Fully Integrated and Multidisciplinary Model of Modern Human Origins: Out-of-America, with ultimate origins from an East Eurasian hominid, and into Africa with admixture with extinct African archaic hominins.
This model that I consider to best reflect core interdisciplinary evidence for modern human origins (ethnology, linguistics, population genetics, paleobiology, archaeology) postulates that behaviorally and anatomically modern humans originated from a population of East Eurasian humans such as Neanderthals (whose geographic reach stretched all the way to the Altai Mountains in southern Siberia and, possibly, beyond the Arctic Circle in the northwestern Urals), Homo erectus or Denisovans (the newly-discovered hominid species attested through a tooth and a pinkie from the Denisova Cave, South Siberia). Between 200,000 and 100,000 years ago, a subset of this original hominid population migrated to the New World (via the Bering Strait Land Bridge), where speciation into modern humans occurred. While, under out-of-America II, the unique social behaviors shared between modern humans and New World primates (pair bonding, paternal investment, cooperative breeding and speech) are interpreted as homoplasies, the fact that such key aspects of human social and cognitive behavior are shared with Platyrrhines suggests that the immediate ancestors of modern humans were exposed to the same New World environment as the New World monkeys.
This migration into a new continent via a relatively narrow land bridge resulted in a population bottleneck still visible in the human genome (as compared to other primate genomes) and in the Amerindian genome. With the next retreat of the ice shield, our ancestors migrated back to the Old World and replaced, possibly with some admixture, all Old World hominids in Eurasia and Africa. As a result of this re-expansion in the Old World, all human populations, with the exception of American Indians (and arguably such isolates as Papua New Guineans, the peoples of the Caucasus and the Hadza of Tanzania), somewhat rebounded from the original bottleneck due to population size growth, waves of intraspecific admixture and, possibly, admixture with archaic hominids in Eurasia and Africa. This replacement of Old World hominids by the modern humans coming out of America corresponds to the emergence of signs of modern human behavior all over the globe around 40,000 years ago. One clear advantage of Out-of-America II over Out-of-Africa is that prolonged geographic isolation is an absolute prerequisite for a speciation event to occur. Africa had been well settled by ancient hominids to allow for the easy and matter-of-fact speciation into modern humans in Africa that’s assumed by the mainstream science of human origins. The emergence of a new hominid species with a radically different, worldwide adaptation based on an advanced system of symbolic thinking and social cooperation followed by the dramatic replacement of pre-existing hominids all over the world is best explained as having its origin in a unique original environment – on a continent previously unexplored by hominids. Another advantage of Out-of-America II over Out-of-Africa is that it’s consistent with ancient DNA results: while we don’t have a single ancient DNA sample to ascertain whether modern African populations are directly related to ancient “anatomically modern humans” (e.g., Omo, Herto, etc.) and hominids in Africa, we do have ancient DNA data (X chromosome, autosomes, blood groups) that document matches between Neanderthal and Denisovan genetic variation, on the one hand, and modern humans in the New World (and in Melanesia). While these matches are currently interpreted as a sign of admixture between Africa-derived modern humans and in-situ hominids, they are the only signs of continuity between archaic and modern humans known to science at this moment.
After speciation in the New World had occurred, early Homo sapiens sapiens colonized first Eurasia and then Africa replacing and admixing with local hominids. Admixture with archaic hominins in Africa was more substantial than in Eurasia, which is reflected in the firmly established excess in intragroup genetic diversity in Africa. This model uses Y-DNA evidence, namely the phylogenetic position of the major African E clade as a subset of the non-African DECF clade, as evidence for the extra-African origin of modern humans. Back in 1998, the Michael Hammer lab published a paper entitled “Out of Africa and Back Again: Nested Cladistic Analysis of Human Y Chromosome Variation,” in which a major back-migration into Africa accounting for the majority of African Y chromosomes was proposed. Haplogroups A and B found exclusively in Africa are explained as either the product of admixture between African hominids and the incoming modern humans or as retentions from the earliest, purely African phase in the modern human evolution. Archaeologically, the presence of sites such as Dabban, with clear Upper Paleolithic roots, in North Africa around 40,000 YBP supports the back-migration idea. From the paleobiological perspective, the Hofmeyr skull in South Africa dated at 36,000 YBP clusters with Upper Paleolithic Eurasians, which, again, suggests that Africa was peopled from Eurasia, not the other way around. Another argument in favor of an extra-African origin of modern humans is the fact that skulls with archaic features survived in various part of Africa (e.g., the Iwo Eleru skull dated at 11-16,000 YBP or the Lukenya Hill calvaria in Kenya at 23-22,000 YBP). If there was indeed continuity between “anatomically modern humans” in Africa that begin to show up in the paleontological record from 200,000 BP on and today’s anatomically and behaviorally modern humans, we would not expect archaic hominins to survive in Africa for almost 180,000 years. Outside of Africa, modern humans needed only a short window to replace all of the Neanderthals. It’s also noteworthy that African megafauna was largely spared in Africa: only 14% (or 7 out of 49 genera) of African megafauna went extinct in the Late Pleistocene. Outside of Africa, megafauna extinctions were much more dramatic, with 86% of megafauna going extinct in Australia, 80% in South America, 73% in North America, 60% in Europe. Under the anthropogenic theory of megafauna extinctions, modern human hunting and ecological disruption are the causes of the extinctions. If Africa was the least affected continent, it’s possible that it was peopled by modern humans later than other continents and/or by smaller numbers of modern humans. But genetics predicts otherwise – Africa must be the oldest and most populous continent, hence modern Africans are more diverse than populations outside of Africa. If anatomically and behaviorally modern humans originated at a place in Sub-Saharan Africa and expanded first across all of Africa (as the distribution of “basal” mtDNA and Y-DNA lineages in current phylogenies seems to suggest), then it’s unclear why the megafauna was not affected by their new and improved hunting practices. But the anthropogenic theory of megafauna extinctions is just one theory out of many and climate change may have been a bigger factor.