Anthropology theories: Multiregional or Out of Africa?

October 28, 2004 2:53 PM

Multiregional or Out of Africa?
posted by four panels to Science & Nature (13 answers total)

Out of Africa is pretty close to a consensus theory, as I understand. The discovery of Homo floresiensis could be consistent with this theory as follows: H. erectus is know to have migrated at least as far as Java. It's possible that certain populations migrated further, either by boats (this would be a big deal, as only H. sapiens is known to have used boats) or by temporary land bridges (common in the Indonesian islands, but not previously thought to be capable of bridging the relatively deep water between Java and Flores). If a population of H. erectus did make it to Flores, and was subsequently isolated, the process known as island dwarfing could have resulted in Flores Man.
posted by mr_roboto at 3:15 PM on October 28, 2004

I thought that back in the day, the going theory is that all of the land masses were one big land mass allowing humans in whatever evolutionary branch they were in at the time to travel.
Also, in all of my genetics and biology classes, they nearly soley present only the Out of Africa theory, though there is a case to be made for the Multiregional theory. I could go more into it, but from skimming, the readings cover the theories better than I can.
posted by jmd82 at 6:07 PM on October 28, 2004

Both?

Basically, as I remember it from first-year anthro., multiregionalism involves 'gene flow' among populations: i.e. interbreeding. So, a multiregionalist would say, as new populations of anatomically-modern humans entered the territories of archaic humans 30-90,000 years ago, to some degree they would have merged through sexual contact, out-marriage, etc.

A strict 'Out of Africa' interpretation, on the other hand, would involve the complete replacement of archaic populations everywhere with no (or insignificant) interbreeding.

Another interesting feature of the dialectic is that 'Out of Africa' adherents tend to specialists on Western-European prehistory, whereas multiregionalists (to the extent that there are any left) tend to be more focussed professionally on China and South East Asia. So you could say that the two camps are generalizing from their own (maybe quite different?) sets of data.

I'd imagine that this new discovery's going to place a lot more focus on (the slightly neglected) field of Asian prehistory, which can only be good for the discipline as a whole.
posted by Sonny Jim at 6:57 PM on October 28, 2004

jmd82, I'm not sure what you're saying about land masses.

At various times in the past 10 million years there have been land bridges between continents due to lower sea levels when lots of water was locked up in ice. So that would allow human migration during those windows. But there hasn't been one big land mass (Pangaea) for a lot longer than that.
posted by i_am_joe's_spleen at 7:18 PM on October 28, 2004

Multiregionalism is essentially dead. Milford Wolpoff is perhaps its only real champion, and John Relethford posits a modified and reduced-importance version (essentially yes, out-of-africa, but with some limited gene flow between indigenous and migrant populations). Nearly everyone else advocates Out-of-Africa again and again.

A likely first movement out of Africa is by the Dmanisi hominids, whether Homo erectus, habilis, or otherwise, around 2 million years ago. Another movement of classic H. erectus after that moving eastward through Asia, which eventually deposited the ancestors of H. floresiensis on Flores. Other movements of H. heidelbergensis followed in the last million years, and H. neanderthalensis probably developed in Europe and the Near East from one of those movements. Humans didn't leave Africa until ~100,000 years ago; an intermediate form between older forms (think H. heidelbergensis) and anatomically modern H. sapiens has been found in East Africa dating to about 150,000 years ago, and the first anatomical moderns are found in Israel dating to about 100,000 years ago, eventually spreading north and west to Europe, forcing out the Neanderthals (or taking over space vacated by dying-off Neanderthals) by 35-40,000 years ago, and moving east, reaching Australia by ~50,000 years ago, and moving to the New World (probably from Siberia) either 30,000 years ago if you believe Monte Verde, and 15-20,000 years ago if you reject it.

The thing is, skeletal studies are, while all we have, notoriously variable. For instance, sheep and goats, while separate non-interbreeding species, share a nearly identical skeletal structure. Other animals, while incredibly variable in skeletal morphology, are able to and do interbreed successfully (think, say, dogs). So everything supported by skeletal evidence needs to be taken with a grain of salt. I don't want to sound too pessimistic, because paleolithic studies is essentially my chosen career, but we have to be realistic about the veracity of our data. Now, genetic studies are doubted out-of-hand by many paleoanthropologists because 1. they don't understand the genetics and 2. they overturn skeletal evidence. Studies by Krings et al (1999 and 2001, I believe) and Ovchinnikov et al (2000, Nature) have essentially shown that, based on numbers of genetic differences in corresponding DNA sequences, Neanderthals are somewhere intermediate between humans and chimpanzees (the average number of differences between humans and Neanderthals is between the average number of differences between humans, and between humans and chimps). More or less, they're closer to us, but still a distinct species. I believe another study, the citation of which I don't have off the top of my head, showed that ancient modern humans (contemporaneous with the Neanderthals studied) grouped with recent modern humans, NOT with the Neanderthals. Relethford dealt with this information semi-convincingly in his "a little multi-regionalism" theory, and showed with mathematical models that interbreeding couldn't be completely ruled out, but it's generally still dismissed as insignificant by paleoanthropologists and archaeologists.

