Human history is not just about where we came from but how we adapted to the ever-changing environments we encountered. A recent review by Steven Abood and Hiroki Oota, published in Journal of Physiological Anthropology, dives deep into the migration of Homo sapiens into East Eurasia, revisiting the evidence that solidified the “Out of Africa” model and highlighting the physiological adaptations that allowed our ancestors to survive in extreme environments.
For most of the 20th century, paleoanthropologists debated between two competing models of human evolution: the “Multiregional Evolution” hypothesis and the “Out of Africa” model. The former suggested that modern humans evolved in different parts of the world independently from local Homo erectus populations, while the latter proposed that Homo sapiens emerged in Africa and later spread across the globe.
By the 1990s, genetic research began to challenge the Multiregional hypothesis. Studies on mitochondrial DNA (mtDNA), which is inherited exclusively from the mother, found that all modern human mtDNA lineages trace back to a common ancestor in Africa, roughly 200,000 years ago. The discovery of this so-called “Mitochondrial Eve” dealt a major blow to the Multiregional theory.
The authors explain how modern genome sequencing further cemented the Out of Africa model. With the completion of the Human Genome Project in 2003 and subsequent projects like the HapMap and 1000 Genomes Project, researchers found that African populations had significantly greater genetic diversity than non-Africans—a key sign that humans originated there before dispersing outward.
While the Out of Africa model was becoming the dominant paradigm, the discovery of Neanderthal and Denisovan DNA in modern humans added an unexpected twist.
“Analysis of the Neanderthal genome revealed that 1 to 4% of the genome in modern humans living outside Africa is derived from Neanderthals,” the study notes.
This means that as Homo sapiens expanded into Eurasia, they interbred with Neanderthals and Denisovans. In East Asians, Neanderthal DNA makes up about 12–20% more of the genome than in Europeans.
This interbreeding provided genetic advantages. Certain Neanderthal-derived genes influence immune system function, while Denisovan DNA played a role in high-altitude adaptation in Tibetans. One famous example is the EPAS1 gene, which helps Tibetans survive in low-oxygen environments at high elevations. This gene, first identified in a Denisovan finger bone from Siberia, suggests that hybridization between archaic humans had real physiological consequences.
Once Homo sapiens left Africa, how did they reach East Asia? This question remains hotly contested, with evidence pointing to two possible routes:
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The Southern Route – following a coastal migration through South Asia and Southeast Asia before moving northward.
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The Northern Route – moving through Central Asia and Siberia before reaching East Asia.
Archaeological evidence, particularly the presence of distinct microblade stone tools, has been used to argue for a northern migration. However, genetic research presents a different picture. The Pan-Asian SNP Consortium found that genetic diversity decreases from Africa to India, then further decreases as one moves eastward, supporting a southern migration as the dominant route.
The genome of the Jomon people, a hunter-gatherer group from ancient Japan, has provided additional insights. The Jomon genome shows strong affinities to populations from Southeast Asia rather than Siberia, reinforcing the idea that early East Asians primarily arrived via the southern route.
One of the most fascinating aspects of human migration is how populations adapted to new and often extreme climates. The study explores several key physiological adaptations seen in East Asians today, many of which stem from evolutionary pressures in Ice Age Eurasia.
For example, a genetic variant in the PER2 gene, which regulates circadian rhythms, has been linked to light sensitivity. As humans moved into regions with long, dark winters, those with a version of PER2 that allowed for better light adaptation had a survival advantage.
Similarly, genetic variations in UCP1, a gene involved in thermogenesis (heat production), helped East Asians develop resistance to cold by improving the function of brown adipose tissue (BAT). This adaptation is still seen today—populations with a history of cold exposure tend to have higher metabolic rates, helping them generate more body heat.
Interestingly, while East Asians, Pacific Islanders, and Native Americans share common ancestry, only East Asians seem to have retained high metabolic rates that contribute to obesity resistance. The reason for this remains an open question.
Living at high altitudes presents unique physiological challenges. In addition to lower oxygen levels, people living in the Himalayas and Tibetan Plateau experience extreme cold, high UV radiation, and nutritional stress.
The EPAS1 gene, inherited from Denisovans, has been a key adaptation for Tibetans. This gene helps regulate red blood cell production, preventing excessive thickening of the blood—a common problem for people unaccustomed to high altitudes.
Studies have also found that Tibetan women with certain physiological traits, like a wider left ventricle and better blood flow regulation, have higher reproductive success. This suggests that natural selection has actively shaped high-altitude adaptations in these populations over thousands of years.
The migration and adaptation of Homo sapiens into East Eurasia is a story that continues to be rewritten as new genetic discoveries emerge. While the Out of Africa model is firmly established, the complexities of interbreeding with Neanderthals and Denisovans, the debate over migration routes, and the nuances of physiological adaptation remind us that human evolution is anything but straightforward.
Perhaps the biggest lesson from this study is that evolution is an ongoing process. The very genes that helped our ancestors survive Ice Age Eurasia may still be influencing our health today—shaping everything from metabolism to immune function to high-altitude endurance. And as we continue to uncover more ancient genomes, our understanding of human history will only deepen.
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Fu, Q., Meyer, M., & Pääbo, S. (2013). DNA analysis of an early modern human from Tianyuan Cave, China. PNAS. DOI: 10.1073/pnas.1221359110
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Huerta-Sánchez, E. et al. (2014). Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA. Nature. DOI: 10.1038/nature13408
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Meyer, M. et al. (2012). A high-coverage genome sequence from an archaic Denisovan individual. Science. DOI: 10.1126/science.1224344
