Planet Earth

'dem bones tell strange tales

Gene ExpressionBy Razib KhanOct 26, 2010 5:16 AM


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There is a new paper in PNAS on remains from China which re-order and muddle our understanding of the emergence of anatomical and behavioral modernity in Eurasia. Human remains from Zhirendong, South China, and modern human emergence in East Asia:

The 2007 discovery of fragmentary human remains (two molars and an anterior mandible) at Zhirendong (Zhiren Cave) in South China provides insight in the processes involved in the establishment of modern humans in eastern Eurasia. The human remains are securely dated by U-series on overlying flowstones and a rich associated faunal sample to the initial Late Pleistocene, >100 kya. As such, they are the oldest modern human fossils in East Asia and predate by >60,000 y the oldest previously known modern human remains in the region. The Zhiren 3 mandible in particular presents derived modern human anterior symphyseal morphology, with a projecting tuber symphyseos, distinct mental fossae, modest lateral tubercles, and a vertical symphysis; it is separate from any known late archaic human mandible. However, it also exhibits a lingual symphyseal morphology and corpus robustness that place it close to later Pleistocene archaic humans. The age and morphology of the Zhiren Cave human remains support a modern human emergence scenario for East Asia involving dispersal with assimilation or populational continuity with gene flow. It also places the Late Pleistocene Asian emergence of modern humans in a pre-Upper Paleolithic context and raises issues concerning the long-term Late Pleistocene coexistence of late archaic and early modern humans across Eurasia.

I read the paper, and I really didn't understand anything between the introduction and discussion. Mostly because it was a detailed exploration of anatomical details, and I've never taken an anatomy class. I basically rely on people like John Hawks to tell me what's going on in that domain. He hasn't blogged the paper (well, as of this writing), but he did give an assessment to National Geographic:

Still, the jaw and three molars were the only human remains retrieved from the Chinese cave, and the jaw is "within the range" of Neanderthal chins as well as those of modern humans, added paleoanthropologist John Hawks of the University of Wisconsin, Madison. "If this holds up, we have to reevaluate" the human migration time line, he said. "Basically, I think they're right, [but] I want to see more evidence," Hawks added. "I really, really hope that there can be some sort of genetic extraction from this [fossil]."

The issue of why this is relevant is covered well in the early portion of the paper:

...In eastern Eurasia, the dearth of diagnostic and well-dated fossil remains...has inhibited more than general statements for that region. Fully modern human morphology was established close to the Pacific rim by ∼40 kya, as is indicated by the fossils from Niah Cave in Sarawak...and especially Tianyuan Cave in northern China...The actual time of the transition has remained elusive, because the age of the latest known archaic humans in the region is substantially earlier...The eastern Eurasian age of the transition has been generally assumed to approximate that of western Eurasia (∼50–40 kya), although there have been claims supporting earlier dates for modern human presence in East Asia.... This scenario implies a long term (>100,000 y) restriction of early modern humans to portions of Africa with a brief ∼90 kya expansion into extreme southwestern Asia, followed by a relatively rapid expansion throughout Eurasia after ∼50 kya...The scenario also implies some form of adaptive threshold, roughly contemporaneous with the emergence of the Upper Paleolithic (sensu lato), and a marked behavioral difference between those expanding modern human populations and regional populations of late archaic humans (14). It is in this context that three fragmentary human remains were discovered in 2007 at Zhirendong, South China...Because it is only well-dated diagnostic human remains that can document the timing and nature of human evolution and dispersal patterns (as opposed to archeological proxies for human biology or imprecise inferences from extant genetic diversity), the Zhirendong remains have the potential to shed light on these ongoing paleoanthropological issues.

OK, so the stylized orthodox model would be that anatomically modern humans arose in Africa ~200,000 years ago, and expanded out of Africa ~100-50,000 years ago. Full behavioral modernity emerged a bit later. In the broad outlines I think we can still go with this, but we have reached a level of fine-grained understanding of the evolutionary history of the human past that we need to consider adding detail to the margins of the story. The Denisova hominin, and perhaps H. floresiensis, are a good clue that the human family tree really was bushy, and that the past was filled with players unknown to us. The likelihood of Neandertal admixture also suggest that the other branches of the bush can't be ignored and considered as without consequence for modern humans. There may be traces of other lineages in other human populations as well; there have long been claims based on inferences from some genetic data of modern populations, but the ability to compare to the Neandertal sequence gave those results from last spring particular credibility. But if the Neandertal admixture results become part of the consensus we should recalculate our probabilities of the other inferences.

So how does it change things? One of the authors of the PNAS paper, Erik Trinkaus, has long made claims of hybridization from the fossil record, and this work falls in line with that tradition. The key here is the fossils seem to exhibit derived features, not ancestral ones. Derived features imply common ancestry of the populations which share the derived traits. If so, Trinkaus and company seem to be pointing to Alan Templeton's "Out of Africa again and again." On the other hand, I can't but help think of Luke Jostins' plot of hominin cranial capacities as a function of time: separate lineages all seemed to be going on the same general path in terms of direction. I don't make the claim here that H. sapiens sapiens was inevitable, but perhaps a common suite of traits which we associate with advanced hominin lineages, in particular the branch of which we are the terminus, were being selected for across the whole clade. In other words, perhaps anatomical modernity exhibits some element of convergent evolution, while behavioral modernity is the true hallmark of H. sapiens sapiens. It sounds crazy, and I don't really believe it necessarily, but we live in crazy times. We had a neat and tidy story for 20 years between 1985 and 2005, but all good things have to end.

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