Two such explanations are the “Out of Africa” and “multi-regional” models. These are actually quite famous (at least for an EvoAnth concept, which granted doesn’t set the bar for fame that high) so I wouldn’t be surprised if you’ve heard of them. If you have and don’t fancy reading an over-simplified and mildly condescending summary of them, feel free to skip ahead to after the next picture.
The out of Africa model compared to the multi-regional model.
The Out of Africa model (or “OoA,” as it is called by those particularly fond of vowels) argues modern humans arose in Africa ~200,000 years ago and eventually spread out of Africa and across the world. Of course, they were not the first hominin to do so as Homo erectus had already left Africa 1.7 million years ago. They met remnants of these migratory populations around the world and promptly drove them to extinction. Whether this was through actually murdering them or just taking their land is contested.
On the other hand the multi-regional hypothesis suggests that such an extinction did not take place. This model comes in two flavours. The first suggests that the various populations of H. erectus which spread across the globe individually developed into H. sapiens (i.e. Asian H. erectus became Asians, European H. erectus became Europeans etc.). However, this would suggest that neanderthals and Europeans always existed together when we know neanderthals were the only species in Europe from ~300,000 – 40,000 years ago. The second variant gets around this issue by conceding that H. sapiens did appear in Africa, but when it spread it across the globe it absorbed the other hominins it found via interbreeding, rather than driving them extinct.
These two competing ideas were happy to duke it out across academia for years until genetics came along and ruined all the fun. By comparing the genes of various populations you can work out who is most closely related to each other, thus drawing a family tree for the entire human species. Such reconstructions show we all descended from an African populations, signalling the death knell for the multi-regional hypothesis.
So it seemed settled, humans appeared in Africa ~200,000 years ago and left ~60,000. Out of Africa was correct and could take up the mantle of “best explanation for how humans spread across the globe.” A prestigious award I assure you.
However, in a very dramatic twist, it was not settled! Finds in the Middle East (including the first burial) indicated that humans had left Africa over 100,000 years ago. Yet humans disappeared from these settlements ~65,000 years ago, being replaced by neanderthals, indicating that it was a failed attempt. Then there are recent genetic studies indicating there was some limited interbreeding with neanderthals. Finally there are the latest finds from China suggesting that there was a successful departure from Africa some time between 500,000 and 60,000 years ago.
All of this has fed into the new, improved “Out of Africa 2: The revenge of multi-regionalism.” It makes some concessions, admitting there could have been other migrations prior to the crucial one 60,000 years ago and that there was some interbreeding with other hominins in different regions. However it is still broadly the same postulating that the migration of humans which resulted in modern populations occurred 60,000 years ago.
So, now can we say it’s settled? After all, we have genetic evidence confirming OoA2, fossil humans only successfully appear outside of Africa after 60,000 years ago and so do human tools. All the evidence we have is consistent with OoA2, so can we hang up our hats and be done with the thing?
Two researchers, Dennell and Petraglia, think not. They’ve written in Quaternary Science Review that whilst the evidence is consistent with OoA2 it does not preclude other explanations. For example, although fossil humans do only appear after 60,000 years ago there is not a sudden abundance of finds. They remain infrequent, with only a handful of teeth and a couple of skull bones being found in China, for example. If there was a sudden increase in number then it would be fairly safe to say that this is evidence of a migration. The relative scarcity of finds, however, means that they do not preclude other explanations. There could’ve been an earlier migration, for example.
A summary of the evidence for early humans for around the world. Note how there is not a sudden explosion of evidence during the grey period, when migration supposedly occurred.
They also note that the genetic studies wouldn’t be able to detect the population history of extinct populations, which may reveal that there was an earlier migration which produced a lineage that only died out recently. There’s also the fact that these genetic studies (for some reason which escapes me so they could be just making this up) can’t detect migration earlier than 60,000 years. As such they may be missing out on crucial earlier migrations.
Further, there are also a handful of finds which don’t appear to fit the classic OoA2 model. A Chinese mandible, for example, that appears to be human was dated to ~125,000 years ago! However, here things start to a bit fishy since it is often hard to draw the dividing line between humans and the various archaic groups which came before them. Whilst the paper discussing the discovery of the Chinese mandible does ultimately label it as human it notes “it also exhibits a lingual symphyseal morphology and corpus robustness that place it close to later Pleistocene archaic humans.” As such I’d take these anomalies with a pinch of salt since they don’t seem to be definitively human. However, they still raise some interesting questions.
Qualms over the anomalies and genetics aside* this paper is essentially a call to not be dogmatic. Whilst OoA2 does fit with the evidence it does not preclude other ideas and so should something appear contrary to it we should not reject it off hand. This is a worthwhile message regardless of what you ultimately think about OoA2. That said, it hasn’t been refuted…yet.
|Robin Dennella, Michael D. Petragliab (2012). The dispersal of Homo sapiens across southern Asia: how early, how often, how complex? Quaternary Science Reviews, 15-22 DOI: 10.1016/j.quascirev.2012.05.002|
*if someone knows more about either of them and whether they support what is being said please do share.
