Although a few holdouts continue to cling to ever more specious variations on the old "multiregional" theory of human origins, the evidence is by now overwhelmingly in favor* of the notion that all extant human populations are descended exclusively from a single ancestral population which existed somewhere in Africa during the last 100,000 years. At this point the only question is when exactly subsets of this population began to migrate out of Africa to populate the world, and, for reasons I shall explain shortly, I have long believed that any migration could not have occurred earlier than 60,000 years at the very most: what's interesting is that a new report in Science is giving some much welcome validation to what I've long been convinced of.
Geneticists and paleoanthropologists disagree about whether the ancestors of modern humans began to leave Africa as early as 100,000 years ago or as recently as 50,000 years ago. Although several recent genetic studies have supported a late exodus, fossil evidence has furnished few clues to the timing. Paleontologists have not found any fossils from the crucial period from the most likely source--sub-Saharan Africa.
It seems that they just had to take a closer look at a stunning skull that's been sitting on a South African museum shelf for more than 50 years. A team of international researchers led by paleoanthropologist Frederick Grine, of Stony Brook University in New York re-analyzed this skull, discovered in 1952 in a dry river bed near Hofmeyr, South Africa. Its age and identity had remained mysterious, because initial attempts to date it failed. Some researchers thought it could be less than 10,000 years old.The reasoning seems pretty convincing to me, and not just because it validates my preconception either. What is also noteworthy is that while the Hofmeyr skull resembles that of European and Asian fossils contemporary to it, none of these skulls look like those of modern Europeans or Africans. I emphasize this for a very important reason, namely the tendency on the part of many popularizers and (especially) illustrators to portray early modern humans in such a way that the impression is left on many minds that modern day Africans are somehow more "primitive" or more "unchanged" from our shared common ancestors (though features like straight hair and thin lips are actually shared with other apes, and therefore clearly "primitive" human characteristics), while simultaneously fostering a delusion that with a shave and a change of clothes, Europeans from 30,000 years ago would be indistinguishable from those of today. Modern "racial" characteristics probably date to no more than 20 or even 10,000 years ago - the earliest date I've yet seen for an identifiably "black" skeleton is only 9,000 years, while the only thing established by the "Kennewick Man" pseudo-controversy over supposedly European settlers preceding the Native Americans is that the ancestors of today's East Asian populations did not begin to look "East Asian" until relatively very recently, perhaps even only shortly before the emergence of the precursor to Chinese civilization some 6,000 years ago (the existence of the Ainu suggests much the same story). Once you understand that 15-20,000 years ago East Asians looked pretty much like Middle Easterners and Europeans, the absurdity of puffing up "Kennewick Man" as a European becomes clear (as does the fawning manner in which so many East Asians worship "white" features while denigrating their own ...)In this week's Science, the team reports that it used state-of-the-art optically stimulated luminescence and uranium series to pinpoint the skull's age to about 36,000 years. Although they could not use radiocarbon dating on the bone itself, the researchers dated the cementlike carbonate that coated the inside of the skull vault. Two different tests confirmed that the soil was deposited at one time, shortly after the fossil was buried.
Further analysis showed that this skull looks more like fossils of modern humans who lived in Europe and Asia about 36,000 years ago than fossils of Africans or Europeans from the past 10,000 years. That result suggests that the Hofmeyr skull is closely related to the modern humans who first swept into Europe and Asia, and that they all were the offspring of a source population in sub-Saharan Africa. The study also provides the first fossil evidence that the ancestral stock of modern humans left Africa recently. "If there was a very late migration out of Africa, you would expect Europeans at that time to look like Africans at that time," says Grine.
But enough about anachronistic racial stereotyping; I want to expand on what I said at the beginning of this post about my reasons for believing that human migration out of Africa couldn't have begun earlier than 60k years before the present. The reason for my stance is very simple, and amounts to this: I simply don't believe that our species is more than 80,000 years old at the most. This may sound heretical, given both the existence of "anatomically modern" skeletons much older than that, as well as the existence of mtDNA giving a most recent common ancestor age of as much as 200,000 years ago, but in the first place I reject out of hand the privileged place accorded mtDNA evidence as opposed to that from other parts of the genome which give much more recent numbers (e.g. the 60-80k given by the nonrecombining portion of the Y chromosome): if one wishes to date the emergence of a species using coalescent techniques, then surely a better candidate for said period will be the most recent date derivable by looking at all lines of genetic evidence, rather than simply picking one non-recombining piece of DNA while ignoring another. I also reject the assumption that the birth of a species as young and as creative as ours can be dated merely to some time when skeletons left behind began to look sufficiently like ours: in a way, this last assumption is the mirror image fallacy to that of the typical racist, in that where one believes that those who look different must by their natures be fundamentally different (and threatening), the other proceeds from the assumption that all those who look sufficiently alike must think and act alike. If modern humans really are 150-200,000 years old, what on earth were they up to during that period up to ~80,000 years ago when we first start seeing traces of evidence for modern human behavior? Why can we find no trace whatsoever of even the minimal amount of creativity to be found in the random graffiti and carvings of bored teenagers the world over? Why do we see instead that same quality of endless repetition on the same few themes to be found with Neandertal and H. erectus stone kits?
