In favor of recent Out-of-Africa (Eriksson et al. 2012)

A new paper in PNAS argues for a recent (~60kya) expansion of modern humans Out-of-Africa. After reading the title, I was not sure what date the authors were arguing for, and I went straight for the movie in the supplemental material, which is a pretty cool depiction of the authors' scenario.

However, I disagree with the conclusions of this paper, for a variety of reasons. First, the climate history of Africa is consistent with older dispersal scenarios. The authors of the current paper follow other researchers into attributing the Skhul/Qafzeh hominins to an Out-of-Africa-that-failed, but that proposition is increasingly untenable.

The halving of the human autosomal mutation rate implies that Eurasians and Africans split before 100 thousand years ago, and African hunter-gatherers may have split as much as 300 thousand years ago. These dates are not set in stone, but can be downsized if one allows for substantial archaic African admixture. But, doing so weakens the case for a sub-Saharan origin of Homo sapiens.

Second, the Nubian Complex is a direct archaeological link between NE Africa and S Arabia pre-100ka. It becomes increasingly difficult to argue that the pre-100ka expansion fizzled when you have the triple evidence of Mt. Carmel, the Nubian Complex, and Jebel Faya, providing a combination of anthropological and archaeological evidence for African-Asian interaction prior to 100ka.

Some of the conclusions of this paper may be influenced by their choice of the dinucleotide mutation rate:

The dinucleotide stepwise mutation model mutation rate for these markers was estimated in the work by Dib et al. (46) to μdi = 1.52 × 10−3 mutations per generation.

But, Sun  et al. seem to report a much slower dinucleotide mutation rate, as well as a deviation from the stepwise symmetrical model that would bias age estimates downwards if that model is used. So, I am not very confident in the age estimates provided in this paper.

There is a good reason to favor a recent human expansion: if Out-of-Africa happened pre-100ka, as I have argued, then what did modern humans wait for to conquer the rest of the planet (a greater than 50ka hiatus until they begin appearing all over Eurasia). However, that problem can be solved if we acknowledge the fact that modern humans prior to 100ka may have been anatomically "like us", but behaviorally they were not much different from other hominins living on the planet at the time. These pre-100ka H. sapiens were just another set of hunter-gatherers: they may have had a chin, a smaller face, and a more globular braincase, but they did not appear to behave in any drastically different way than other humans who lacked these features.

There are three factors that drive migration: curiosity, need, and ability. One may wonder "what's on the land beyond the sea", but one needs the ability to build a lasting boat to find out. One may have the ability to build a boat, but has no need to do so, if there is game-a-plenty around camp and a beautiful woman with a few beautiful babies in the shelter.

I think that 2-3 reasons contributed to the hiatus:

1. The going was good in Arabia prior to the climate crisis of ~70kya
2. The way north was blocked by Neandertals
3. Whether or not modern humans had the genetic capacity to outcompete the Neandertals, they did not yet have the behavioral expression needed to achieve this

I don't know to what extent changes in the modern human lineage made the mental hardware of early Homo sapiens something like a transistor-based computer that had to compete against the older triode-based models that filled the planet. As we sample more ancient hominins, we may eventually find out whether our wiring was really much improved.

But, one does not really need the best of wiring to conquer a planet. Few would argue today, I suspect, that English mental hardware was superior to e.g., Bavarian mental hardware, but the English brought half the planet under their domination, partly because of their fortunate geographical position which gave them (and other West Europeans) access to the lands beyond the sea. Similarly, few would argue that the Mongols had an innate ability to conquer half of Eurasia, but they happened to have a combination of drive, leadership, organization, and military hardware that allowed them to do so.

This is what I suspect was the real cause for the success of modern humans: they may have had some genetic advantage over others, but their success was partly unintended (the consequence of the drying up of the Sahara-Arabia region that forced them out c. 70kya), and partly the result of them having some vital technological "edge" over other Homo populations.

There may have been interplay between the "need" and "ability" causes of the great human diaspora: as modern humans were pushed out by the advancing desert, they had to adapt to dwindling resources, the challenge of new environments, and the challenge of contact and competition with archaic hominins in both Eurasia and Africa. Adversity does not always breed success: it most often results in failure. But, while many long-forgotten peoples may have faced formidable challenges during the long aeons of geological time, at least one of them had the combination of luck and the "right stuff" to rise to the occasion, and we are their descendants.


PNAS doi: 10.1073/pnas.1209494109

Late Pleistocene climate change and the global expansion of anatomically modern humans

Anders Eriksson et al.

The extent to which past climate change has dictated the pattern and timing of the out-of-Africa expansion by anatomically modern humans is currently unclear [Stewart JR, Stringer CB (2012) Science 335:1317–1321]. In particular, the incompleteness of the fossil record makes it difficult to quantify the effect of climate. Here, we take a different approach to this problem; rather than relying on the appearance of fossils or archaeological evidence to determine arrival times in different parts of the world, we use patterns of genetic variation in modern human populations to determine the plausibility of past demographic parameters. We develop a spatially explicit model of the expansion of anatomically modern humans and use climate reconstructions over the past 120 ky based on the Hadley Centre global climate model HadCM3 to quantify the possible effects of climate on human demography. The combinations of demographic parameters compatible with the current genetic makeup of worldwide populations indicate a clear effect of climate on past population densities. Our estimates of this effect, based on population genetics, capture the observed relationship between current climate and population density in modern hunter–gatherers worldwide, providing supporting evidence for the realism of our approach. Furthermore, although we did not use any archaeological and anthropological data to inform the model, the arrival times in different continents predicted by our model are also broadly consistent with the fossil and archaeological records. Our framework provides the most accurate spatiotemporal reconstruction of human demographic history available at present and will allow for a greater integration of genetic and archaeological evidence.

Link