There are two branching hypotheses on the origin of the human species. The most widely accepted is the “out of Africa” (OOA) theory, which holds that archaic Homo sapiens evolved into anatomically modern humans solely in Africa between 200,000 and 60,000 years ago . This hypothesis further proposes that members of one branch of H. sapiens left Africa at some point between 125,000 and 60,000 years ago, and that over a long period, these H. sapiens replaced more “primitive” populations of other hominins in Asia or Europe, such as Homo neanderthalensis and Homo erectus .
The competing theory is the multiregional evolution hypothesis , which argues that some or all of the genetic variation among contemporary human races is attributable to genetic inheritance from either other H. sapiens subspecies or from other hominid species. In the multiregional model, all archaic human forms worldwide, such as H. erectus and Neanderthals, as well as modern forms, subsequently evolved together into the diverse populations of modern H. sapiens, which are considered to make up a single, continuously gradient (as distinct from categorically separate) human species.
DNA analysis demonstrating the existence of “Mitochondrial Eve” has strongly corroborated the recent African origin model of OOA by providing crucial support to the theory that H. sapiens moved from Africa to replace residing hominin populations elsewhere . Mitochondrial Eve is the most recent matrilineal common ancestor of all humans currently alive. Women pass along mitochondrial DNA unchanged during sexual reproduction, and the DNA of this most recent woman from whom all currently living humans descend through an unbroken line on their mother’s side proves that modern humans only evolved once, most likely in East Africa, sometime between 150,000 and 200,000 years ago.
At the same time, however, studies suggest that Neanderthals, our closest-known evolutionary relatives, coexisted with H. sapiens on Earth for more than 5000 years and frequently interbred with modern humans . According to researchers, at least one-fifth of the Neanderthal genome may lurk within modern humans, influencing traits including the appearance of the skin and hair people have today and the diseases they get. This finding indicates that a true “extinction” of Neanderthals may not have occurred , but that they may have been absorbed into H. sapiens. Genetic evidence shows that other archaic hominins, such as the Denisovans, also interbred with H. sapiens . The most current version of the OOA hypothesis emphasizes the African origin of most human populations but allows for the possibility of local contributions/interbreeding between humans and other hominins . Consequently, this article mainly discusses Neanderthal–human interbreeding, while also explaining other admixtures of archaic humans with hominins who were their contemporaries, such as Denisovans and H. heidelbergensis.
Origin of race: human interbreeding with Neanderthals and Denisovans
Neanderthals are an extinct species of human (in the genus Homo), related to modern humans . Traces left by Neanderthals include bone and stone tools, which have been found all over Eurasia, from Western Europe to central and northern Asia. Neanderthals are generally classified by biologists as H. neanderthalensis, and sometimes as Homo sapiens neanderthalensis.
Denisovans are another extinct species of humans, similar to Neanderthals. The Denisova Cave is located in southwestern Siberia, in the Altai Mountains near the Russian border with China and Mongolia . Research shows that Denisovans shared a common origin with Neanderthals but were genetically distinct.
Recent genetic studies have shown a higher Neanderthal admixture in East Asians compared with Europeans , most likely indicating that at least two independent gene-flow events must have taken place in early modern humans and that the early ancestors of East Asians experienced more admixture than those of Europeans after the divergence of these two groups ; to put it in another way, studies seeking to explain why East Asians inherited 15–30 % more Neanderthal DNA than Europeans have concluded that East Asians interbred with Neanderthals in two waves .
The first interbreeding with Neanderthals occurred in the Middle East before the ancestors of modern non-Africans spread out across Eurasia. The ancestors of modern Europeans and Asians then split out of this migrant group , and the ancestors of East Asians interbred again with Neanderthals after the split. The first humans with proto-Neanderthal traits are believed to have existed in Eurasia as early as 350,000–600,000 years ago, with the first “true Neanderthals” appearing between 200,000 and 250,000 years ago.
As this implies, Neanderthals and Denisovans were likely more closely related to one another than either was to modern humans . Although the range covered by Denisovans is argued, studies have confirmed the impact of Denisovan ancestry in the islands of Oceania, particularly Papua New Guinea, and some parts of mainland Asia, such as Tibet.
Why does Neanderthal ancestry appear to a higher degree in Asia?
All H. sapiens living today have interbred to some degree with Neanderthals, Denisovans, or other hominins, and as outlined above, we know that these hominin groups lasted longer and interbred more in some parts of the world than in other areas.
Most hominins other than Denisovans and Neanderthals were simply replaced by H. sapiens that migrated out of Africa, but sufficient interbreeding occurred with Denisovans and Neanderthals in Eurasia to leave a significant mark on modern human DNA . Because Neanderthals ranged only from Europe to West Asia, the question of why there were two waves of interbreeding between East Asian H. sapiens and Neanderthals remains a mystery. The answer to this question lies in differences in behavior, and in particular aggressiveness, between groups of hominins.
Homo heidelbergensis, which exhibited proto-Neanderthal traits, existed in Eurasia as early as 350,000–600,000 years ago, while the first Neanderthals appeared between 200,000 and 250,000 years ago . At varying times, Neanderthals inhabited the region from Western Europe to Central Asia; their eastern and northern range extended to Okladnikov in the Altai region and Byzovaya in the Ural region of present-day Russia . Neanderthals started to disappear/interbreed with H. sapiens from the time the latter migrated to Europe. Fossil findings have indicated brutality and violence among H. sapiens living 10,000 years ago . The evidence has shown that in addition to interbreeding, Neanderthals were also very often killed by H. sapiens, and in related findings, genetic studies have shown that the mutations in ADSL, GLDC, and SLITRK1 genes, which are associated with hyperactivity and aggressive behavior in modern humans, were not found in Neanderthals [19–21]. Thus, by multiple methods, H. sapiens were responsible for the extinction of Neanderthals, who were more cooperative and less aggressive than H. sapiens according to studies from various fields.
