A large swathe of modern humans have Ancient North Eurasian (ANE) admixture, but ANE ancestry is especially high in northern Europeans. ANE were a Caucasian/Mongoloid hybrid peoples from northern Siberia who intermixed with the Yamnaya tribe, who moved across Europe and Asia and spread their Indo-European language. It’s possible the ANE may have derived their Mongoloid admixture from a distant ancestor. A hybrid hominin species of Homo Heidlbergensis and Homo Erectus called the Homo Denisovan was found to live in southern Siberia, the same region the ANE later inhabited.
A human group known to have Denisovan ancestry are the Australian Aborigines. Many Aboriginals and Papuans have fair hair, much like northern Europeans, that contrasts their black skin. Fair hair is thought to have evolved in Siberia. We know very little about the Denisovans as a fingerbone and a few teeth are the only fossils found to date, but it is highly probable that the trait for fair hair is derived from them, as Austrailian Aborigines have no ANE and very little Neanderthal ancestry.
Denisovans interbred with Neanderthal populations in southern Siberia where they both inhabited. Nordic Europeans carry up to 4% Neanderthal DNA while Western Hunter Gatherers were found to have no Neanderthal admixture, so it can only be these Siberian Neanderthals that make up the Neanderthal genes of Nordic Europeans today. The ANE was recently discovered to be genetically West Eurasian, but Denisovan ancestry is also prevalent in East Asian populations today and it is perhaps through their Denisovan common ancestor that northern Europeans acquired their Mongoloid features. Pakistani and the Pashtun peoples of Afghanistan (both of whom incidentally are mostly derived of the same Haplogroup R1a as the Slavic nations) have as much, if not more, ANE ancestry as northern Europeans; the Balkan peoples have less ANE ancestry than southern Europeans, however, the higher degree of Mongoloid admixture in both the Slavic and Balkan populations can be explained by an evolutionary process called the Founder Effect. The Founder Effect is the loss of genetic variation with the the genes more adaptive to the environment gradually replacing the maladative ones, resulting in newer populations which are genetically distinct from the parent population.
Aside from Europe, another continent where the Yamnaya spread their ANE ancestry is the Indian subcontinent. Modern South Asians are a mixture of peoples stemming from three waves of migrations; the first being the first migration out of Africa by Homo Erectus along the coast of the Indian Ocean across Asia into Oceania, the second wave comprises Middle Eastern farmers, who had also populated much of Europe. The final and major wave comes from the Yamnaya who brought with them the Indo-European languages as well as their ANE admixture into the Indian subcontinent. Although the fair complexions of northern Europeans was not environmentally advantageous in the tropical climate of the Indian subcontinent, but possibly because these were deemed attractive phenotypes and thus granted more reproductive success, the modern peoples of this subcontinent still possess a significantly lighter complexions compared to other fellow Austrailoid populations, including the Andamanese Islanders, from whom they were genetically cut off from for thousands of years, despite their close geographic proximity. Although rare, South Asians can be born with the fair complexion possessed by northern Europeans, but it is not so advantageous over darker complexions that it can trigger the Founder Effect amongst the native population of the Indian subcontinent.
https://www.nationalgeographic.com/science/2019/04/enigmatic-human-relative-outlived-neanderthals/ (Note: I cannot use on here a picture of an Indonesian woman named ‘Chika’ from the article as it is protected by copyright, but it is interesting to note her pale white skin and high cheekbones, traits shared by northern Europeans)
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