Is the Out of Africa model of human migration right?

ResearchBlogging.orgHomo 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.

Out of Africa and Multi-regional model

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.

And OoA reigned supreme!

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.

summary of evidence

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.

About these ads

17 thoughts on “Is the Out of Africa model of human migration right?

  1. Doesn’t it seem strange to anyone else that humans (of the just-like-us variety) would simply hang around East Africa for that huge expanse of time — 200,000 to 60,000 years — before any band of adventurers among them would strike out in a northwesterly direction? One hundred forty thousand years, and not one of them with the Magellan spirit combined with being able to persuade a few to follow him/her?

    • Humans appear to have originated underneath the Sahara and so could’ve only left Africa when that desert was more humid and thus tolerant of life. Such periods of humidity did occur around the same time as the first Middle Eastern finds appear (~135-115 kya) suggesting that people did leave Africa almost as soon as they were able to.

      Of course, this early migration appears to have failed and it was not until later that it became humid again and another could be attempted (assuming some people didn’t move to North Africa during the first period of humidity and thus could’ve migrated whenever they wanted, regardless of what the Sahara was doing).

      • i think a more humid climate would if anything make living in Africa more bearable, and thus make humans less likely to want to migrate out?

        • People naturally migrate because populations grow and it’s pretty beneficial to find an area without competition. The changing climate simply provided new competition free areas for them to find.

  2. “Yet humans disappeared from these settlements ~65,000 years ago, being replaced by neanderthals”

    Neandertals are humans, as well as denisovans or other ancient human populations.

    • Surely what you call them is ultimately irrelevant. Whether it’s “humans” and “neanderthals” or “human group A” and “human group B,” there was a populating in the Middle East that vanished and another appeared in its place.

      That said, I’m unsure whether neanderthals can really be categorised as humans in the strict sense. On the other, I’m not sure they’re completely distinct. Taxonomy is irritating like that.

  3. Bear in mind that nobody decided to migrate. They just followed game & good habitats. Sometimes that would work out; sometimes it wouldn’t. We would expect small groups to go in any given direction in search of food but small groups also often go extinct through random chance. They also probably would move on if a place wasn’t good – maybe not much rain for a few years or whatever.

    The identity of the Chinese “early modern humans” is unclear and, in some cases, so are the dates.

    • If I recall correctly the spread of farming from the fertile crescent into Europe was via similar means. I think there is a distortion in popular culture over how we perceive expansion. Everyone knows Columbus but nobody remembers Joe who moved to the next valley so there’d be more space for his cows, despite the fact the latter is actually the driving force of human expansion.

  4. Attention : Adam Brenton.

    Yes: Another Model known as the Kumari Model exists and is described in the book “Aryans-Who Are They ?, by A.R.Vasudevan ” . In simple terms it is like this, Africa and Kumari Land (now submerged in the Indian Ocean) had land connection before 60,000 years ago. Modern humans rose in the Kumari Land some 200,000 years ago. As population grew in size, it broke into two groups, the breakaway group moved towards the Antarctic continent and settled in the polar region. Till 60,000 years ago there was free movement of people between Kumari Land and Africa and the African population was a subset of the Kumari population. If Kumari Land had not been swallowed by the sea,we should be having the same archeological and genetic evidence in Kumari Land as in Africa. The breakaway group rejoined the ancestral group of Kumari after a long time , after which a third group with Mongolian features emerged due to interbreeding.

    The Kumari Model described above might not have seen the light of the day but for a significant genetic finding by the Genographic Project. The finding is that the human species almost broke into two before rejoining after a separation of 100,000 years. This fits in perfectly with the Kumari Model and we can explain many things not known so far. The group that stayed in Kumari grew darker and the group which lived in the polar region acquired near Nordic features during the 100,000 years of separation. The colour problem of modern humans has been troubling many brilliant scientists and now we have an answer.
    There is another Genographic finding which is a radical departure from earlier assumptions. In the OoA model the Europeans should be genetically closer to Africans than South Indians. According to the new finding the South Indians are closer to Europeans than to Africans. Accordingly the project has redrawn the migration map showing the Eurasian migration starting from India. This is what exactly the Kumari Model says.