The Flores guys are a perfect corollary to out-of-Africa. Homo erectus lands on the island 800+ thousand years ago, and because of isolation, evolves anagenetically (probably through drift and selection) into what we see up to 18,000 years ago with the Flores skeletons.

So, anyway, in sum: out-of-Africa again and again.
posted by The Michael The at 7:50 PM on October 28, 2004

and another thing, re: jmd82 and spleen-meister... spleen is right.

Yeah, Pangaea (that one-continent thing) hasn't been around for a few billion years, and for all of hominin evolution (since the split from chimps) the earth has been essentially in the form it is now, save for the aforementioned rising and lowering of sea levels creating and hiding land bridges and other land masses (at one point, the Mediterranean was completely or nearly dry!).

Thus, when considering hominin migrations, think of the world as it is now, with sometimes a little more land and sometimes a little less.
posted by The Michael The at 7:55 PM on October 28, 2004

Although, wouldn't sea-level be a lot more important in when considering South East Asian prehistory, what with Sahul and all, and the exposure of other significant landbridges during periods of low sea level?

Just saying.
posted by Sonny Jim at 8:11 PM on October 28, 2004

One of the reasons given for ruling out possible interbreeding between Neanderthals and modern Homo Sapiens is based on mitochondrial DNA studies. But, first, I believe that it's based on a very small sample of Neanderthal DNA, and also, all that that would tell us is that no modern Homo Sapiens are descended from female Neanderthals. Mitochondrial DNA does not speak to a possible male line of ancestry.

I think personally that we don't have enough evidence one way or another. But if pressed, I think it's both. I think that the spread of erectus and archaic Homo Sapiens to multiple regions does not preclude gene flow from one region to another. In other words, archaic populations in Asia and Europe may have had gene flow from African modern Homo Sapiens over a relatively short period of time, if the genes gave a survival advantage.

It's kind of controversial, but there may be shared characteristics between archaic and modern skulls in Asia. That would argue for a continuity between the archaic and modern populations that would not be explained by the strictly Out of Africa Hypothesis.
posted by geekhorde at 10:25 PM on October 28, 2004

> Thus, when considering hominin migrations, think of the world as it is
> now, with sometimes a little more land and sometimes a little less.

Here's a world map and a timeline. The australopithecines (African just-prehumans) are late pliocene.

Paleospecies migration in the period we're talking about would be largely influenced by the question "can I walk from here to there?" and although the miocene-pliocene-pleistocene world map basically looks modern, TMT's "sometimes a little more land and sometimes a little less" often took the form of the presence (or absence) of land bridges between areas otherwise separated by bodies of water. These bridges, which came and went, would obviously be critical for the walk/don't-walk issue. Here's George Gaylord Simpson's famous paper on the subject.
posted by jfuller at 6:02 AM on October 29, 2004

Thanks for the corrections, all.

Mitochondrial DNA does not speak to a possible male line of ancestry.

I'm not sure if I'm reading this right, but DNA analysis suggests the Y chromosome is a derivative of the X chromosome and the defualt regulatory pathways in humans is the female pathway. This further suggests that we arise from females, not the other way around.
posted by jmd82 at 11:11 AM on October 29, 2004

In other words, archaic populations in Asia and Europe may have had gene flow from African modern Homo Sapiens over a relatively short period of time, if the genes gave a survival advantage.

A survival advantage has nothing to do with interbreeding: these are ancient humans and pre-humans we're talking about, who had no idea whatsoever about genetics and selective advantages. Any interbreeding would have to get around morphological differences, which is essentially a nice way of saying "you don't look like me, so I'm not going to fuck you." Given how picky people are about mating in general now within a species, it's difficult at best to assume that it was a happy sex-a-thon between species with some drastically different features, especially in terms of cranial and facial morphology ("She had beautiful eyes, but that sagittal crest just turned me off..."). Which isn't to say that interbreeding didn't happen, but rather that we shouldn't assume that because it could have, it probably did.

The arguments that geekhorde makes about the interaction of migration and gene flow are made by, again, Relethford, so if you're interested in his justifications, check out his Genetics and the Search for Modern Human Origins.
posted by The Michael The at 12:49 PM on October 29, 2004

I'm not sure if I'm reading this right, but DNA analysis suggests the Y chromosome is a derivative of the X chromosome and the defualt regulatory pathways in humans is the female pathway. This further suggests that we arise from females, not the other way around.

That's kinda irrelevant here, as the X-Y duality had arisen way before hominins (~7-8 mya), mammals, or even vertebrates. As far back as the split from other apes, we've had both X and Y chromosomes, and there are traceable lineages of both (and thus traceable lines of X- and Y-linked traits).
posted by The Michael The at 12:57 PM on October 29, 2004

I don't know that people are really all that picky about interbreeding. Everywhere that different 'races' of people have spread, despite morphological differences, people have interbred. Also, not to be crass, but many people will have sex with anything that moves.
posted by geekhorde at 12:58 PM on October 31, 2004 [1 favorite]