Monday, January 18, 2010

Very low human population size in the Lower Pleistocene

From the NY Times, reporting on a study estimating the "effective" population size of hominins ca. 1.2 million years ago based on three versions of the human genome:

They put the number at 18,500 people, but this refers only to breeding individuals, the “effective” population. The actual population would have been about three times as large, or 55,500.

Comparable estimates for other primates then are 21,000 for chimpanzees and 25,000 for gorillas. In biological terms, it seems, humans were not a very successful species, and the strategy of investing in larger brains than those of their fellow apes had not yet produced any big payoff. Human population numbers did not reach high levels until after the advent of agriculture.

That notion that humans were not very successful is an interesting one, especially considering how far-ranging they were by that point. By that tims, hominins were found all over Africa, all the way to Indonesia, along the northern Mediterranean rim and even as far septentrionally as the Republic of Georgia (i.e., the Dmanisi specimens). I'll be curious to see how the study accounts for this, unless this is it:

But that estimate would apply to the worldwide population only if there were inbreeding between the humans on the different continents. If not, and if modern humans are descended from just one of these populations, like Homo ergaster in Africa, then the estimate would apply only to that.

Richard G. Klein, a paleoanthropologist at Stanford, said it was hard to believe the population from which modern humans are descended was as small as 18,500 “unless they were geographically restricted to Africa or a small part of it.”

Can't wait to read the paper.


Maju said...

For some reason I don't seem able to access NYT (neither with Firefox nor Epiphany browsers). But for what I've read here and elsewhere it seems indeed that they are talking of the direct ancestors of H. sapiens, as not even the Neanderthal genome seems to have been taken into account (right?). That would mean some 50,000 H. ergaster at that time leading to our species. That population size seems reasonable to me for Africa or rather parts of it (i.e. the Eastern/Southern Africa most likely ancestral homeland).

In fact Atkinson-2007 suggests even smaller effective population sizes for our species before the Eurasian expansion. In comparison this estimate appears quite large.

Toos said...

Maju, take a look at about this subject. However interesting, but to you especially in relation to your last sentence?

Maju said...

Sure, interesting, Toos. I always try to keep a healthy skepticism and I look forward to that paper. They are just reconstructions: nothing is written on stone in this matter.

Andrew Oh-Willeke said...

The low populations are striking, but less so when one considers the sparse populations, e.g., of Cape Khoisan at the time they first encountered Europeans at the Cape of Good Hope in 1652, or populations densities of other top predator species. Hunter-gatherer societies don't get very dense. They make North Dakota look positively urban by comparison.

The African population only caveat is also notable. Co-evolution makes it likely that Africa large mammals would have been evolved to deal with hominin threats better than those of Eurasia. So, the population densities for the Africa populations from which we evolved may have been lower than that of Eurasian Homo Erectus populations.

It also seems fair to guess that Homoe Erectus weren't as effective at hunting as (and hence prone to have lower populations densities than) either modern hunter-gatherers, or the Neanderthals (whose effective population size can be estimated at an order of magnitude level from the handful of DNA samples we have from them and is also quite low).

The other thing a low population size does, as we try to imagine historically, is to emphasize the importance of individuals, which so often gets lost in pre-history.

In a society that small (comparable to the population of a single state house or city council district), you don't have to be somebody all that extraordinary to be a consequential pre-historical figure, leading your band to places that have never seen hominids before, overcoming a natural disaster or disease outbreak (or not), dealing with extended family (to the extent that mattered).

Maju said...

The low densities of Neanderthals and Sapiens in Paleolithic Europe (before Magdalenian) were surely similar, as I discussed here.

Otherwise I agree.