Homo sapiens arose in Africa 195,000 – 160,000 years ago, emerging from our archaic predecessors. Nowadays we’re an international species, with an outpost on every continent (and even one in space). How we changed from one state of affairs to the other is the subject of great debate, with many competing hypotheses attempting to explain how we transitioned from being a purely African ape to a worldwide one.

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 (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.

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.

Quaternary Science Reviews , 15-22 DOI: Robin Dennella, Michael D. Petragliab (2012). The dispersal of Homo sapiens across southern Asia: how early, how often, how complex?, 15-22 DOI: 10.1016/j.quascirev.2012.05.002

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*if someone knows more about either of them and whether they support what is being said please do share.