This week’s issue of Nature devotes several articles (free access) to the state of the genome on the tenth anniversary of the publication of the first draft sequence.
Eric Lander’s review article which leads the special section is interesting because he touches on how new knowledge of the genome in the past decade has expanded our view of human origins.
Well before 2000, studies of genetic variation at handfuls of loci such as mitochondrial DNA and blood groups across worldwide populations had given rise to an intellectual synthesis according to which modern humans arose in Africa, dispersed from the continent in a single migration event 50,000–100,000 years ago, replaced resident archaic human forms, and gave rise to modern populations largely through successive population splits without major mixture events. It was difficult, however, to reconstruct the details of these events from the limited data. In addition, little was known about the role that positive selection may have played in shaping the biology of human populations as they migrated and expanded.
A decade later, genomic data have radically reshaped our understanding of the peopling of the globe, yielding a vastly richer picture of population mixing and natural selection. These studies have been made possible by the growth in catalogues and maps of genetic variation among human populations, as well as differences with our closest relatives, such as Neanderthal and chimpanzee.
Shifting the focus from stories based on individual loci (such as ‘mitochondrial Eve’) to large collections of genetic markers has provided powerful new insights. It is now clear that the migration of humans out of Africa was more complicated than previously thought, and that human history involved not just successive population splits, but also frequent mixing.
There is now strong genetic evidence, for example, showing that south Asians are the product of an ancient mixture. European populations have mixed so extensively with their neighbours that their genes mirror geography, rather than reflecting the paths of human migrations or language families.
Most strikingly, genome analysis showed that anatomically modern humans mixed with the Neanderthals. Europeans and Asians (but not Africans) have all inherited 1–4% of their genome from Neanderthals, indicating gene flow in the Middle East on the way out of Africa.
This is all the more interesting in light of this week’s issue of New Scientist, whose cover article (subscription required) covers recent discoveries suggesting biologists may be close to finding one of the genes responsible for speciation itself.
That gene is at the heart of a crucial but seemingly irrelevant process called genetic recombination. During the production of eggs and sperm, chromosomes pair up, cross over and swap segments of DNA, mixing the genes you inherited from your mother and father (see diagram). This shuffling of the genetic pack is the source our individual uniqueness. But no one suspected that recombination might also play a role in generating new species.
The breakthrough came with the discovery that the gene that controls genetic exchange, which goes by the prosaic name of PR-domain containing 9, or Prdm9, is also implicated in generating reproductive incompatibility between different members of the same species. Dubbed “the speciation gene”, if Prdm9‘s two roles turn out to be linked, it could be evolution’s missing X factor.
It will be interesting to see what other evolutionary biologists make of this as we enter the next decade of research.