    It is heartening to note that archeological and other evidences are mounting day by day in favour of Kumari Model, which bodes well for clearing unsolved riddles.

    • Do you have any other academic sources that provide evidence for what your saying? The stuff you’ve mentioned so far is far from convincing. For starters you’ve got the fact that since Europeans and Indians are part of the same out of Africa migration; they’re going to be more closely related to each other than to Africans under the OoA model (in fact all non-Africans are more closely related to each other than they are to Africans).

      Further the geneographic project supports the OoA and has Europeans travelling through Central Asia en-route to Europe, not originating in India. Also there seems to be no evidence of the mystical continent hidden under the Indian ocean. 200,000 years ago there was a land bridge between Asia and America, even one between Britain and continental Europe, but nothing in the Indian Ocean.

      Finally you got my name wrong.

  5. Attention : Adam Brenton.

    Please note a small correction to my above post,

    In the above description , the sentence,
    “According to the new finding the South Indians are closer to Europeans than to the Africans”.

    to be read as
    ” According to the new finding the Eurasians are closer to South Indians than to Africans”.

    A.R.Vasudevan

  6. Attn: Adam Benton.

    Many thanks for your prompt response. Sorry for getting your name wrong.

    I have just said what is the Kumari Model about without giving any supporting evidence. The book contains all the necessary evidence and arguments to support the model.

    Luigi Luca Cavalli- Sforza has said “Cultural transmission is thus an important object of study, one that has been dramatically neglected.”

    Therefore, in my book, I have heavily leaned on the cultural side, developed a hypothesis and validated the same through population genetics. It is this approach that has been advocated in a paper by David Reich et al which is as follows.

    “We warn that models in population genetics should be treated with caution. Although they provide an important framework for testing historical hypothesis, they are oversimplifications”.

    Earlier the Geographic Project did support the OoA and said Europeans travelled through Central Asia enroute to Europe, not originating in India. But in their press release dated 02 Nov 2011, it is stated “The divergence of common genetic history between populations showed that Eurasians groups were more similar to populations from Southern India than they were to those in Africa. This supports a southern route of migration from Bab-el-Mandab strait in Arabia before any movement heading north and suggests a special role for south Asia in the out of Africa expansion of modern humans”. Accordingly they have redrawn the map of migration where migration of Eurasians starts from South India. This is a radical departure from their earlier stand where the northern route went to Central Asia straight rather than through South Asia. This makes the Eurasian migration shown in the book gain scientific acceptance.

    Another finding from Genographic Project is “Ancient humans started down the path of evolving into two separate species before merging back into a single population. The two populations lived in isolation for 10,000 years”. According to the Kumari Model, the African population was a subset of the Kumari population till 60,000 years with a common genetic history. Therefore the significant finding of Genographic Project fits in perfectly with the Kumari Model which says that the split group lived in the Antarctic region. When a group of modern humans lived in the polar region for 100, 000 years, they should have acquired Nordic features, whatever was their skin colour to start with. This is a significant result, solving the colour and racial problem which has been troubling many outstanding scientists.

    There is yet another Genographic finding which states that Southern India’s caste system predates arrival of Indo-Europeans. This is again in conformity with what is said in the Kumari Model. According to Kumari Model caste system was necessitated by the Neolithic revolution and being indigenous had nothing with the arrival of Indo-Europeans.

    Now coming to the question of mystical continent, one has to look at the Indian Ocean in any atlas. There is a long line of islands starting from Lacadives, Maldives, to Chagos and also further on. It has been established that around 14,000 years ago the sea level was 130 meters lower than it is now which means a large habitable area was exposed. Some authors have drawn the inundation maps showing the unsubmerged area which is quite substantial to support the Kumari Model.

    Just because plate tectonics, sea floor separation, continental drift etc were suggested, it does not mean that other disasters cannot occur. The Toba eruption was a mega eruption spewing out 3000 cubic kilometers of magma and ash. When such a huge quantity comes out at some place the earth’s top crust at some other place could have come down. Recently I was reading as to how the English Channel was formed. It is said that some areas are still slowly sinking at a slow rate. We do not know for certain what havoc the Tobo incident caused.

