, but there is no reason to think that the trait associations found in that condition are relevant to the evolution of an earlier hominid.

With the quadrupedal story, they are on safer ground. But in that case, if the claims are not so bold, neither are the possible conclusions if they are right.

The PR

That some kind of PR machinery started this story is obvious. After all, a single-reference paper in an obscure neurological journal by a respected but obscure Turkish scientist would not ordinarily make international headlines. Nor would a story about a single family carrying a very rare mutation, regardless of what it did (indeed, the genetic paper by the German team in December did not create any press). And the fact that the documentary has clearly been timed to coincide with the release of Tan's paper is pretty blatant.

But this is the interesting part: Even the hoax-like appearance of it is part of the PR. The first news stories could not have broken through internationally without the sensationalist claims -- the "evolution running backward". To be sure, this story has had unusually long legs because newspaper editors are drawn to its freakshow quality (generally showing little apparent regard for its unfortunate subjects). And there is the video showing the people lurching around, which has become a popular e-mail chain mail. It is the same force that makes people tune into the Discovery Health Channel to watch "Trash Can Full of Skin", and "200 Pound Tumor".

Let's face it: World Science (slogan: "Long before it's in the papers") is not the New York Times. It seems like a great place to float a strange story. And after the first story, they followed up several times, first with news about the genetics.

By this time, blogs were on the story, and it might have been expected that I might write about it. I certainly got several e-mails from colleagues passing the story around. Thanks to all those who sent references and pictures!

It was not clear at this point that there was a documentary. That story emerged during the first week of March, with the BBC2 release, the producers at Passionate Productions themselves, and another World Science story about the feud between Tan and the British researchers. This story raised the question of payments to the family associated with the research and documentary, quoting bioethicist Arthur Caplan.

At this point, mainstream science writers picked up the story, notably including The Times online and a skeptical Carl Zimmer. And in what I think may be the first case of active blog-publicity-managing by a key player in an emerging paleoanthropology story, Zimmer retracted his post after a correspondence with Nicholas Humphrey [see update below]. And National Geographic News covered the story with perhaps the most unintentionally hilarious quote I've seen in a long time:

This bizarre case is not a hoax, according to experts who have studied the family.

The cynical part is that the entire progression of the story was predictable without any necessity of orchestrating it. You see, it has all happened before, with almost exactly the same sequence of events. It is the Flores story reborn.

Consider: a rare, unique, and strange discovery comes to light. Immediately many experts are skeptical, but it takes time for them to organize a response. In the interim, a few strong backers explain that unique evolutionary principles can explain the find, and some experts cautiously agree. Then, two things happen: first, there is a conflict between the Western backers of the find and a senior local scientist; second, a high-profile documentary appears in which the discoverers can take their case directly to the public.

It's an autocircus.

Only the documentary itself can save us from this madness. At least that seems to be the import of this quote from one of the producers, on Gene Expression:

Our film will, I hope, redress the balance and be seen as a sensitive, thorough and thought-provoking record of the family/phenomenon. We made it not for voyeuristic reasons (although of course the quadrupedal siblings are visually arresting) but because we thought it raised all kinds of fascinating scientific, and many other, questions. I think the reason it's created such a fuss because bipedality is something that defines us as human beings - separate and distinct from beasts - and their existence is challenging philosophically. You only have to look at the Bible, for instance, to see how the word "upright" is loaded with meaning about purity, morality etc. That's ingrained very deeply in us.

Yeah. Um-hmm. "Upright" is loaded with meaning about purity, morality, etc. Ingrained deeply. Yep.

UPDATE (3/14/2006): Carl Zimmer wrote me about his earlier comments, and wanted to make sure his post and retraction weren't taken out of context by either me or by readers who might not follow the link to his explanation. He writes:

As I explained, I retracted my post specifically because I realized I shouldn't be making speculations about the relationship between the research and the show if I hadn't done the reporting to back them up. In retrospect, I'd now says that if the post had been limited to my skepticism about their scientific claims, it would be still up now. But I screwed up, and I took responsibility for it. I don't appreciate your implication that I was being managed, which is not supported by the fact.

Definitely nothing sinister implied on my part, just that it is novel that somebody is watching blogs!

References:

Tan U. 2006. A new syndrome with quadrupedal gait, primitive speech, and severe mental retardation as a live model for human evolution. Int J Neurosci 116:361-369. DOI link

Turkmen S, Demirhan O, Hoffmann K, Diers A, Zimmer C, Sperling K, Mundlos S. 2005. Cerebellar hypoplasia and quadrupedal locomotion in humans as a recessive trait mapping to chromosome 17p. J Med Genet DOI link

This show on the Discovery Channel (3/4/06) seems like it should have some promise -- at least compared to the National Geographic Explorer version from last year.

