Four Waves Of Denisovans And Humans In Tibet

A new review article recaps the four waves of settlement of the Tibetan Plateau by modern humans and archaic hominins. 

The first wave was by archaic hominins called Denisovans reached Tibet in the Middle Stone Age. Denisovans are a sister clade of Neanderthals, and are closer to both Neanderthals and modern humans than the dominant hominin species outside of Africa that preceded all three of these hominin species, Homo eretcus which emerged from Africa about 1.8 million years ago after evolving in Africa about 2 million years ago. 

Some Denisovans evolved genetic adaptations to the low oxygen levels at high altitudes that were passed to modern human populations in Asia in the Upper Paleolithic era, through admixture (i.e. via mixed Denisovan and modern human children) around 47,000 years ago, probably somewhere in the lowland of East Asia near Tibet.

The second wave was made up of Upper Paleolithic modern human hunter--gatherers around 40,000 years ago.

The third wave was made up of Mesolithic modern human hunter-gatherers around 16,000 years ago.

The languages of these two waves of hunter-gatherer people in Tibet (let alone any Denisovan language) probably died and were lost forever around the time that herders and farmers arrived in Tibet.

The last wave was made up of Neolithic modern human herders and farmers around 8,000 years ago who probably brought the Sino-Tibetan languages to the Tibetan Plateau from a source somewhere in Northern China. We know that they continuously occupied the Plateau, but don't know if the previous hominin inhabitants of the Tibetan Plateau settled there year round or occupied it continuously.

Science Daily has the press release:

Denisovans were archaic hominins once dispersed throughout Asia. After several instances of interbreeding with early modern humans in the region, one of their hybridizations benefited Tibetans' survival and settlement at high altitudes. . . . 

Peiqi Zhang, a UC Davis doctoral student who has participated in excavations of an archaeological site above 15,000 feet (4,600 meters) . . . review[ed] . . . evidence of human dispersal and settlement in the Tibetan Plateau, integrating the archaeological and genetic discoveries so far. . . .  

Archaeological investigations suggest four major periods of occupation, beginning with Denisovans about 160,000 years ago and followed by three periods of humans who arrived starting around 40,000 years ago, 16,000 years ago and 8,000 years ago.

"Based on archaeological evidence, we know that there are gaps between these occupation periods," Peiqi Zhang said. "But the archaeological work on the Tibetan Plateau is very limited. There's still a possibility of continuous human occupation since the late ice age, but we haven't found enough data to confirm it." . . . 

"From the genetic studies, we can detect that all East Asians, including the Tibetans, interbred with two distinct Denisovan groups, with one of such events unique to East Asians (and the other shared with other South Asians). . . . Since all East Asians show the same patterns, we have reason to believe that this interbreeding event (the one that's unique to East Asians) happened somewhere in the lowland instead of on the plateau."

Zhang and Zhang propose two models of human occupation of the Tibetan Plateau as a framework for scholars that can be tested by future discoveries: 

* Intermittent visits before settling there year-round about the end of the ice age, about 9,000 years ago. 

* Continuous occupation beginning 30,000 to 40,000 years ago.

In either model, Denovisans could have passed the EPAS1 haplotype to modern humans about 46,000 to 48,000 years ago.

"The main question is whether they're staying there all year-round, which would mean that they were adapted biologically to hypoxia. . . . Or did they just end up there by accident, and then retreated back to the lowlands or just disappeared?"

It's unclear when Denisovans went extinct, but some studies suggest it may have been as late as 20,000 years ago.

Neanderthals probably went extinct about 29,000 years ago. 

The paper and its abstract are as follows:

The peopling of the Tibetan Plateau is a spectacular example of human adaptation to high altitudes as Tibetan populations have thrived for generations under strong selective pressures of the hypoxic environment. Recent discoveries are leading to paradigmatic changes in our understanding of the population history of the Tibetan Plateau, involving H. sapiens and the archaic hominin known as Denisovan. 

Archaeological and genetic studies provide essential insights into behavioral and biological human adaptations to high elevations but there is a lack of models integrating data from the two fields. Here, we propose two testable models for the peopling process on the plateau leveraging evidence from archaeology and genetics. 

Recent archaeological discoveries suggest that both archaic Denisovans and Homo sapiens occupied the Tibetan Plateau earlier than expected. Genetic studies show that a pulse of Denisovan introgression was involved in the adaptation of Tibetan populations to high-altitude hypoxia. These findings challenge the traditional view that the plateau was one of the last places on earth colonized by H. sapiens and warrant a reappraisal of the population history of this highland. Here, we integrate archaeological and genomic evidence relevant to human dispersal, settlement, and adaptation in the region. We propose two testable models to address the peopling of the plateau in the broader context of H. sapiens dispersal and their encounters with Denisovans in Asia.

Peiqi Zhang, et al., "Denisovans and Homo sapiens on the Tibetan Plateau: dispersals and adaptations" Trends in Ecology and Evolution (December 1, 2021). DOI: https://doi.org/10.1016/j.tree.2021.11.004