Analyzing Human Expansion

Given what I recently had to say about my thoughts on the age of our species, I thought at least some people might find this genetics paper interesting. Following is an excerpt from the abstract:

Here, we present a dynamic genetic model of human settlement history coupled with explicit geographical distances from East Africa, the likely origin of modern humans. We search for the best-supported parameter space by fitting our analytical prediction to genetic data that are based on 52 human populations analyzed at 783 autosomal microsatellite markers. This framework allows us to jointly estimate the key parameters of the expansion of modern humans. Our best estimates suggest an initial expansion of modern humans ∼56,000 years ago from a small founding population of ∼1,000 effective individuals. Our model further points to high growth rates in newly colonized habitats. The general fit of the model with the data is excellent.

This is so thoroughly in agreement with my thoughts on the matter that one might wonder if I were a co-author of this paper if one didn't know better: we are a very young species which has been successful from the very beginning of its inception. Especially pleasing is that rather than rely on the old Y chromosome and mtDNA standbys, these researchers actually looked at a robustly large number of autosomal markers collected from a healthy number of different populations. Later on in the course of the paper, its authors have the following to say:

Consideration of only the simulations within the 95% CI leads to an average colonization time of the world of 2,243 ± 227 generations. Under the assumption of a 25-year generation interval, this translates into an estimate of the initial expansion of modern humans from East Africa ∼56,063 ± 5,678 years ago. This estimate suggests that humans started expanding shortly before they crossed into Eurasia (an event believed to have occurred ∼45,000–75,000 years ago), a long time after the earliest fossil evidence for anatomically modern humans (∼160,000–195,000 years ago).

In other words, the genetic evidence tells a story much more in keeping with my own ideas about human origins and subsequent expansion than the traditional take one gets by looking at bones alone: something happened 50-80,000 years ago to set the very small collection of individuals who were our shared ancestors on a course of rapid population expansion, shortly after the onset of which some of them started to pour out into Eurasia and beyond. Whatever that change was, the data says clearly that it cannot have been merely cultural, while the archaeological evidence also makes clear that it wasn't something one could read merely from looking at brow ridges, molars and so forth. My bet is that it was a cognitive change, and if the attempts to reconstruct the Neandertal genome bear fruit, I imagine that this assumption will be confirmed surprisingly soon.

PS: An anthropologist by the name John Hawks has interesting things to say about the uniformity of Acheulean stone technology across Africa, Europe and Asia over more than 1 million years.

The maintenance of a single cultural tradition across much of three continents over a million years by exclusively social transmission seems incredible. Some have suggested that the handaxe is hardwired into the human genome, a proposition that seems even less credible (at least, to me). Absent these means of transmission, we are left with the proposition that the handaxe did not fade from the earth because of its functional utility -- either it was the tool that did the job the best, or it was the best tool that humans were capable of making that did the job adequately.

Now, the question you have to ask yourself is just how "human" creatures incapable of bettering the simple stone handaxe over a million years could possibly be; they may have looked like us, but it's clear they didn't think like us, and the timespans under consideration rule out "culture" as the limitation here. Indeed, as Hawks suggests, at this point it isn't even clear that such a thing as "culture" (and its attendant variation across time and space) existed in a meaningful sense until about 80,000 years ago.

Comments

[gene berman]

Could you lay out a supposition of some cognitive change stimulating out-migration that would show up as the result of reconstruction of the N genome? I can think of a number of developments with such stimulant-potential but they're all in the category of social (rather than some specific, physically-related cognitive) advancement.

[Abiola]

"Could you lay out a supposition of some cognitive change stimulating out-migration that would show up as the result of reconstruction of the N genome?"

I'd guess that it would either be a change in our ability to use language, or else a newfound propensity to want to try new things, even when what we know seems to work well enough. Perhaps the male intoxication with "new, better" gadgetry for its own sake is a manifestation of this new trait ...

"I can think of a number of developments with such stimulant-potential but they're all in the category of social (rather than some specific, physically-related cognitive) advancement."

The problem with the social argument from where I stand is that one then has to explain why our ancestors from so relatively recently had only an effective population size of ~1,000 people, even though we know closely related groups of Homo were around at the time, many skeletally indistinguishable from this one. One also then has to explain what possible social change would lead even toddlers to engage in the sorts of random creative acts that supposedly fully modern humans didn't for more than 100,000 years - look at the stone toolkits of these "modern" humans and you'll be shocked by just how uniform they are over such an enormously long period, as if it simply never bothered them to keep doing the same thing over and over, generation after generation, in the way it would bother almost any of us. Perhaps it really didn't, and the capacity for boredom is in fact a very recent addition to our mental kit.

[gene berman]

There's a Yiddish word--nooge--I guess related to "nudge." And that was my intention: to gently nudge you in the direction you've chosen (above), which (I'd guess) should properly work to get you back as a contributor or commenter on such matters over at GNXP.

[Abiola]

"should properly work to get you back as a contributor or commenter on such matters over at GNXP"

As long as the likes of "godlesscapitalist" and Jason Malloy are still using GNXP as a soapbox for their obsessions with "proving" black mental inferiority, there's absolutely no chance of my doing so. Human population genetics is a fascinating subject, but I'm not going to do anything to legitimize those willing to use all sorts of specious arguments and junk data to push their racist agendas.

[sophia]

Abiola,

John Hawks has published several papers which do seem to support a modified multiregional hypothesis ("adaptive introgression", possibly from Neandertals, or other archaic human pop'ns). I wonder what you make of them. I admit that I am out of my depth on these subjects and thus do not feel confident to venture an opinion.

Sophia

[Abiola]

Hawks has lots of ideas, some of them on the mark and some less so. I put his claims for soft multiregionalism in the latter category, as there's not a shred of evidence for it, and to the extent he buys into such theories he's definitely in a tiny and ever-diminishing scientific minority.

The basic problem with multiregionalism-lite is that it's like the "God of the gaps" argument, in that no matter how much evidence one musters against it, one can never definitely disprove the assertion that Neandertals interbred with humans but the evidence just hasn't been found yet, or that the Neandertal genes were simply overwhelmed by the human contribution (the net effect of this latter claim would be exactly the same as no interbreeding). Soft multiregionalists can always immunize their position against falsification in a way the more hardcore types cannot, and to the extent that they do their claims have left the realm of science (I haven't read all of John Hawks' work, so I'm not about to accuse him of belonging to this group).

[radek]

I can't say that I know about the genetics but the paper itself makes a lot of sense. The way I understand it is this;
Say human population in 8000BC is 5 mil. Then
(5mil/L0)^(1/n)=1+r
where L0 is the founding population, n is the time between the initial outmigration and r is the average growth rate of world population, taken over all demes weighted appropriately. L0, n and r are then parameters to be estimated. The authors then use the structure of the model and genetic variance to find the best fitting values of these parameters and that's how they get their numbers.
A lot of the parameter values though make sense from point of view of economic/demographic considerations. The estimated growth rate of population fits pretty well with what other folks've estimated for example. Furthermore "Our model further points to high growth rates in newly colonized habitats" fits pretty well with a Malthusian type model with migration - I've got a dinky one of my own that I made up for other purposes but it definetly fits this pattern.
The assumptions of an assumed pop growth rate (logistic is plausible but why?) - which should be a function of Malthusian pressure - and of constant migration rate which should respond to pop/resource differentials - as the authors themselves note - could be relaxed. Also the thing about carrying capacity - but this would involve some serious value judgements and here I think assuming same K for all demes is a safe way to go.