Review
Denisovans and Homo sapiens on the Tibetan Plateau: dispersals and adaptations

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Highlights

  • 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.

Section snippets

High-altitude settlement and selective pressures

Tibetans are the largest indigenous population living in high-altitude environments who have developed a suite of traits to cope with the harsh environment of the Qinghai–Tibetan Plateau (Tibetan Plateau, hereafter, TP) [1., 2., 3.]. With an average elevation of 4000 m above sea level (masl), it is surrounded by natural barriers with a cold and arid environment and significant seasonal variations [4]. During the Late Pleistocene (see Glossary), the highly frequent millennial-scale oscillations

The archaeology of human occupations and paleoclimates

Archaeological research on TP began in the 1960s but well-documented excavations and dated sites are limited so far [10,11,22] (Figure 1). The available archaeological data suggest that there were four major periods of human occupation on the TP.

Genetic adaptations: archaic introgression and natural selection

Geographic isolation from lowlands and the selective pressure of hypoxia are essential factors for the shaping of fitness of Tibetan populations. Hence, genetic studies have focused on identifying the beneficial genes for high-altitude adaptations and inferring the Tibetan demographic history. Another key component in the high-altitude adaptation of Tibetans is the role of adaptive introgression from Denisovans [18], while the timing and geographical range of the introgressions remain

Human occupations of the TP: discontinuity or continuity?

The earliest occupation of the TP is likely by the Denisovans, as is indicated by data from Baishiya Cave; the Denisovans visited the northeast margin several times between 160 ka and 60 ka, possibly earlier or later [21,24]. Nevertheless, we still do not know if these Denisovans were biologically adapted to high altitudes as no Denisovan nuclear DNA that includes the EPAS1 locus is known from the highland. Future discoveries are critical to examine whether they had direct contacts with H.

Concluding remarks

New discoveries accumulate rapidly and continue to improve our understanding of the population history of the TP, while the few existing syntheses relied heavily on archaeological findings [10,11,22]. Using a cross-disciplinary approach, we propose two parsimonious but distinct models for the population history of the high-altitude TP. The models are constructed to reach the consistency between archaeology and genetics, but the challenges specific to each field remain. For example,

Acknowledgments

We thank Brenna Henn, Randy Haas, Natalie Swinford, Giulia Gallo, the UC Davis Paleoanthropology group, and the two anonymous reviewers for their helpful feedback, and John Darwent for the assistance on figure illustrations. P.Z. was supported by Baldwin Fellowship of The Leakey Foundation. XJ.Z. was supported by NIH Grant 1K99GM143466-01 and NIH Grant R35GM119856 (to Kirk Lohmueller at UCLA). XL.Z. was supported by the Pan-Third Pole Environment Study for a Green Silk Road (Pan-TPE), Grant No.

Declaration of interests

There are no interests to declare.

Glossary

Admixture
the exchange of genetic materials between two previously isolated populations, often facilitated by migrations.
Blade
a type of stone flake whose length is at least twice the width. It often shows two parallel edges, and the negatives of previous blade removals on its upper face show the systematic character of the production.
Holocene climate optimum
a period of temperate climate usually dated from 8 ka to 4 ka years ago.
Hypoxia
a condition that occurs when there is insufficient oxygen

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    These authors have contributed equally to the manuscript

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    Twitter: @Xinjun90 (X. Zhang) and @emiliahsc (E. Huerta-Sanchez).

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