Archaic Admixture in Modern Humans - Continued
The original Out-of-Africa theory posited a single origin population with zero admixture occurring between archaic and modern populations, because biologically they apparently couldn’t. In fact, modern humans picked up archaic admixture as they spread out of Africa and into Eurasia. Modern Eurasians have 1-4% Neanderthal admixture, and Melanesians have an additional 4-6% from the Denisovans:
Denisova Admixture and the First Modern Human Dispersals into Southeast Asia and Oceania:
… Aboriginal Australians, Near Oceanians, Polynesians, Fijians, east Indonesians, and Mamanwa (a “Negrito” group from the Philippines) have all inherited genetic material from Denisovans, but mainland East Asians, western Indonesians, Jehai (a Negrito group from Malaysia), and Onge (a Negrito group from the Andaman Islands) have not. These results indicate that Denisova gene flow occurred into the common ancestors of New Guineans, Australians, and Mamanwa but not into the ancestors of the Jehai and Onge and suggest that relatives of present-day East Asians were not in Southeast Asia when the Denisova gene flow occurred.
Modern sub-Saharan Africans appear to be the most archaic-influenced of all modern humans. About 2% of their gene pool comes from a now-extinct member of the Homo genus that broke away from the modern human lineage around 700,000 years ago. This implies that archaic admixture in modern sub-Saharan African populations extends beyond the H. heidelbergensis clade, which is ancestral to modern humans and Neanderthals, and may encompass late H. erectus populations (expanded on in this post):
Genetic evidence for archaic admixture in Africa:
… contemporary African populations contain a small proportion of genetic material (≈2%) that introgressed ≈35 kya from an archaic population that split from the ancestors of anatomically modern humans ≈700 kya. Taken together our results suggest that polymorphisms present in extant populations introgressed via relatively recent interbreeding with hominin forms that diverged from the ancestors of modern humans in the Lower-Middle Pleistocene.
A further 13% of archaic admixture in sub-Saharan Africans comes from a population that may be related to the Skhul-Qafzeh (dated to 120,000 – 80,000 BP) early (anatomically) modern humans of the Levant.
The original Out-of-Africa theory also posited that modern humans simply replaced archaic populations, but this did not happen, at least not everywhere. In western and southern Africa, the remains of the archaic populations that contributed to the genome of African populations lived into the later stone age, some ~13,000 years ago. Around the time when farming had begun in the Middle-East:
The Later Stone Age Calvaria from Iwo Eleru, Nigeria: Morphology and Chronology
Similarly, remains of early modern humans from southwest China date to date to ~14.5-11.5 thousand years ago (the image of this post is a reconstruction of the remains found in China):
Human Remains from the Pleistocene-Holocene Transition of Southwest China Suggest a Complex Evolutionary History for East Asians:
The fossils are of a people with a highly unusual mix of archaic and modern anatomical features and are the youngest of their kind ever found in mainland East Asia. Dated to just 14,500 to 11,500 years old, these people would have shared the landscape with modern-looking people at a time when China’s earliest farming cultures were beginning,
This "unusual mix" is likely reminiscent of the Skhul-Qafzeh "almost modern” humans found in the Levant. Also similar to the early modern human populations which were found in North Africa:
Curnoe et al., 2012: Our analysis suggests two plausible explanations for the morphology sampled at Longlin Cave and Maludong. First, it may represent a late-surviving archaic population, perhaps paralleling the situation seen in North Africa as indicated by remains from Dar-es-Soltane and Temara, and maybe also in southern China at Zhirendong. Alternatively, East Asia may have been colonised during multiple waves during the Pleistocene, with the Longlin-Maludong morphology possibly reflecting deep population substructure in Africa prior to modern humans dispersing into Eurasia
As the prior study observed, there is archaic material present in living Africans, but DNA rapidly degrades in tropical climates so they may never retrieve DNA from the aforementioned archaic skulls of North Africa.
Authors of the study also note that a population candidate for the archaic Chinese remains could be the Denisovans, the population which contributes to the genome of Melanesians and which inhabited East Asia (as well as Siberia and tropical Asia at large) around the time that Neanderthals inhabited Europe and central Asia:
Curnoe et al., 2012: DNA extracted from a >50 ka hominin fossil from Denisova Cave in Central Asia belonging within the Neandertal lineage shares features exclusively with Aboriginal Southeast Asians and Australasians. This has been interpreted as: 1) evidence for interbreeding between the ‘Denisovans’ and the earliest modern humans to colonise the region; and 2) implying occupation of Southeast Asia by this archaic population during the Upper Pleistocene.