And it starts this way: "Will the hobbit be one of science's greatest discoveries or a monumental mistake?

7:01: OMG! IT'S THE BBC HORIZON PROGRAM!

7:05: Peter Brown featured to good effect.

7:07: "Homo australopithecus"... not a good sign... The 3-d phylogeny and map are kind of cool, though. It's like human evolution on a Risk board.

7:08: Tools and pygmy stegodon brought in with the archaeologists Morwood and Roberts.

7:11: All of them converging on how significant the discovery is..."and then the bones disappeared!" Cut to commercial. That's kind of cool.

7:17: Jim Phillips explaining why the tools are those of modern humans. Good explanation, well laid out, including scale pictures of the LB tools along with very near matches from his own collection.

7:20: Bob Martin explaining brain-body allometry. Predicts brain size of 750 ml for hobbit-sized body. But LB1 brain is only 400, consistent with stature "size of a meerkat".

7:23: Voiceover explains that "islands hold a special place in scientific law". "Islands enable evolution to trick the laws of scaling to enable brains to shrink as much as bodies." Gee, I'd like to trick the laws of scaling, too!

7:24: "The idea is fallacious" -- Jim Phillips. "It's not something magical about islands, it's something magical about Flores, and that doesn't strike me as science at all." -- Bob Martin.

7:29: Ann MacLarnon looking through the Royal College of Surgeons collection. She's found a tiny three-year-old microcephalic skeleton. Not a great comparison -- they're talking about the twin-rooted premolars, but they're showing deciduous molars. Another sectioned adult microcephalic has the brain size of LB1. "A huge breakthrough for the hobbit skeptics"

7:33: On to Ralph Holloway, explaining endocasts. "What I am seeing in the microcephalic is very different from what I'm seeing with the hobbit's brain."

7:34: Martin brings the Royal College specimen's endocast, and they do a CT scan. Holloway: "I'm really quite struck by the lack of pathology I'm seeing on the microcephalic endocast." So this is a microcephalic that doesn't show obvious endocast deformation.

7:40: The village of Rampasasa..."Could these people be the hobbit's living descendants?" Now, this is some old school anthropometry. The village elder arrives to be measured, "He's rumored to be over a hundred years old." "At 4 feet 4 inches, he is especially short. So short, he qualifies as a unique kind of human -- a pygmy."

7:44: "The case against the hobbit as a new species seems conclusive." "The anthropological sensation of the century was in danger of becoming an embarrassment."

7:45: "The hobbit team is back. Their mission: to find fresh evidence to rescue their theory of a new hominid." "They're operating on a tip from a local. Since they were here, Liang Bua cave ... has been closed. They need to find a new cave to dig."

7:47: Meanwhile, Peter Brown has LB2 back and is X-raying...cut to commercial. I have to say, this is really a well-put-together program, in terms of making the different interpretation clear and keeping the energy up.

7:54: A second lower jaw, identical to the first. Roberts: "Like some kind of leper colony .... The probability's got to be vanishingly small."

7:56: "The case of the hobbit's true identity is closed...or is it?" And now, as MC Hammer would say, we break it down -- the brain and tools are still modern human, the "population" of small bones is incredible as some kind of pathology.

7:58: They've got another rock shelter to dig. They will return to start excavations. "It will take years, or even decades, before the picture is clear and the hobbit's identity is established."

Yippee.

All in all, this is a much better effort than the National Geographic versions. I like it.

In Nature a couple of weeks ago, Robin Dennell and Wil Roebroeks had a provocative paper exploring the possibility that early humans (i.e. Homo erectus) originated in Asia rather than Africa.

The paper is all speculation of course; there is no evidence of any earlier hominid in Asia.

But it is the good kind of speculation. Although maybe not quite this big:

Most probably, we are on the threshold of a profound transformation of our understanding of early hominin evolution that might prove as far-reaching as the demise of the notion of Man the Hunter in the early 1960s (Dennell and Roebroeks 2005:1103).

Here's the abstract:

The past decade has seen the Pliocene and Pleistocene fossil hominin record enriched by the addition of at least ten new taxa, including the Early Pleistocene, small-brained hominins from Dmanisi, Georgia, and the diminutive Late Pleistocene Homo floresiensis from Flores, Indonesia. At the same time, Asia's earliest hominin presence has been extended up to 1.8 Myr ago, hundreds of thousands of years earlier than previously envisaged. Nevertheless, the preferred explanation for the first appearance of hominins outside Africa has remained virtually unchanged. We show here that it is time to develop alternatives to one of palaeoanthropology's most basic paradigms: 'Out of Africa 1' (Dennell and Roebroeks 2005:1099).