Researchers have produced new DNA evidence that almost certainly confirms the theory that all modern humans have a common ancestry. The genetic survey, produced by a collaborative team led by scholars at Cambridge and Anglia Ruskin Universities, shows that Australia's aboriginal population sprang from the same tiny group of colonists, along with their New Guinean neighbours.
The research confirms the “Out Of Africa” hypothesis that all modern humans stem from a single group of Homo sapiens who emigrated from Africa 2,000 generations ago and spread throughout Eurasia over thousands of years. These settlers replaced other early humans (such as Neanderthals), rather than interbreeding with them.
Academics analysed the mitochondrial DNA (mtDNA) and Y chromosome DNA of Aboriginal Australians and Melanesians from New Guinea. This data was compared with the various DNA patterns associated with early humans. The research was an international effort, with researchers from Tartu in Estonia, Oxford, and Stanford in California all contributing key data and expertise.
The results showed that both the Aborigines and Melanesians share the genetic features that have been linked to the exodus of modern humans from Africa 50,000 years ago.
Until now, one of the main reasons for doubting the “Out Of Africa” theory was the existence of inconsistent evidence in Australia. The skeletal and tool remains that have been found there are strikingly different from those elsewhere on the “coastal expressway” – the route through South Asia taken by the early settlers.
Some scholars argue that these discrepancies exist either because the early colonists interbred with the local Homo erectus population, or because there was a subsequent, secondary migration from Africa. Both explanations would undermine the theory of a single, common origin for modern-day humans.
But in the latest research there was no evidence of a genetic inheritance from Homo erectus, indicating that the settlers did not mix and that these people therefore share the same direct ancestry as the other Eurasian peoples.
Geneticist Dr Peter Forster, who led the research, said: “Although it has been speculated that the populations of Australia and New Guinea came from the same ancestors, the fossil record differs so significantly it has been difficult to prove. For the first time, this evidence gives us a genetic link showing that the Australian Aboriginal and New Guinean populations are descended directly from the same specific group of people who emerged from the African migration.”
At the time of the migration, 50,000 years ago, Australia and New Guinea were joined by a land bridge and the region was also only separated from the main Eurasian land mass by narrow straits such as Wallace's Line in Indonesia. The land bridge was submerged about 8,000 years ago.
The new study also explains why the fossil and archaeological record in Australia is so different to that found elsewhere even though the genetic record shows no evidence of interbreeding with Homo erectus, and indicates a single Palaeolithic colonisation event.
The DNA patterns of the Australian and Melanesian populations show that the population evolved in relative isolation. The two groups also share certain genetic characteristics that are not found beyond Melanesia. This would suggest that there was very little gene flow into Australia after the original migration.
Dr Toomas Kivisild, from the Cambridge University Department of Biological Anthropology, who co-authored the report, said: “The evidence points to relative isolation after the initial arrival, which would mean any significant developments in skeletal form and tool use were not influenced by outside sources.
“There was probably a minor secondary gene flow into Australia while the land bridge from New Guinea was still open, but once it was submerged the population was apparently isolated for thousands of years. The differences in the archaeological record are probably the result of this, rather than any secondary migration or interbreeding.”
The study is reported in the new issue of Proceedings of the National Academy of Sciences.
Australia's archaeological record provides several apparent inconsistencies with the “Out Of Africa” theory. In particular, the earliest known Australian skeletons, from Lake Mungo, are relatively slender and gracile in form, whereas younger skeletal finds are much more robust. This robustness, which remains, for example, in the brow ridge structure of modern Aborigines, would suggest either interbreeding between homo sapiens and homo erectus or multiple migrations into Australia, followed by interbreeding.
The archaeological data also indicates an intensification of the density and complexity of different stone tools in Australia during the Holocene period (beginning around 10,000 years BP), in particular the emergence of backed-blade stone technology. The first dingos arrived at around the same time, and it is thought both were brought to the continent by new human arrivals – leading to theories of a secondary migration that has resulted in disputes regarding the single point of origin theory.
Materials provided by University Of Cambridge. Note: Content may be edited for style and length.
Cite This Page:
University Of Cambridge. "New Research Confirms 'Out Of Africa' Theory Of Human Evolution." ScienceDaily. ScienceDaily, 10 May 2007. <www.sciencedaily.com/releases/2007/05/070509161829.htm>.
University Of Cambridge. (2007, May 10). New Research Confirms 'Out Of Africa' Theory Of Human Evolution. ScienceDaily. Retrieved March 11, 2018 from www.sciencedaily.com/releases/2007/05/070509161829.htm
University Of Cambridge. "New Research Confirms 'Out Of Africa' Theory Of Human Evolution." ScienceDaily. www.sciencedaily.com/releases/2007/05/070509161829.htm (accessed March 11, 2018).