NEW YORK – Combined ancient DNA analysis and geological testing indicates early modern humans arrived in southern China about 45,000 to 50,000 years ago, a new study has found.
Homo sapiens evolved in Africa by about 315,000 years ago, and anatomically modern humans spread out of Africa about 45,000 to 65,000 years ago and arrived in China thereafter, according to recent molecular data. But some paleoanthropologists have instead suggested based on analyses of fossils found in caves that anatomically modern humans may have been in southern China earlier, by about 120,000 years ago.
To address this discrepancy, a Fudan University-led team of researchers estimated the ages of anatomically modern human (AMH) fossils from five caves in southern China with a combination of ancient DNA analysis and a multi-method geological dating strategy. The findings, published Monday in the Proceedings of the National Academy of Sciences, highlighted discrepancies between the sample ages determined originally through uranium-thorium dating and the ages the researchers' new analysis uncovered: they reported that the samples from these regions were much younger than previously suspected, suggesting anatomically modern humans likely appeared later in China.
"In light of our findings, we conclude that claims for an early arrival of AMHs in southern China as seemingly documented at Huanglong, Luna, and Fuyan caves cannot be substantiated at present," senior author Hui Li, a professor at Fudan, and his colleagues wrote in their paper.Â
They collected samples from five caves in southern China where anatomically modern humans were thought to have been early arrivals. They extracted DNA from human remains from two caves, Yangjiapo Cave and Fuyan Cave. Samples from Fuyan were indirectly dated previously to be between 80,000 and 120,000 years old based on U-Th dating of flowstone, deposits of calcite or other minerals that are made in caves. This earlier finding also was extrapolated to Yangjiapo Cave based on the type of fossils found there.
The researchers sequenced ancient DNA samples from eight teeth from Yangjiapo Cave and two from Fuyan Cave. By analyzing these and other modern and ancient human samples as well as Neanderthal and Denisovan samples, the researchers found the samples' maximum age was less than 15,600 years old, dating to the Holocene Epoch and not earlier.
At the same time, the researchers analyzed sediment, charcoal, and other fossil samples from all five caves using a combination of optically stimulated luminescence, U-Th, and carbon dating.Â
The approaches, though, sometimes provided differing age estimates. For Huanglong Cave, OSL dating of six sediment samples that harbored human fossils indicated they were more than 215,000 years old and previously published U-Th dating suggested they were between 81,000 and 103,000 years old — indicating a Pleistocene Epoch origin — but AMS 14C dating of the teeth suggested younger ages, ranging between 8,450 years and 26,700 years old.
Meanwhile, samples from Fuyan Cave from the layer above where the teeth were uncovered were estimated to be between 95,000 and 168,000 years ago, using U-Th dating. The layer containing the teeth, meanwhile, dated to between more than 302,000 years old to about 200,000 years ago using OSL. However, AMS 14C dating of the teeth themselves — including one that also underwent ancient DNA analysis — dated them to between 9,160 and 13,590 years old. The tooth FY-HT-1, they noted, was dated twice to similar times.
The findings, the researchers noted, indicate that initial U-Th dating of cave flowstone may have incorrectly estimated old ages for anatomically modern human fossils found there, possibly due to complex depositional histories of those flowstones.
This, they added, underscores the issues that may arise when relying on a single approach to gauge timeframes in human evolution. "Moving forward, there is an urgent need for researchers working in the region to routinely adopt a multimethod strategy, including the targeting of human remains for direct AMS 14C dating and aDNA analysis," the researchers said.