The paper does not prove that climate has not played an important role in the shaping of human cranial variation. Rather it shows that it does not play an important role in the level of diversity found in different populations. But, I really can't think why anyone would propose that.
Let's take a particular trait, say nose breadth or cranial breadth. It has been demonstrated that the former tends to be narrower and the latter broader in more northerly compared to more equatorial regions. But, this is a difference in the means and not a difference in the variance of the traits in question.
The authors write:
All the most informative traits were distributed in the anterior region of the cranium (yet not all anterior traits were highly informative). This came as a surprise as the anterior regions of the cranium are generally assumed to be more affected by natural selection (i.e. facial area and cranial breadth), while the temporal bone and some traits of the neurocranium are considered to reflect population history (Beals et al. 1984; Franciscus & Long 1991; Roseman 2004; Harvati & Weaver 2006).
Diversity is reduced in either of two ways:
- Serial bottlenecks, when a subset of the variation present in the ancestral population migrates to new regions
- Selection, which reduces diversity in the traits under selection
Under the scenario of selection, however, it is expected that traits under selection will be more affected by the new ecological conditions that occur away from the cradle.
The finding that the anterior region of the cranium is more informative in terms of reduced diversity is actually consistent with selection and not with bottlenecks, since it is in this part, in nasal features, facial flatness, prognathism, etc. that populations have developed strongly differentiated types under selection.
Furthermore, in this paper the study is limited to skulls from the last 2,000 years. But, if bottlenecks are indeed responsible for the reduction of diversity, then this reduction of diversity would be visible in the earliest Homo sapiens skulls from the various regions, as these would be descended from the few bottlenecked migrants. Are Upper Paleolithic Europeans, for example, more or less diverse than Africans of equivalent age, or even modern Africans?
On the contrary, recent skulls may be less diverse than the earliest ones, due to a longer period of selection, i.e., the tens of thousands of years between the earliest Homo sapiens in Europe or Asia and the ones of the last 2,000 years. Moreover, this selection ought to have been strongest in regions further from Africa, as this is correlated with different environments (although not necessarily the 3 climate variables considered here).
- Within-population variance decreases with distance from Africa, but this does not measure between-population difference in mean trait values
- Both bottlenecks and selection result in reduced variance
- A stronger variance-reduction signal in the anterior cranium is more consistent with selection than with bottlenecks
- The bottlenecks theory should be more visible in early, not recent skulls as those studied in this paper. Recent skulls should have reduced variance compared to their more ancient counterparts due to a long period of selection.
Proceedings of the Royal Society B doi:10.1098/rspb.2008.1563
Distance from Africa, not climate, explains within-population phenotypic diversity in humans
Lia Betti et al.
The relative importance of ancient demography and climate in determining worldwide patterns of human within-population phenotypic diversity is still open to debate. Several morphometric traits have been argued to be under selection by climatic factors, but it is unclear whether climate affects the global decline in morphological diversity with increasing geographical distance from sub-Saharan Africa. Using a large database of male and female skull measurements, we apply an explicit framework to quantify the relative role of climate and distance from Africa. We show that distance from sub-Saharan Africa is the sole determinant of human within-population phenotypic diversity, while climate plays no role. By selecting the most informative set of traits, it was possible to explain over half of the worldwide variation in phenotypic diversity. These results mirror those previously obtained for genetic markers and show that ‘bones and molecules’ are in perfect agreement for humans.