Evidence that two main bottleneck events shaped modern human genetic diversity — Proceedings B
Evidence that two main bottleneck events shaped modern human genetic diversity — Proceedings B
There is a strong consensus that modern humans originated in Africa and moved out to colonize the world approximately 50 000 years ago. During the process of expansion, variability was lost, creating a linear gradient of decreasing diversity with increasing distance from Africa. However, the exact way in which this loss occurred remains somewhat unclear: did it involve one, a few or a continuous series of population bottlenecks? We addressed this by analysing a large published dataset of 783 microsatellite loci genotyped in 53 worldwide populations, using the program ‘Bottleneck’. Immediately following a sharp population decline, rare alleles are lost faster than heterozygosity, creating a transient excess of heterozygosity relative to allele number, a feature that is used by Bottleneck to infer historical events. We find evidence of two primary events, one ‘out of Africa’ and one placed around the Bering Strait, where an ancient land bridge allowed passage into the Americas. These findings agree well with the regions of the world where the largest founder events might have been expected, but contrast with the apparently smooth gradient of variability that is revealed when current heterozygosity is plotted against distance from Africa.
Jacobs et al ‘08 – Ages for the Middle Stone Age of Southern Africa: Implications for Human Behavior and Dispersal
In today's Science, Jacobs et al discuss Ages for the Middle Stone Age of Southern Africa: Implications for Human Behavior and Dispersal:
The expansion of modern human populations in Africa 80,000 to 60,000 years ago and their initial exodus out of Africa have been tentatively linked to two phases of technological and behavioral innovation within the Middle Stone Age of southern Africa—the Still Bay and Howieson's Poort industries—that are associated with early evidence for symbols and personal ornaments.
Establishing the correct sequence of events, however, has been hampered by inadequate chronologies. We report ages for nine sites from varied climatic and ecological zones across southern Africa that show that both industries were short-lived (5000 years or less), separated by about 7000 years, and coeval with genetic estimates of population expansion and exit times.
Comparison with climatic records shows that these bursts of innovative behavior cannot be explained by environmental factors alone.
Y-chromosomal evidence of a pastoralist migration through Tanzania to southern Africa
Henn et al. in P.N.A.S. present Y-chromosomal evidence of a pastoralist migration through Tanzania to southern Africa (doi):
Although geneticists have extensively debated the mode by which agriculture diffused from the Near East to Europe, they have not directly examined similar agropastoral diffusions in Africa. It is unclear, for example, whether early instances of sheep, cows, pottery, and other traits of the pastoralist package were transmitted to southern Africa by demic or cultural diffusion.
Here, we report a newly discovered Y-chromosome-specific polymorphism that defines haplogroup E3b1f-M293. This polymorphism reveals the monophyletic relationship of the majority of haplotypes of a previously paraphyletic clade, E3b1-M35*, that is widespread in Africa and southern Europe.
To elucidate the history of the E3b1f haplogroup, we analyzed this haplogroup in 13 populations from southern and eastern Africa. The geographic distribution of the E3b1f haplogroup, in association with the microsatellite diversity estimates for populations, is consistent with an expansion through Tanzania to southern-central Africa.
The data suggest this dispersal was independent of the migration of Bantu-speaking peoples along a similar route. Instead, the phylogeography and microsatellite diversity of the E3b1f lineage correlate with the arrival of the pastoralist economy in southern Africa. Our Y-chromosomal evidence supports a demic diffusion model of pastoralism from eastern to southern Africa ~2,000 years ago.
The Migration History of Humans: DNA Study Traces Human Origins Across the Continents
Scientific American covers the human diaspora in a nice broad-brush overview:
Fifty or sixty thousand years ago a small band of Africans—a few hundred or even several thousand—crossed the strait in tiny boats, never to return.
The reason they left their homeland in eastern Africa is not completely understood. Perhaps the climate changed, or once abundant shellfish stocks vanished. But some things are fairly certain. Those first trekkers out of Africa brought with them the physical and behavioral traits—the large brains and the capacity for language—that characterize fully modern humans. From their bivouac on the Asian continent in what is now Yemen, they set out on a decamillennial journey that spanned continents and land bridges and reached all the way to Tierra del Fuego, at the bottom of South America.
