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July 15, 2003

The Great Leap

This New York Times article on the "Great Leap Forward" is a pretty good survey of the issues at hand. As stated, "anatomically modern humans" existed in Africa 100,000 years ago, but it wasn't until ~50,000 years B.P. that there was a ratchet up toward the hallmarks of modern sapiency. The breakneck speed of cultural evoution and the profusion of forms & styles that characterize our species seems to have lagged our outward humanity by tens of thousands of years.

What was the catalyst? Some theorists posit that an important regulatory gene-FOXP2-played a crucial role, in particular, in the development of the capacity for language. I'm pretty agnostic on this issue. Dr. Henry Harpending has indicated that it might be that not all modern humans made the cultural transition in toolkits that characterizes the "Great Leap," suggesting that if modern humans are genetically very similar (the result of a radiative expansion from African 50,000 years ago), it is more than just an alteration in in the FOXP2 gene in the African context.

Posted by razib at 06:14 PM

While the details of the genetic changes necessary for modern sapiency are still not known with certainty, W.H.Calvin's study of the climatic "crash boom boom" cycle gives a compelling explanation of the conditions that made the changes possible or necessary. (Summarized neatly here.)

Posted by: Michael Gersh at July 16, 2003 03:11 PM

That's a neat hypothesis, but why do you consider it a compelling explanation? Seems to me I could come up with about a half-dozen similarly plausible theories in an afternoon. Does the guy have any evidence for his theory? For example some citations to back up his claims on climate change, maybe some sediment cores that would show the changes in flora, or other evidence from the recent fossil record? That would at least show that the changes in the enviroment that he posits took place, though proving that they drove human evolution would still be a stretch...

Posted by: bbartlog at July 16, 2003 06:24 PM

well-it seems like spencer wells in the journey of man is leaning toward a calvinian explanation of some facets of human evolution. cavlin's specific theory might not be correct-but i think his method/direction has something going for it....

Posted by: razib at July 16, 2003 06:37 PM

Harpending has done plenty of excellent work, and he's the sort of researcher I'm willing to cut some slack in the hypothesizing department. In any case, my own reading has tended me towards the same conclusions. Just because we have fossils from 150,000 years ago that look like us doesn't mean the people in question actually thought like us. This is a point that Richard Klein has been making for quite some time, and in fact there is increasing genetic evidence that our species didn't just loll about for 100,000 years before suddenly undergoing a "creative explosion."

Interesting papers that support Harpending's hypothesis can be found here and here. Why everyone seems to have bought into the age for our species suggested by the mitochondrial MRCA (Most Recent Common Ancestor) is beyond me; it ought to be clear to anyone who knows what he's about that every gene will give you a different MRCA, so there's nothing sacred about the number indicated by mtDNA. Were we to look only at Cytochrome P450, we would obtain an age for the human species of several hundred million years, but I'd bet nobody'd be willing to buy that conclusion!

My own best estimate for the age of our species is that we've been around for no more than 80,000 years, tops.

Posted by: Juvenal at July 17, 2003 01:02 PM

This all depends upon one's definition of sapiency, and, does a less sapient being with our exact genetic makeup actually constitute a different species? Do we look at the development of better and more diverse tools, language and syntax, or the creation and appreciation of art as dispositive of the appearance of a new species?

Until or unless we can come up with a way to examine fossil DNA we will have to rely upon behavioral markers, which are discernible in the fossil record. We know that environmental changes brought on behavioral changes. Whether these were genetic changes is, at this point, conjecture.

Posted by: Michael Gersh at July 17, 2003 02:05 PM

"This all depends upon one's definition of sapiency, and, does a less sapient being with our exact genetic makeup actually constitute a different species?

Ah, but isn't the argument that they didn't have "our exact genetic makeup"?

Once we begin talking about populations as closely genetically related as all offshoots of erectus must have been, the traditional definition of "species" in breeding terms breaks down. I'd wager that we and the neanderthals, as well as any vestige populations of erectus that still existed when we came on the scene, would have been biologically capable of bearing fully fertile offspring, but the behavioral differences would have been such to prevent this actually occurring.

For my purposes, using the greatest lower bound for MRCA obtainable as the age of our species is as good a number as we're ever likely to get. Outside of the Rift Valley, Africa's soils aren't very conducive to fossilization, and there's no good reason to believe that our species didn't actually originate elsewhere on that continent. Consequently, the odds of our finding conclusive fossil evidence on this issue look slim to me - but maybe that's my bias as a computational biologist speaking :-)

Posted by: Juvenal at July 17, 2003 02:45 PM

But we gotta look at autosomes too...

i thought they did some prelim analysis of the autosomal DNA (regions of) and tended to support the mtDNA conclusions (i believe in the PNAS). of course, nothing conclusive....

i tend to support "eve" and its permutations as the most plausible, but i think that triumphulism of stringer et al. is premature and gives the public the wrong impression when they already ideological factors that favor eve (easy to fit into religious expalnations compared to more complex hypotheses like multiregionalism).

Posted by: razib at July 17, 2003 08:23 PM