Thirty three-dimensional (3D) landmarks registered with Microscribe in four cranial samples (Western and Eastern pygmies and non-pygmies) were obtained. Multivariate analysis (generalized Procrustes analysis, Mahalanobis distances, multivariate regression) and complementary dimensions of size were evaluated with ANOVA and post hoc LSD. Results suggest that important cranial shape differentiation does occur between pygmies and non-pygmies but also between Eastern and Western populations and that size changes and allometries do not affect similarly Eastern and Western pygmies. [. . .] Although not directly related to skull differentiation, the diversity among pygmies would probably suggest that the process responsible for reduced stature occurred after the split of the ancestors of modern Eastern and Western pygmies.
Cranial differentiation of eastern and western pygmies
Diversity among African Pygmies (PLoS One):
That was so jargony I got exactly Zero from reading it!
ReplyDeleteActually, it seems the last sentence I can partly decipher.
ReplyDeleteDo I read it correctly that the African pygmies were once two groups of normal-sized Negroids who concurrently evolved towards pygmization? That is too incredible to believe. Like lightning striking the same person twice.
Also of interest: African Pygmies have the widest noses of any human group.
ReplyDeleteBasically, yes, they are claiming the eastern and western populations may have become pygmied independently. If this is the case, they say, the underlying genetic variants causing dwarfing should not be identical in both populations. Whether this will turn out to be the case (or whether the authors' observations can be explained by gene flow from surrounding populations), I don't know. But there are pygmies outside of Africa (e.g., in the Andaman islands), so it's not outlandish that evolutionary pressures could have caused reductions in stature independently in two populations in Africa.
ReplyDelete"Although our results agree with suggestions that African pygmies share a more recent ancestor [13]–[15], differences in cranial morphology suggest a post-split independent skull differentiation. There is no direct evidence to associate independent skull differentiation with independent body size reduction, but we cannot disregard the possibility that the process of pygmeisation occurred after the split of the ancestors of modern Eastern and Western pygmies. Patin et al. [14] suggested that the split of pygmy's ancestor into Easter and Western populations occurs around 20,000 years ago. Our knowledge of this original population and the habitat in which they lived is very limited [56] but some clues may be found in studying climatic changes. OIS 2 represents a cooling event which starts around 24,000 years ago and finished around 12,000 years ago. Global temperature minima are associated with reduced rainfall in tropical Africa with subsequent reduction in the areas occupied by the rainforest [57]–[59]. Such decrease of the rainforest certainly happened during OIS 2. The global warming that followed led to the replacement of open, grassy vegetation by rain forest and by about 8,000 years ago, rainforest reached maximum extension [see 60]. Patin et al. [14] have suggested that the split between Eastern and Western pygmies occurred at the same time as the rainforest retreated into refugia. However, they observed that gene flow has continued and only stopped more recently. However, one can also hypothesize that around 20.000 years ago, a population living around the periphery of the rain forest, moved to the East and West following the shrinkage of the forest and the expansion of the savanna, which at this point was nearing the Equator as a consequence of climatic changes associated with global cooling event (OIS 2). With warmer conditions at the end of the Pleistocene, the rain forest extended, and due to demographic pressures, populations inhabiting the periphery cannot move to the north and have to adapt to live in the rain forest. Gene flow between Eastern and Western populations stops at that moment (Bagyeli and Baka pygmies who inhabit in South Cameroon, in territories separated by few hundred kilometers, were never in contact and did not know the existence of each other until few years ago). The process of pygmeisation started at that time, in this new habitat, but it developed differently in Eastern and Western regions. Such scenario agrees with the archeological evidence which points to the first presence of settlements in the forest at the end of Pleistocene [61] and it would explain why a) gene flow between Eastern and Western pygmies' ancestors does not stop around 20,000 years but later, b) why pygmies do not appear as a homogenous group in relation to non-pygmies populations, probably also due to a different degree of gene flow with non-pygmies groups [14], and c) why Eastern and Western pygmies differ in cranial and skeletal morphology [24], [28, this study]. This scenario also predicts that potential biological mechanisms that produce differentiation in pygmies (e.i. GH/IGF) would not be the same for all African pygmies."
