March 24, 2009

Variability of anatomically modern humans and Out of Africa

This paper touches on a theme I have blogged about before, namely the fact that early Africans did not form a single mating population, a subset of which filtered into Eurasia, but were rather subdivided themselves. This is evidenced by the substantial variability of the first anatomically modern humans:
Our results thus revealed that shape variability of early AMH was highest among all tested groups, i.e., within a sample of the genus Homo embracing the last 1.8 million years. The shortest connections between early AMH are either with other specimens of this group or recent modern humans, for instance, Omo 2 [recently dated to ~195 ka (1)] and LH 18, two of the earliest east African candidates for the emergence of modern human morphology (18), and the Levantine Qafzeh 6 connect with recent Australian aboriginals (cf. ref. 19). We also find a connection between 3,500-km-distant sites in the Levant and northwest Africa, i.e., between the more archaic looking Jebel Irhoud 1 and Skhul 5, whereas Jebel Irhoud 2 connects to recent Europeans. Qafzeh 9 (Levant) is linked to a European UP specimen. We find, however, no single link between Neanderthals and AMH, including Upper Paleolithic specimens.

[...]
Our data on neighbors and variability is unsupportive of the strict forms of a single-origin model but does not conflict with another approach, the model of ‘‘isolation by distance,’’ which predicts that genetic and phenotypic dissimilarity increases with geographic distance (24, 29–31). The metapopulation framework would predict the same because frequency and magnitude of genetic exchange would follow the likelihood of 2 populations to meet, which declines with geographical distance from the early AMH epicenter in Africa. Our fossil AMH data, however, suggest that before there was isolation by distance from Africa, there already existed (at least temporally) isolation by distance within Africa during the Pleistocene.
But what about the observed low genetic diversity found in humans?
Genetic diversity among living modern humans is known to be very low when compared with extant apes (32, 33). To reconcile this observation with our proposed metapopulation model within Africa, it is necessary to assume that genetic diversity of early AMH(and maybe even earlier fossil groups of Homo) must have been relatively low as well. The only fossil human group for which such genetic data are available, the Neanderthals, support this contention; their level of genetic variability also is low when compared with living apes (34, 35).
UPDATE: John Hawks discusses the paper.

PNAS
doi:10.1073/pnas.0808160106

Early modern human diversity suggests subdivided population structure and a complex out-of-Africa scenario

Philipp Gunz et al.

Abstract

The interpretation of genetic evidence regarding modern human origins depends, among other things, on assessments of the structure and the variation of ancient populations. Because we lack genetic data from the time when the first anatomically modern humans appeared, between 200,000 and 60,000 years ago, instead we exploit the phenotype of neurocranial geometry to compare the variation in early modern human fossils with that in other groups of fossil Homo and recent modern humans. Variation is assessed as the mean-squared Procrustes distance from the group average shape in a representation based on several hundred neurocranial landmarks and semilandmarks. We find that the early modern group has more shape variation than any other group in our sample, which covers 1.8 million years, and that they are morphologically similar to recent modern humans of diverse geographically dispersed populations but not to archaic groups. Of the currently competing models of modern human origins, some are inconsistent with these findings. Rather than a single out-of-Africa dispersal scenario, we suggest that early modern humans were already divided into different populations in Pleistocene Africa, after which there followed a complex migration pattern. Our conclusions bear implications for the inference of ancient human demography from genetic models and emphasize the importance of focusing research on those early modern humans, in particular, in Africa.

Link

7 comments:

AK said...

In his book "The real Eve: modern man's journey out of Africa" Stephen Oppenheimer proposed a single successful migration along the east coast of Arabia ca 80KYA. He also posits an earlier, unsuccessful immigration into West Arabia ca 120KYA. In his model, the population(s) involved in this earlier migration became extinct.

The information in this article would be consistent with some success in this earlier migration, which wouldn't have been along such a narrow corridor as the eastern coastal route.

terryt said...

I've always had trouble accepting the idea of a single migration from Africa, especially one confined to the extremely exposed Hadramawt and Makran coastlines. To me the single migration theory owes too much to the Garden of Eden myths, and the southern migration explanation seems concocted simply to fit with the single origin theory.

"Genetic diversity among living modern humans is known to be very low when compared with extant apes".

Easily explained if we accept the possibility that extant apes may have never undergone a bottleneck over their entire population, merely local ones, for the last 5-6 million years whreas humans may have suffered one. Not necessarily at the origin of modern humans. We would certainly expect there to have been some sort of bottleneck at the change from Australopithecus to Homo.

ren said...

Isolation by distance would mean a large population, which is pretty impossible for the rapid speciation into Sapiens.

Multiple waves out of Africa would also fail to account for why all Eurasians descend from M and N, which are closely related in an L3 sub-branch.

Kosmo said...

Ren makes a good point. The entire human population outside of Africa is descended from only two closely related sub-branches of L3.

This hardly evidence for multiple Out of Africa events.

In fact, if there were multiple Out of Africa events... those populations leaving Africa were likely themselves closely related, and were probably descended from the same small slice of African diversity-- which in a sense would make it, again, a single Out of Africa event, as it would still be only one population that left.

onur said...

From what I read in the passages quoted by Dienekes, I don't see a contradiction in this paper with Ren's and Kosmo's points. It says that the early AMH have the highest variation among the Homo populations of the last 1.8 million years, at least in cranium shape. That means they also had more variation than modern humans. So we can presume modern humans as a sub-group of the early AMH (probably as a result of bottlenecks). The even lower variation in the modern non-Africans can, again, be explained as a result of further bottlenecks after the successful Eurasian dispersals.

Kosmo said...

Onur, I agree.

terryt said...

"The entire human population outside of Africa is descended from only two closely related sub-branches of L3".

Surely you don't really believe that all humans outside of Africa descend from just two women, accompanied in their migration by a similarly minute number of men? Survivors of a flood perhaps?

We can't even actually say for sure that the two modern mtDNA branches emerged from Africa at the same time as the modern human Y-chromosome haplogroups.

All we can say is that the mtDNA lines M and N probably became fixed in one or more more populations that had emerged from Africa at some time. The haplogroups could easily have become fixed in hybrid populations formed between this emerging African group and human populations already outside that continent. These earlier outside Africa populations may or may not have carried modern human haplogroups. The hybrid populations would have expanded, in turn forming hybrids with other groups it ran into. At least that's what appears to have happened in more modern times. This scenario would certainly result in the claimed diversity of the ancient human population.

"the early AMH have the highest variation among the Homo populations of the last 1.8 million years".

And when we go back beyond 1.8 million years the diversity is so great that the collection of fossils found so far has been classified into a dozen or more separate species. It appears that the development of Homo around 1.8 million years ago had in fact lowered that previous diversity considerably, another bottleneck during our evolution.