It is worth reviewing exactly what "Out of Africa 1" is supposed to be. The paradigm is that emergence of hominids from Africa required increases in brain size and/or body size, coincident with the emergence of hominids like KNM-ER 3733, KNM-WT 15000, and others. The motivation for this hypothesis is simple: australopithecines have not been found outside of Africa. Nor has anything like Homo habilis, which is australopithecine-sized but has larger brains.

Of course, it is questionable just how basic this paradigm is. Consider what I (and my colleagues) were able to write only seven years ago:

The problem is that significant range expansion out of Africa occurred a half million years or more later than the first H. sapiens [corresponding to others' H. erectus or H. ergaster]. Population size before then may have remained small, and this is not an inconsequential time span, being one quarter of the time H. sapiens has existed. An important date in behavioral evolution is 1.5 MYA because it is marked by the earliest appearance of the Acheulean, the ubiquitous hand-axe industry of the Early and Middle Pleistocene.... Before this time, humanity was limited to Africa and immediately adjacent sections of Asia such as the Levant (Hawks et al. 2000:7).

Evidence for large body size in Late Pliocene humans (notably KNM-WT 15000 but also many others) made it very plausible that larger bodies were necessary for dispersal from Africa. But without good evidence for such dispersal before around 1.4 million years ago (and arguably not before 1 million years), larger bodies could not be assumed to be a sufficient condition for dispersal. Writing about the origin of humans, we had to consider all these alternatives -- at a time when the Dmanisi sample consisted of a single uncertainly dated mandible and the Mojokerto date stood alone with very questionable provenience.

Now we know that hominids did leave Africa by at least 1.8 million years ago. Dmanisi has almost singlehandedly changed the perspective.

And in doing so, it made much more convenient the hypothesis that large body size was both necessary and sufficient for dispersal from Africa. If the date of dispersal and the date of human origins are the same, then it is natural to propose that the coincidence is more than chance.

I would say this is more of a convenient hypothesis (and an easy story to tell) than it is a basic paradigm. The idea that large body size caused dispersal from Africa may have been a local minimum in terms of parsimony (at least as long as the body size of the Dmanisi fossils was not known), but it was only one alternative among many still in play.

And it remains a plausible hypothesis -- after all, the Dmanisi remains are a bit larger than australopithecines, and they might well have shrunk from a larger early-human-like size after reaching Asia instead of before.

But Dennell and Roebroeks give motivations for examining some alternatives.

The only reason why the earliest tool assemblages in Asia are attributed to H. erectus s.l. is that palaeoanthropologists have already decided that, in effect, it was the only hominin capable of migration out of Africa, and with sufficient Wanderlust to do so (Dennella and Roebroeks 2005:1099).

Homo erectus sensu lato (s.l.) means Homo erectus "in the loose sense", which would include not only the "strict sense" (sensu stricto) H. erectus. from Java and China, but also hominids like OH 9 and KNM-ER 3733 from Africa, and presumably the Dmanisi hominids.

A long passage reviews the total faunal evidence from Asia during the Late Pliocene. The thrust of the passage is that there are very few sites with extensive fauna, and of these most preserve mainly large-bodied herbivores. There are a few hints that a hominid-friendly fauna may have existed, including the presence of baboons. But there are no hominids of any kind at the vast majority of Asian localities -- Dmanisi is a real exception in the Plio-Pleistocene record.

This is the key taphonomic argument: if we have only found Early Pleistocene humans from continental Asia within the past ten years, then how can we preclude there having been australopithecines there? Dennell and Roebroeks argue that if there were australopithecines, we shouldn't necessarily expect to have found them yet -- we just haven't looked extensively enough.

A close read of the section raises a caution, though. One of the main arguments for the incompleteness of the Asian record is that sites don't preserve each others' fauna.

It is also likely that the full range of taxa is incomplete for the Indian subcontinent, because Megantereon and Pachycrocuta are not recorded in India but are present in Pakistan; in Pakistan, there is no evidence of Camelus and small primates, and in neither country is Homotherium recorded, although this is present to the west at Dmanisi, to the north at Kuruksay, central Asia and to the east at Longuppo, south China (Dennell and Roebroeks 2005:1100).

Of course, all of these species are recorded in Asia taking all the sites in aggregate; this is hardly an argument for the overall weakness of the record -- just an argument that no individual site is an adequate record of the continent's fauna.

To me, the important question is not whether australopithecines as currently known from Africa were in Asia. A more troubling possibility is that the australopithecines that we now know from Africa were not the only (or main) manifestations of early hominids in Africa. Large parts of Africa that we might expect to be congenial to hominids, like the Zambesi basin, have few or no fossils at all. The recovery of the Bahr el Ghazal mandible (Brunet et al. 1994) certainly makes clear that hominids were living across a much larger area than we have adequately sampled. But that mandible is, although not identical, certainly very similar to known contemporary hominids in its adaptation.

