Zhirendong puts the chin in China

A 100,000-year-old modern human from China? That's the claim made by Liu and colleagues Liu:Zhirendong:2010, who report on a mandible and isolated teeth from Zhirendong, in South China. The remains lie under a thin flowstone with a uranium-series date of 106,000 years ago. The mandible was reported in the science press last fall; I wrote about it at the time.

The most notable aspect of the mandible is its chin:

The Zhiren Cave human remains, securely dated to at least 100 kya (early MIS 5), therefore represent the oldest evidence of derived modern human morphology in East Asia. The Zhiren 3 mandible in particular presents an anterior symphyseal morphology and orientation which aligns it with other early modern humans and distinct from all Middle and Late Pleistocene archaic humans. It is nonetheless a robust mandible, with a lingual symphyseal contour, symphyseal cross-section, and lateral corporeal breadth that distinguish it from most (but not all) Late Pleistocene early modern humans.

The authors accept the morphology as evidence for an African influence on the population of China at this time. They focus on the hypothesis that substantial gene flow from Africa had begun before 100,000 years ago into the South Asian and ultimately East Asian populations, because they argue that the mandible combines a mixture of more robust or archaic morphology with the modern human chin.

So, are they right?

Underlying the paper is the assumption that a chin is the work of African genes. That assumption is questionable in the Late Pleistocene record.

For one thing, few Africans as old as 100,000 years have a chin. The Klasies River Mouth sample, dating to over 90,000 years ago, includes several mandibles with the central portion preserved. These range in morphology from an extreme with no substantial symphyseal development to one with a quite prominent chin (probably exaggerated by alveolar resorption).

In West Asia, the situation is simpler because all the specimens with chins are asserted to be modern humans. Where the same site (e.g., Tabun) has a mandible with a chin and one without, the chin is assumed to be a modern human.

European Neandertals usually didn't have a chin. But a few of the latest Neandertals actually did have one -- like Vindija 206 and Saint-C├ęsaire.

In this paper, those Neandertal mandibles are included within the "Late Pleistocene archaic humans" sample. The presence of a few chinned Neandertals is enough to place the symphyseal profile of Zhiren 3 within the range of those European Neandertals. Zhiren 3 stands apart from the earlier, Middle Pleistocene, sample but not from the Neandertals in its symphyseal morphology.

Is it then an archaic human? If we ask which sample the Zhiren 3 mandible is most like, it is closest to the mean of the African and Skhul-Qafzeh samples. If a chin is your definition of a modern human, this is one. The issue is with the definition and the assumption that these samples cannot overlap. They do overlap in their morphology.

My opinion: Late Pleistocene populations shared developmental trajectories that differed by relatively few changes. The African and West Asian populations were closest to having chins as an expected outcome -- but Neandertals were not far removed from it, and evolved closer over time. The chin is not highly distinctive as a unique evolutionary outcome; it is a threshold that we notice at one point in a continuous range of variation.

Until we know how genes build the mandible, we won't know how much gene flow is implied by this shared feature. Do some later Neandertals have a chin because of gene flow from other populations, because their face reduced in overall size relative to the vault, or both? Did Zhiren 3 have genes recently derived from Africa, or is it at one end of a range of variation with a long East Asian heritage?

If we had many more specimens, we might not address these questions any more effectively. But we could compare other characters, either confirming the pattern or rejecting it.

As it stands, the Late Pleistocene record in China is sparse. Dennis Etler provides a nice list of the Chinese fossils, the relevant time period is occupied by samples from Xujiayou, Ordos, and a few other sites with even fewer bone fragments. None of the remains are strongly diagnostic about their phylogenetic position. The Liujiang skull -- a modern human by any definition -- has a U-series date of 68,000 years ago, but some uncertainty about whether the specimen is really that old.

With many bone fragments around China, I expect there is a good prospect for DNA recovery from somewhere. Zhirendong's combination of age and latitude may put it beyond the extreme limit for DNA preservation, but maybe we can hope.