Human populations vary genetically, however, much of this variation is not divided into discrete groups but rather is distributed as a cline, or gradually, with one population smoothly transitioning into another. Genetic isolation in most human populations is therefore primarily dictated by distance. Also, the human propensity for roaming and spreading into new environments means that humans spread their genes widely and tend to genetically homogenize populations. However, there are some exceptions to these general patterns. Some human populations have experienced relatively long periods of genetic isolation.
Pygmies of West and Central Africa are known for their comparatively short stature. Pygmies do not comprise a single group but rather approximately two dozen groups with a diverse array of different languages. While on average adult male height is about 5 feet, comparatively small for most human populations, mean height can vary by as much as 8 inches among different pygmy groups. Pygmies are among the last groups in Sub-Saharan Africa to practice the hunter-gatherer lifestyle typical of our earliest human ancestors and as such understanding their evolutionary history can provide important clues to the history and diversification of our species.
Paul Verdu of Paris' Centre National de la Recherche Scientifique and colleagues in the latest issue of the journal Current Biology examined the genetic history of 9 pygmy populations (Bezan, Central Baka, Eastern Baka, Southern Baka, Gabonese Baka, Kola, Koya, Eastern Bongo, and Southern Bongo) and 12 neighboring non-pygmy populations using 28 genetic markers across 604 individuals. As a group, pygmy populations in this study showed large amounts of genetic diversity and, with the exception of the Eastern and Southern Bongo pygmies, pygmy genetic variation clustered together, and separate from non-pygmy genetic variation. Eastern and Southern Bongos showed comparatively more evidence of gene flow with non-pygmy populations, hence their tendancy to cluster within non-pygmy genetic variation.
These genetic data support a common genetic heritage of all West African pygmy populations in the study and an evolutionary split from other West African peoples some 54,000-90,000 years ago. Within the pygmy lineage, divergent populations appear to have arisen more recently, as recent as 2,600-2,900 years ago, the same time agricultural peoples in the region were undergoing expansion. These conclusions are in line with previous results employing other types of genetic markers. Interestingly, Bongo pygmies not only genetically clustered closely to non-pygmy populations but they exhibit the tallest mean height, further evidence of gene flow from non-pygmy populations to Bongo pygmies. Much of this gene flow could be relatively recent and may eventually swamp out the pygmies unique genetic lineage.
The expansion of agricultural peoples in Sub-Saharan Africa likely drove ancestral pygmy populations into increased isolation from one another therefore driving their genetic divergence. Culture and language of indigenous peoples are not the only parts of their history under threat of extinction, but, as human history shows, their very genetic lineage too can be lost to population extinction or genetic exchange among populations. Modern molecular genetics are a powerful tool for uncovering evolutionary history. Being a widespread and adaptable species human populations vary in their genetic history with some being more characterized by isolation and others by mixing. Evolutionary theory provides the critical framework to make sense of rapidly growing genetic data from human populations and shows that humans are subject to the same evolutionary forces that shape the rest of nature.
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Paul Verdu, Frederic Austerlitz, Arnaud Estoup, Renaud Vitalis, Myriam Georges, Sylvain Théry, Alain Froment, Sylvie Le Bomin, Antoine Gessain, Jean-Marie Hombert (2009). Origins and Genetic Diversity of Pygmy Hunter-Gatherers from Western Central Africa Current Biology, 19 (4), 312-318 DOI: 10.1016/j.cub.2008.12.049
Roberta Kwok (2009). Pygmies share a recent common ancestor Nature DOI: 10.1038/news.2009.82