The question is whether hominids had adapted to other ecologies that are much less satisfactorily sampled than the East African rift. They probably weren't living where chimpanzee and gorilla ancestors did, but where else might they have been? Some such ecologies -- like the coasts -- would make early dispersal very plausible.

(In this regard, early humans are not the only hominids who lack a satisfactory ancestor. Who was the ancestor of A. aethiopicus? In what ecology did the first robust hominid arise?)

So what is the broader set of hypotheses that we should consider? Dennell and Roebroeks suggest:

If the above taphonomic review suggests that we cannot show the absence of hominins from areas in Asia at a time before the little evidence we have indicates their presence, we need to consider alternatives to the current Out of Africa [that is, their "Out of Africa 1"] model. There are three issues here. The first is when hominin(s) first left Africa -- might they, for example, have left shortly after they acquired the ability to make stone tools, the earliest of which are currently 2.6 Myr old? Or could they have left even earlier, about 3.0Ð3.5 Myr ago, when some australopithecines were already living in the African grasslands? The second issue is whether we yet know the full range of hominins that inhabited both Africa and Asia in the Late Pliocene and Early Pleistocene. Even in east Africa, several new taxa have been claimed in the past decade (for example, A. anamensis, A. garhi, Ardipithecus ramidus and Kenyanthropus platyops) and doubtless more will be found. (An indication of how little we know about Pleistocene east Africa is that only recently has the first fossil evidence for chimpanzee been found.) In Asia, the recent discoveries of H. georgicus and H. floresiensis should make us very wary of assuming that H. erectus s.l. was the only player on the Asian stage in the Early Pleistocene. Third, Asia might not have been the passive recipient of whatever migrated out of Africa but might have been a major donor to speciation events, as well as dispersals back into Africa. Such two-way traffic is well documented for other mammals in the Pliocene and Early Pleistocene, such as Equus and bovids, with more taxa migrating into than out of Africa. There is no reason why hominin migrations were always from Africa into Asia, and movements in the opposite direction might also have occurred, as has been suggested for the Olduvai OH9 (refs 13, 58) and Daka specimens. We should even allow for the possibility that H. ergaster originated in Asia and perhaps explain its lack of an obvious east African ancestry as the result of immigration rather than a short (and undocumented) process of anagenetic (in situ) evolution (Dennell and Roebroeks 2005:1100-1101).

Of course, most of the evidence indicating the presence of hominids is not fossil but archaeological. On this topic, Dennell and Roebroeks have much to say:

Any stone tool assemblage in Asia dated as older than 1.9 Myr ago (the earliest date that Homo is supposed to have left Africa) is either dismissed or (more usually) ignored; undated Oldowan tools are assumed to date from after 1.9 Myr ago and not from 2.6 Myr ago (the date of their first appearance in east Africa); and stone tool assemblages in Asia dated to the Olduvai Event (1.77Ð1.95 Myr ago) and not associated with hominin remains are automatically attributed to Homo erectus s.l. However, there is no reason why Oldowan assemblages in Arabia cannot be older than 1.9 Myr old, or why the tools from Ain Hanech (Algeria) or Erq el Ahmar (Israel) were made by H. erectus s.l. [instead of other hominids] (ibid:1102, references omitted).

There is a section about what exactly absence of evidence can tell, a short critique of using continents as proxies for biogeographic units:

As noted earlier, Pliocene grasslands extended all the way from west Africa to north China, and 'Savannahstan' might prove a more useful spatial unit for modelling early hominin adaptations and dispersals within them than simply an undifferentiated 'Africa' or 'Asia'. For example, the African hominins 1.9Ð1.7 Myr ago at Koobi Fora (Kenya) and Ain Hanech (Algeria), and their slightly later counterparts in Asia at 'Ubeidiya (Israel), and Majuangou (north China) were all living in broadly comparable grassland environments, and it makes sense to place them within the same frame of reference.

I think there is much of value to consider here; but it is less a revolution and more a statement of the field in transition. There are also alternatives that are not considered in this paper but that may be equally plausible -- most notably, the idea that early humans themselves may have been substantially polymorphic (witness KNM-ER 42700), or that brain size rather than body size may have been a prerequisite to dispersal (since habilines, Dmanisi, and H. erectus s.l. are all allometrically similar in brain size).

National Geographic News also has an article about the paper.

References:

Dennell R, Roebroeks W. 2005. An Asian perspective on early human dispersal from Africa. Nature 438:1099-1104. Full text (subscription)

Hawks J, Hunley K, Lee S-H, Wolpoff M. 2000. Population bottlenecks and Pleistocene human evolution. Mol Biol Evol 17:2-22.