Hawks on Handling Exponential Growth in Demographic Models
John Hawks has a nicely detailed discussion about handling exponential growth in demographic models. Very interesting, and hopefully he'll keep them coming:
Exponential growth is a feature of current human populations, and was may represent how the human population behaved during some episodes of its demographic history. However, "exponential" can mean different things to different people, if you're not used to thinking mathematically about growth. So I need to lay out some definitions...
The dawn of human matrilineal diversity
Out in the AJHG, is the next in the National Genographic line of studies. This one, by Behar et al. assesses 624 mitochondrial genomes from Khoisan hunter gatherers in Africa (doi). The abstract says:
The quest to explain demographic history during the early part of human evolution has been limited because of the scarce paleoanthropological record from the Middle Stone Age. To shed light on the structure of the mitochondrial DNA (mtDNA) phylogeny at the dawn of Homo sapiens, we constructed a matrilineal tree composed of 624 complete mtDNA genomes from sub-Saharan Hg L lineages.
We paid particular attention to the Khoi and San (Khoisan) people of South Africa because they are considered to be a unique relic of hunter-gatherer lifestyle and to carry paternal and maternal lineages belonging to the deepest clades known among modern humans. Both the tree phylogeny and coalescence calculations suggest that Khoisan matrilineal ancestry diverged from the rest of the human mtDNA pool 90,000-150,000 years before present (ybp) and that at least five additional, currently extant maternal lineages existed during this period in parallel.
Furthermore, we estimate that a minimum of 40 other evolutionarily successful lineages flourished in sub-Saharan Africa during the period of modern human dispersal out of Africa approximately 60,00070,000 ybp. Only much later, at the beginning of the Late Stone Age, about 40,000 ybp, did introgression of additional lineages occur into the Khoisan mtDNA pool. This process was further accelerated during the recent Bantu expansions. Our results suggest that the early settlement of humans in Africa was already matrilineally structured and involved small, separately evolving isolated populations.
ScienceDaily has some more breathless coverage about this - quoth Spencer Wells: Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world. Truly an epic drama, written in our DNA.. Epic, indeed.
Boskops – a figment of anthropologists’ imaginations
No doubt you've all seen some of the hype surrounding the new book, Big Brain: The Origins and Future of Human Intelligence by Gary Lynch and Richard Granger. The book argues that a long-extinct hominin species, the "Boskops", were much smarter then we are:
Our big brains, our language ability, and our intelligence make us uniquely human. But barely 10,000 years ago--a mere blip in evolutionary time--human-like creatures called "Boskops" flourished in South Africa. They possessed extraordinary features: forebrains roughly 50% larger than ours, and estimated IQs to match--far surpassing our own. Many of these huge fossil skulls have been discovered over the last century, but most of us have never heard of this scientific marvel.
Prominent neuroscientists Gary Lynch and Richard Granger compare the contents of the Boskop brain and our own brains today, and arrive at startling conclusions about our intelligence and creativity. Connecting cutting-edge theories of genetics, evolution, language, memory, learning, and intelligence, Lynch and Granger show the implications of large brains on a broad array of fields, from the current state of the art in Alzheimer's and other brain disorders, to new advances in brain-based robots that see and converse with us, and the means by which neural prosthetics-- replacement parts for the brain--are being designed and tested. The authors demystify the complexities of our brains in this fascinating and accessible book, and give us tantalizing insights into our humanity--its past, and its future.
It seems to be getting glowing reviews all over the place - Cognitive Neuroscientist Michael Gazzaniga says it's "like mixing gas with fire. In this book there are big, explosive ideas by two ingenious brain scientists". William Calvin reviews it in New Scientist ("I've been waiting for someone to write this book for years"). Discover Magazine tells us that the Boskops probably had "an internal mental life literally beyond anything we can imagine".
....and unsurprisingly it's all crap. John Hawks discusses the anthropological evidence for these Boskops and, well, it ain't that good:
This category became untenable as further information about the archaeology of South Africa came to light. Ronald Singer (1958) reviewed the "Boskop race" evidence as it existed by the 1950's. He concluded that there was no reason to maintain that any "big-headed, small-faced group" had existed in prehistory, separate from the current biological variability of "Bushman, Hottentot and Negro." But that view is unsupportable -- in fact, what happened is that a small set of large crania were taken from a much larger sample of varied crania, and given the name, "Boskopoid." This selection was initially done almost without any regard for archaeological or cultural associations -- any old, large skull was a "Boskop". Later, when a more systematic inventory of archaeological associations was entered into evidence, it became clear that the "Boskop race" was entirely a figment of anthropologists' imaginations.