It should be noted that compared to Africa, Eurasia lacks predators that could have presented a threat to hominin species. The increase in species richness or biodiversity that occurs from the poles to the tropics is often referred to as the latitudinal diversity gradient (LDG), and the greatest biodiversity is found in the tropics . The African continent lies almost entirely within the tropics and extends equally to the north and south of the equator, which creates favorable conditions for wildlife, including large predators . The rich vegetation in Africa, where edible fruits and nuts are abundant, results in a diversity of animal species, and many carnivores, such as hyenas, lions, vultures, crocodiles, and cheetahs, reside exclusively in this biologically diverse region, where they once posed a major threat to human species.
In the Eurasian region, in contrast, where Neanderthals evolved and ultimately ranged, there were no carnivores that regularly preyed on humans. Thus, the Neanderthals that ranged in Eurasia evolved toward peaceful behavior.
Thus, the H. sapiens that came after H. heidelbergensis began their journey in East Africa, where they had to compete with other animals, including archaic hominins, and watch out for dangerous predators. The hyperactivity and violence of H. sapiens, which distinguish them from Neanderthals, were an essential part of their survival, because they had to fight and often kill predators and competitors.
Edible insects and Asia
Although Neanderthals never inhabited East Asia, East Asians have more Neanderthal DNA than Europeans do. The peaceful nature of Neanderthals would have been advantageous in East Asia due to the large of amount edible insects available—considerably higher than in Europe.
Before a rise in human population density forced people to turn to agriculture, hominins were hunter-gatherers, whose diet mostly consisted of fruits, nuts, and insects. Prior to the domestication of animals in approximately 9000 BCE, hominins would have relied on hunting and scavenging to obtain what meat they ate. Due to the instability of meat supplies obtained from hunting, partly a result of unreliable tools, hominins largely depended on insects for protein; coprolites from caves in the United States and Mexico, containing ants, beetle larvae, lice, ticks, and mites , have evidenced this prehistoric entomophagy. Similar to other great ape species that eat insects, then, the evolutionary precursors of H. sapiens were also entomophagous, and staple insects represented an important part of their diet.
Prehistoric entomophagy practices have persisted over time. Cave paintings in Altamira, North Spain, dating from approximately 30,000–9000 BCE, depict the collection of edible insects and wild bee nests, seeming to suggest an entomophagous society. Cocoons of wild silkworm (Theophila religiosae) found in ruins in the Shanxi Province of China dateback to 2000–2500 years BCE. The cocoons were discovered with large holes, suggesting that the pupae had been eaten. The eating of insects is still observed in the developed nations of modern-day Asia .
The ancestors of East Asians interbred with Neanderthals a second time after the earlier interbreeding in the Middle East, as mentioned above. Compared with Europe, the insect-abundant regions of East Asia were able to support larger populations of hominins. In such an environment, aggressiveness and violence would have been more disadvantageous for survival (Fig. 1).
Denisovans, which were related more closely to Neanderthals than to humans, also interbred with H. sapiens. Genetic variation of Denisovans is low compared to that of H. sapiens, but Denisovans were present in large parts of Asia for possibly more than 110,000 years, allowing H. sapiens in Asia to obtain Denisovan traits from interbreeding .
Evidence indicates that the highest Denisovan admixture is found in Oceanian populations, followed by many Southeast Asian populations, but recent research has also found indications that parts of mainland Asia, such as Tibet, have small traces of Denisovan DNA .
Denisovans were adapted to surviving at high altitudes, and Denisovan fossils have been found in high caves in Siberia; researchers have further discovered that Tibetans are inheritors of the ancient Denisovan trait of being able to regulate blood oxygenation . The highest levels of interbreeding with Neanderthals, which were genetically closer to Denisovans than H. sapiens were, occurred in East Asia, and East Asians show a small, relatively insubstantial fraction of Denisovan ancestry. Significant levels of Denisovan genes, however, remain in non-East Asian populations (Southeast Asians and Melanesians) residing far from the Denisova Cave in Siberia.
Interbreeding with Denisovans significantly affected H. sapiens populations on the island of New Guinea, where the highest mountains and highlands in Australasia are found. In general, populations that proliferated in these mountain ranges would have also spread to nearby regions, leaving remnants of Denisovans throughout Asia. At the end of the Ice Age, the separation of the Sahul and Sunda shelves from mainland Asia, caused by rising sea levels, resulted in the local population of Oceania (and parts of Southeast Asia) being less affected by the admixture of Neanderthals with H. sapiens. Thus, traces of Denisovans were more perceptibly preserved in these regions.
Why Neanderthals lost out to Homo sapiens
Although Neanderthals did interbreed with H. sapiens, the majority of their population went extinct from competition with H. sapiens. As follows from the possession by H. sapiens of a mutated gene related to aggression, fossil evidence reveals that Neanderthals were killed by H. sapiens in acts of violence . Furthermore, although Neanderthals possessed brain development enabling greater visual acuity than H. sapiens, the latter had better language-processing abilities . In general, because Neanderthal brains were devoted to vision and spatial memory, this left less area for cognition and social interactions.