There is a reasonable way to resolve all these questions, and it is to start with the premise that ours is a *very* young species, which may have looked outwardly very much like other humanoid populations extant in Africa at the time, but nonetheless thought differently from them, in that special way which sets even Joe Plod apart from every other ape which previously walked upright on two legs. Furthermore, I maintain that not only is our species a very young one, but also that it has by evolutionary standards been a wildly successful one from the very beginning of its emergence, undergoing exponential** growth from day one rather than just from the dawn of agriculture, when all that changed was the exponential taking on a larger value. In short, this new (though outwardly unchanged) species exploded in numbers right across Africa's savannas and then beyond in the blink of an eye in evolutionary terms, taking no more than 10-20,000 years to undergo this whole process, and that is why we both see no evidence of modern creativity before 80,000 years ago while seeing what has often been claimed to be a "bottleneck" dating to around that period: without prior data to go on, there is nothing to distinguish a true genetic bottleneck from what one sees when a small founder population explodes in numbers.
Of course, I could very well be wrong, and I don't have the authority of a university chair to throw behind my assertions, but I'd be lying if I said I had a high expectation of being shown to have been mistaken in the essentials of my argument. The "anatomically modern" humans of 150,000 years ago could well have been mistaken for racially exotic humans of our day based purely on their appearance, and I'm almost certain they used spoken language to communicate, but that they thought in the way we do, or used language in the same creative manner all of us do, is something I consider out of the question, as is the idea that they could have been taught to do so if only they had the necessary teachers, and I feel certain the science will eventually back me up.
*If you have the necessary access, read this ScienceNow article, from which the following passage is quoted:
In the new study, geneticist François Balloux and colleagues at the University of Cambridge show that geographic distance from Ethiopia (the place where the oldest human remains have been found) correlates with the genetic diversity of 51 present human populations distributed worldwide. The research gives support to the theory that, as humans left Africa, some versions of their genes became progressively lost over the migration routes. Thus, populations farther from Ethiopia are characterized by lower genetic variability.Multiregionalism is dead, except amongst holdouts like Wolpoff who have the merit of their life's work at stake, and racist cranks who are even willing to believe in Neandertal ancestors rather than accept that their genes are more than just a mere subset of those of "lesser" races ...The team, which publishes its results today in Current Biology, further observed that genetic diversity decreases very smoothly along ancient colonization routes with increasing distance from Africa. Under a multiregional origin scenario there is no way such a decline in genetic diversity with increasing distance from East Africa could be observed, says Balloux.
**Albeit a very low exponential, and that from a minute base.
"and racist cranks who are even willing to believe in Neandertal ancestors rather than accept that their genes are more than just a mere subset of those of "lesser" races ..."
Abiola,
I love the thought of African ancestors (I really do!) but if Lahn is to be believed, we all got a little Neandertal in us too. I don't see how one theory (the basic African-origin one) precludes the possibility that 37,000 years ago, Neandertals introduced some genes into the human mix that were advantageous. What's the problem with that?
Posted by: sophia | January 23, 2007 at 07:33 PM
The problem with Lahn's hypothesis is that it isn't actually supported by the slightest shred of evidence, while there's tons of contradictory evidence both before and after he made his proposition. See, for instance, the report on recent progress by the team led by Svante Paabo, which has so far managed to reconstruct 1 million base-pairs of Neandertal genome sequence.
www.nature.com/nature/journal/v444/n7117/full/nature05336.html
The word from Paabo's lab is that absolutely *no* genetic sequence has been found which is shared between Neandertals and Europeans without the rest of humanity having it as well. Why then should we believe Lahn, when the overwhelming weight of the evidence contradicts his wild claims?
Lahn definitely has an eye for provocative claims which catch the media's attention, but the substance to them is often less than ... substantial. Common sense alone ought to have told people that Lahn's claim for Neandertal brain genes would be most unlikely to be true, seeing as the Neandertals who supposedly carried such genes were the ones who went extinct, not the humans who didn't. That so many people should have swallowed wholesale a hypothesis that a markedly less intelligent, extinct species gave brain genes to a more intelligent and flourishing one, somehow making it smarter in the process, is precisely what tells me that lots of folks will believe anything that can somehow set them apart from Africans.
Posted by: Abiola | January 23, 2007 at 07:47 PM