Multiregional Hypothesis Human Evolutionary Theory

The Multiregional Hypothesis model of human evolution (abbreviated MRE and known alternatively as Regional Continuity or Polycentric model) argues that our earliest hominid ancestors (specifically Homo erectus) evolved in Africa and then radiated out into the world. Based on paleoanthropological data rather than genetic evidence, the theory says that after H. erectus arrived in the various regions in the world hundreds of thousands of years ago, they slowly evolved into modern humans. Homo sapiens, so MRE posits, evolved from several different groups of Homo erectus in several places throughout the world. However, genetic and paleoanthropological evidence gathered since the 1980s has shown conclusively that that simply cannot be the case: Homo sapiens evolved in Africa and dispersed out into the world, somewhere between 50,000-62,000 years ago. What happened then is quite interesting. Background: How Did the Idea of MRE Arise? In the mid-19th century, when Darwin wrote Origin of Species, the only lines of evidence of human evolution he had were comparative anatomy and a few fossils. The only hominin (ancient human) fossils known in the 19th century were Neanderthals, early modern humans, and H. erectus. A lot of those early scholars didnt even think those fossils were humans or related to us at all. When in the early 20th century numerous hominins with robust large-brained skulls and heavy brow ridges (now usually characterized as H. heidelbergensis) were discovered, scholars started to develop a wide variety of scenarios about how we were related to these new hominins, as well as Neanderthals and H. erectus. These arguments still had to be tied directly to the growing fossil record: again, no genetic data was available. The predominant theory then was that H. erectus gave rise to Neanderthals and then modern humans in Europe; and in Asia, modern humans evolved separately directly from H. erectus. Fossil Discoveries As more and more distantly-related fossil hominins were identified in the 1920s and 1930s, such as Australopithecus, it became clear that human evolution was much older than previously considered and much more varied. In the 1950s and 60s, numerous hominins of these and other older lineages were found in East and South Africa: Paranthropus, H. habilis, and H. rudolfensis. The predominant theory then (although it varied greatly from scholar to scholar), was that there were nearly independent origins of modern humans within the various regions of the world out of H. erectus and/or one of these various regional archaic humans. Dont kid yourself: that original hardline theory was never really tenable -- modern humans are simply too much alike to be evolved from different Homo erectus groups, but more reasonable models such as those put forward by paleoanthropologist Milford H. Wolpoff and his colleagues argued that you could account for the similarities in human beings on our planet because there was lots of gene flow between these independently evolved groups. In the 1970s, paleontologist W.W. Howells proposed an alternate theory: the first Recent African Origin model (RAO), called the Noahs Ark hypothesis. Howells argued that H. sapiens evolved solely in Africa. By the 1980s, growing data from human genetics led Stringer and Andrews to develop a model that said that the very earliest anatomically modern humans arose in Africa about 100,000 years ago and archaic populations found throughout Eurasia might be descendants of H. erectus and later archaic types but they were not related to modern humans. Genetics The differences were stark and testable: if MRE was right, there would be various levels of ancient genetics (alleles) found in modern people in scattered regions of the world and transitional fossil forms and levels of morphological continuity. If RAO was right, there should be very few alleles older than the origins of anatomically modern humans in Eurasia, and a decrease in genetic diversity as you get away from Africa. Between the 1980s and today, over 18,000 whole human mtDNA genomes have been published from people all over the world, and they all coalesce within the last 200,000 years and all the non-African lineages only 50,000-60,000 years old or younger. Any hominin lineage that branched off from the modern human species prior to 200,000 years ago did not leave any mtDNA in modern humans. An Admixture of Humans With Regional Archaics Today, paleontologists are convinced that humans evolved in Africa and that the bulk of modern non-African diversity is recently derived from an African source. The exact timing and pathways outside of Africa are still under debate, perhaps out of East Africa, perhaps along with a southern route from South Africa. The most startling news from a human evolution sense is some evidence for mixing between Neanderthals and Eurasians. Evidence for this is that between 1 to 4% of genomes in people who are non-Africans are derived from Neanderthals. That was never predicted by either the RAO or the MRE. The discovery of a completely new species called the Denisovans threw another stone in the pot: even though we have very little evidence of Denisovan existence, some of their DNA has survived in some human populations.​ Identifying Genetic Diversity in Human Kind It is now clear that before we can understand the diversity in archaic humans, we have to understand the diversity in modern humans. Although MRE has not been seriously considered for decades, now it seems possible that modern African migrants hybridized with local archaics in different regions of the world. Genetic data demonstrate that such introgression did occur, but it is likely to have been minimal. Neither Neanderthals nor Denisovans survived into the modern period, except as a handful of genes, perhaps because they were unable to adapt to the unstable climates in the world or competition with H. sapiens. Sources Disotell TR. 2012. Archaic human genomics. American Journal of Physical Anthropology 149(S55):24-39.Ermini L, Der Sarkissian C, Willerslev E, and Orlando L. 2015. Major transitions in human evolution revisited: A tribute to ancient  DNA. Journal of Human Evolution 79:4-20.Gamble C. 2013. In: Mock CJ, editor. Encyclopedia of Quaternary Science (Second Edition). Amsterdam: Elsevier. p 49-58.Hawks JD, and Wolpoff MH. 2001. The four faces of Eve: hypothesis compatibility and human origins. Quaternary International 75:41-50.Stringer C. 2014. Why we are not all multiregionalists now. Trends in Ecology Evolution 29(5):248-251.