It came from Guangxi

Science journalist Richard Stone writes in the current Science about new Late Pleistocene skeletal remains from Guangxi: “Signs of Early Homo sapiens in China?”

The big prize is the Homo mandible, whose owner would have had a chin that curved ever-so-slightly outward. H. erectus had an inward-sloping chin, whereas modern human chins generally jut out farther than the Guangxi specimen's. Jin's group classifies the fossil as primitive H. sapiens and says the intermediate chin suggests interbreeding with H. erectus. Uranium isotope dating by R. Lawrence Edwards of the University of Minnesota, Twin Cities, indicates that the fossil-bearing layer is about 110,000 years old, in a paper that will appear in the November issue of Chinese Science Bulletin.

The article frames the discovery as a challenge to the “Out of Africa” hypothesis of modern human origins, thereby giving the “out of Africa” defenders several chances to rebut. That framing comes from the discoverers, who are pushing the “early modern” aspects of the jaw.

I don’t think the modern/nonmodern classification is very productive. Here’s a mandible with a chin – a small chin, but apparently a real mental trigone (the article is accompanied by a photo, not the greatest). It’s no less “modern”-looking than most of the 100,000-year-old Klasies River Mouth mandibles (thanks to a reader for noting that one). Some Neandertals had chins, and some much earlier humans came pretty darned close. A chin is not a diagnosis, it’s a symptom.

Here’s a better question: what explains the epidemic? The same anatomical feature, showing up in widespread geographic locations within the past 100,000 years? If these populations were isolated with no gene flow between them, the chin must have appeared coincidentally by convergent evolution. The other alternative is that these ancient human populations traded some genes.

How unlikely is the chin, really? Is it a developmental side effect of a single genetic change? Would that make it more likely, because no combination of factors is required? Or less likely, because a single mutation causing such a strange effect would be very improbable to begin with?

References:

Stone R. 2009. Signs of Early Homo sapiens in China? Science 326:655. doi:10.1126/science.326_655a</p>