    The Andaman – Nicobar Islands is home for ancient aboriginals. They are called living fossils, who were cut off from the main population around 60,000 years ago. Obviously there should have been a land connection which was cut off isolating them from the main body of people in Kumari. Why cannot we accept a similar situation with respect to Africa and say that the land connection was cut off around 60,000 years ago.

    A.R. Vasudevan.

    • Firstly, the genographic project shows that the migration into Europe split off from the main thrust out of Africa in the Middle East, Western Asia and Northern Asia (resulting in Mediterranean, Central Europeans and Scandinavians respectively). It does not indicate that any of these population movements started in India. Go look at the map yourself, there’s no arrow leading from India to Europe. https://genographic.nationalgeographic.com/human-journey/

      Secondly, as I said before the genetic bottle-necking effect associated with migration means all non-Africans will be more closely related to each other than Africans. Thus the fact you’ve discovered Indians are more closely related to Europeans than Africans is neither here nor there.

      Thirdly, the populations which were isolated and almost split into two species are two African populations, not African and non-Africans as your model requires. Thus this discovery doesn’t provide any support for the Kumari model either.

      Then there’s the claim that the caste system existed in India prior to Indo-European migrations. This is true, but the migrations they’re referring to are those which occurred in the Holocene, over 50,000 years after the Out of Africa movement. As such these results do not indicate that India was inhabited my modern humans prior to OoA.

      Finally, whilst I’m no oceanagrapher the topographic maps of the Indian Ocean I’ve seen show that the sealevel would have to drop by 3,000 metres to reveal a landmass even half the size of something you’re talking about. And such a decrease has not occurred in the entirety of human existence, so we can’t have evolved there.

      http://upload.wikimedia.org/wikipedia/commons/3/36/Carlsberg.svg

      If the Kumari hypothesis is valid it would cause a fundamental re-evaluation of human evolution. So I urge you to provide evidence for it, rather than repentantly mis-interpreting the results of the genographic project.

  7. I’m not sure of why this is so hard for me to write on, I have a midterm essay about the two competing hypotheses, and what Southeast Asia has contributed to the argument. I know there are some pretty important finds within SE Asia, but for some reason I am hitting a wall. I’m a bioanth major so I should just spit this out! Can you please help?! So, Multi-regional evidence entails what? H. sapiens left Africa 1.8 mya and meet up with erectus and interbreed to produce modern H. sapiens? And I don’t understand the evidence behind this.

    • Multiregionaism has been settled for a few decades now. Any recent work only mentions it as a historical curiosity, the same way an evolution textbook might talk about Lamarkism. If you want decent resources on multiregionalism you have to look for old stuff. Early 90s at a minimum, much earlier if possible.

      The idea of multiregionalism is that regional variants of earlier hominins led to the modern humans living there today. Asian Homo erectus evolved into modern Asians, European H. erectus into Europeans etc. The main evidence for multiregionalism was fossil continuity, with many claiming that the regional variants of Homo erectus on each continent are connected to the modern humans native to that region. The wikipedia page on “multiregional origin of modern humans” has some nice case studies that might be a good starting point.

      SE Asia was important as it was one of the last regions to “abandon” the multiregional hypothesis. By the 1980s most people were skeptical that European species (like Neanderthals) gave rise to modern humans. See stuff like Mellars’ paper “Major issues in the emergence of modern humans” for details on why. It took a lot longer for Asia to reach the same conclusion.

      The evidence from SE was also a fair bit stronger, and so was a bigger blow to multiregionalism when it was finally defeated. Also, a fair bit of the data in support of out of Africa (particularly genetic) comes from there as well.

      Hope that helps.

  8. Attn : Benton

    A.R.Vasudevan:
    You are seeing the following migration map which I am not referring to.

    Your remarks are : “The discovery that they are talking about is the bottom arrow leading to Australia. Note how it does not go to Europe. Also note the lack of arrow leading from India to Europe”.
    The map that I am talking about is the one given in the Genographic press release “ IBM News room -2011-11-02”, as shown below. You can see the arrow marks from India to Europe. Please check and confirm.

    • So your basing your idea on an out-of-date map of human migration? That would certainly explain a lot. And even putting that aside, it still shows a large chunk of the migration splitting off prior to Southern India; which is not what would be expected given the Kumari hypothesis.

You evolved too. Have a say.

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s