Australia: The Land Where Time Began

A biography of the Australian continent 

Denisovans - Who They Might have Been

Wolpoff & Hee propose that the problem of identifying the Denisovan anatomy could be addressed indirectly. The highest frequencies of Denisovan nDNA are found in Australian Aboriginal people among populations of the present (Cooper & Stringer, 2013). The Australian continent appears to have been only sparsely populated (30) for a good proportion of its prehistory, and significant expansions to populations\ levels of the Late Holocene began less than 10,000 years ago (Williams, 2013). The key importance of low population sizes in the Pleistocene for most of Australian prehistory is in the potential of even a low magnitude of gene flow on the current nDNA variation (31). The significant population size expansions that occurred in Australia in the Late Holocene are similar to the histories of populations in most regions of the world (Cochran & Harpending, 2009; Hawks et al., 2007; Wolpoff & Caspari, 2013). This is 1 of the reasons that Wolpoff & Hee expect that many of the genes that entered Australia were adaptive genes that spread rapidly under selection through populations of Australian Aboriginal people, however low the magnitude of the gene flow in Australia from the rest of the world may have been (32).

Wolpoff & Lee suggest that from Siberia of the past to Australia in the present, it can be inferred that Denisovan haplotypes are quite widespread in the Late Pleistocene:

“Denisovan genetic material [is] present in eastern Southeast Asians and Oceanians (Mamanwa, Australian Aboriginals, and New Guineans) … Our evidence for a Southeast Asian location for the Denisovan admixture this suggests that Denisovans were spread across a wider ecological and geographic region – from the deciduous forests to the tropics – than any other hominin with the exception of modern humans.” (Reich et al., 2011: 23; italics by Wolpoff & Lee)

During the earlier period when populations were low, in one of the earlier dispersals of the Late Pleistocene, some of the ancient nuclear haplotypes that are known across Asia must have reached the Australian continent in high enough numbers to remain at the present in spite of later dispersals (Reyes-Centeno et al., 2014). It is posited by Wolpoff & Lee that haplotypes which remain in Australia from the Late Pleistocene include Denisovans, now at 6 %. But there are also Neanderthal haplotypes that are present across East Asia.

Across Asia the frequencies of Neanderthal genes do not differ much from those across Europe. According to Wolpoff & Lee the source of the Neanderthal haplotypes that are present in Australia, must surely be East Asians. There are also introgressions from Denisovans in mainland East Asia (Huerta-Sanchez et al., 2014), though overall, there is only about 0.2 % of Denisovan contribution to populations from mainland Asia and the Americas (Prüfer et al., 2014). Even at the Tianyuan site, dating to about 40 ka, near Zhoukoudian in China (Shang et al., 2007) , nDNA that has been recovered from skeletal material no discernible Denisovan DNA has been found (Fu et al., 2013). The contribution to nDNA of populations from Australia and New Guinea from Denisovan nDNA is about 25 times greater than for the contributions to mainland Asia and the Americas mentioned above (Prüfer et al., 2014). It has been demonstrated (Meyer et al., 2012) that the highest frequencies of Denisovan nDNA that has been found in living populations are present in indigenous Australians, about 6 %) and similar and slightly less in New Guineans. More broadly, significant amounts of Denisovan nDNA are found only east of Wallace’s Line (Cooper & Stringer, 2013). A mix of Denisovan and Neanderthal haplotypes was found by sequencing of the nDNA of a 100-year-old indigenous Australian (Rasmussen et al., 2011). There was much more Denisovan admixture in this Australian Aboriginal than in continental Asians, though admixture with Neanderthals was about the same proportions as Neanderthal admixture that has been found in Asian sequences.

According to Cooper & Stringer (2013: 322) the apparent lack of Denisovan introgression in mainland populations of the present is most easily explained by overwriting by DNA of incoming East Asian populations in areas other than Island Southeast Asia. Wolpoff & Lee’s expectation is for Denisovan introgression in indigenous Australians and New Guineans to represent descent from earlier populations in Australia that had a significant Denisovan presence. It has been concluded by Rasmussen et al. that their findings support the hypothesis that Australian Aboriginal people of the present descended from the earliest humans who occupied Australia (33), possibly representing one of the oldest continuous populations outside Africa (Rasmussen et al., 2011: 94). The Australian Aboriginal people of the present descend from these earlier Aboriginal people, as well as from a number of other Asian populations that entered the region, as described above.

It is uncertain who the earlier people were that carried the Denisovan haplotypes to Australia. There are no diagnostic skeletal remains from the Denisova Cave to help with this, as noted above, and there is only limited inference that is possible from the Denisovan range. But if the beginning assessment that the immediate ancestry of Australian Aboriginals from populations that inhabit regions that are closest to Australia, one of the key conclusions of this research, and remember that the Denisovan haplotypes are not likely to be from populations in East Asia or Southeast Asia as they are constituted at the present, The Ngandong population are the other potential nearby source of Denisovan haplotypes. No DNA has been recovered from Ngandong skeletal material, but as the result of the anatomical comparisons that have been presented by Wolpoff & Lee, as well as the geography of the region, where else could the significant contribution have come from?

Sources & Further reading

Milford H. Wolpoff and Sang-Hee Lee PB WLH 50: How Australia Informs the Worldwide Pattern of Pleistocene Human Evolution- PaleoAnthropology 2014: 505−564