Genetic data suggest that ancestral East Asians diverged from ancestral Europeans long after the African/non-African split (source). This timeline, however, seems to be challenged by archaic DNA that is reputed to be 40,000 years old.
When did the ancestors of Europeans and East Asians part company? In my opinion, the divergence must have happened long after the time (c. 50,000 BP) when modern humans began to spread out of Africa. It probably occurred near the onset of the last ice age (25,000 – 10,000 BP), when advancing ice sheets and glacial lakes restricted gene flow between the western and eastern ends of Eurasia (Rogers, 1986).
This timeline is supported by several pieces of evidence:
1. Human skin began to lighten some 30,000 years ago in a population that was ancestral to both Europeans and East Asians (Beleza et al., 2012). A second phase of skin lightening, which affected only Europeans, occurred between 19,000 and 13,000 years ago. Proto-Eurasians must have therefore begun to diverge into two groups somewhere between 30,000 BP and 19,000 BP.
2. A Y-chromosome study suggests that all North Eurasian peoples descend from a common ancestral population dated to about 15,000 BP (Stepanov & Puzyrev, 2000; see also Armour et al., 1996; Santos et al., 1999; Zerjal et al., 1997).
3. The language families of northern Eurasia, particularly Uralic and Yukaghir and more generally Uralic-Yukaghir, Eskimo-Aleut, Chukotko-Kamchatkan and Altaic, share deep structural affinities that point to a common origin and not simply to word borrowing (Cavalli-Sforza, 1994, pp. 97-99; Fortescue, 1998; Rogers, 1986).
4. Archeological evidence (characteristic lithic technology, grave goods with red ocher, and sites with small shallow basins) shows the presence of a common cultural tradition throughout Europe and Siberia 20,000 to 15,000 years ago (Goebel, 1999; Haynes, 1980; Haynes, 1982).
5. Dental and cranial remains from Mal’ta (23,000-20,000 BP) in southern Siberia indicate strong affinities with Upper Paleolithic Europeans (Alexeyev & Gokhman, 1994; Goebel, 1999).
Back to the drawing board?
Nonetheless, this timeline now seems disproved by a recent study of archaic DNA:
We have extracted DNA from a 40,000-y-old anatomically modern human from Tianyuan Cave outside Beijing, China. […] The nuclear DNA sequences determined from this early modern human reveal that the Tianyuan individual derived from a population that was ancestral to many present-day Asians and Native Americans but postdated the divergence of Asians from Europeans. (Fu et al., 2013)
So ancestral Europeans and East Asians had already begun to diverge from each other by 40,000 BP. Considering that modern humans entered the Middle East around 46-47,000 BP, the time of divergence must have been close to the initial split between Africans and non-Africans (Schwarcz et al., 1979). Yet the genetic data argue otherwise.
When a new finding seems inconsistent with other data, one should take a second look. Do those human remains from Tianyuan Cave really date back to 40,000 years ago? Actually, they were initially dated to 25,000 BP, by means of uranium series dating of deer teeth from the same cave layer (Tong et al., 2004). Because this dating method is considered problematic when applied to organic remains, radiocarbon dating was later used to get a firmer date, which turned out to be 39,000 – 42,000 BP (Shang et al., 2007).
The two dating methods differed by 15,000 years. That’s a big discrepancy, and it may be why Shang et al. (2007) repeated their radiocarbon dating on several organic remains from the same layer. Such an approach, however, doesn’t rule out the possibility of a shared source of error, either in the remains themselves or in the testing laboratory.
There are two other reasons for doubting the estimate of 40,000 BP:
Associated faunal remains
The modern human remains from layer III were associated with the remains of other fauna. In general, the faunal assemblage indicates a significantly colder climate than the one that now prevails around Beijing. On the one hand, layer III had remains of the Siberian musk deer (Moschus moschiferus), which now lives farther north in the taiga of southern Siberia and northern Manchuria. On the other hand, layer III had no remains of warm climate species, i.e., the rhesus macaque (Macaca mulatta) and the masked palm civet (Paguma larvata), even though these species were present in the uppermost Holocene layer (Shang et al., 2007). The faunal evidence is thus consistent with the colder climate that existed when the last ice age began 25,000 years ago. It is not consistent with the warmer climate that prevailed 40,000 years ago in southern Siberia and northern China during the Malokheta Interstade of the Karga Interglacial (33,000 - 43,000 BP). At that time, average annual temperatures were as much as 2-3° C warmer than they are today (Goebel, 2004).
An outlier among finds of early East Asians
With a dating of 40,000 BP, these remains are much older than all other known finds that might be ancestral to present-day East Asians. The oldest rivals to Tianyuan Cave are Yamashita-cho, Okinawa (≈32,000 BP), Zhoukoudian Upper Cave, China (24,000 – 29,000 BP), Pinza-Abu, Okinawa ( ≈26,000 BP), and Minatogawa, Okinawa (≈18,000 BP) (Shang et al., 2007).
Conclusion
Archaic DNA promises to revolutionize our understanding of human origins. Unfortunately, it may also confer an aura of false certainty on new findings, thereby discouraging the healthy skepticism that makes good science possible. The Tianyuan Cave remains are undoubtedly those of an early East Asian and thus promise to shed much light on the beginnings of this branch of humanity. There are, however, reasons for doubting the date of 40,000 BP, and such doubts will probably become more insistent as we retrieve archaic DNA from other East Asian remains.
References
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