To summarize in layman's language as best I understand it:
ReplyDelete1.) The pygmies of today form two distinct paragroups, a western one in the Congo and an eastern one near the "great lakes" region [Map].
2.) The proposed theory is that these pygmies do not share a common ancestral stock (contrary to what one would think).
3.) 20,000 years ago, (according to the theory), the ancestors of the eastern and western pygmies were two distinct, non-interbreeding, normal-sized Negroid populations.
4.) Both these Negroid groups lived at the periphery of the central African jungle.
5.) Ancient climate change caused a large area of former jungle in the Congo to disappear, opening that area up to settlement.
6.) These areas were settled by the two nearby groups of Negroids.
7.) The climate changes that pushed back the jungles proved temporary. A few thousand years after moving onto what-was-then "savannah", these Negroids found themselves living in the jungle, without having moved.
8.) Life in the jungle is much harder, and for those Negroids who stayed, they eventually pygmicized as an adaptation to the (returned) extreme heat and humidity. The same mechanism was at work 1,500 miles apart in east- and west-central Africa.
9.) Concurrent pygmicization rather than descent from a common proto-pygmy stock, then, explains certain observed differences in the east vs. west African pygmies in the present day.
10.) #9 may be wrong, and the African pygmies may actually really be descended from a common stock after all, and low-level admixture with neighboring groups may explain observed genotypic differences.
I like the term "Procrustes analysis".
ReplyDeleteMol Biol Evol (2010) doi: 10.1093/molbev/msq294
ReplyDeleteInsights into the demographic history of African Pygmies from complete mitochondrial genomes
Chiara Batini et al.
Pygmy populations are among the few hunter-gatherers currently living in sub-Saharan Africa and are mainly represented by two groups, Eastern and Western, according to their current geographical distribution. They are scattered across the Central African belt and surrounded by Bantu-speaking farmers, with whom they have complex social and economic interactions. To investigate the demographic history of Pygmy groups, a population approach was applied to the analysis of 205 complete mitochondrial DNA (mtDNA) sequences from ten central African populations. No sharing of maternal lineages was observed between the two Pygmy groups, with haplogroup L1c being characteristic of the Western group, but most of Eastern Pygmy lineages falling into sub-clades of L0a, L2a and L5. Demographic inferences based on Bayesian coalescent simulations point to an early split among the maternal ancestors of Pygmies and those of Bantu-speaking farmers (∼70,000 ya, years ago). Evidence for population growth in the ancestors of Bantu-speaking farmers has been observed, starting ∼65,000 ya, well before the diffusion of Bantu languages. Subsequently, the effective population size of the ancestors of Pygmies remained constant over time and ∼27,000 ya, coincident with the Last Glacial Maximum, Eastern and Western Pygmies diverged, with evidence of subsequent migration only among the Western group and the Bantu-speaking farmers. Western Pygmies show signs of a recent bottleneck 4,000 – 650 ya, coincident with the diffusion of Bantu languages, while Eastern Pygmies seem to have experienced a more ancient decrease in population size (20,000 - 4,000 ya). In conclusion, the results of this first attempt at analysing complete mtDNA sequences at the population level in sub-Saharan Africa not only support previous findings but also offer new insights into the demographic history of Pygmy populations, shedding new light on the ancient peopling of the African continent.
The ultra-jargony language these academics use annoys me.
ReplyDeleteConsider:
A. "No sharing of maternal lineages was observed between the two Pygmy groups"
B. "∼27,000 ya, coincident with the Last Glacial Maximum, Eastern and Western Pygmies diverged"
So which is it -- A common Proto-Pygmy ancestral stock or not?