Among Many Peoples, Little Genomic Variety
Monday, June 22, 2009
There is a simplicity and all-inclusiveness to the number three -- the triangle, the Holy Trinity, three peas in a pod. So it's perhaps not surprising that the Family of Man is divided that way, too.
All of Earth's people, according to a new analysis of the genomes of 53 populations, fall into just three genetic groups. They are the products of the first and most important journey our species made -- the walk out of Africa about 70,000 years ago by a small fraction of ancestral Homo sapiens.
One group is the African. It contains the descendants of the original humans who emerged in East Africa about 200,000 years ago. The second is the Eurasian, encompassing the natives of Europe, the Middle East and Southwest Asia (east to about Pakistan). The third is the East Asian, the inhabitants of Asia, Japan and Southeast Asia, and -- thanks to the Bering Land Bridge and island-hopping in the South Pacific -- of the Americas and Oceania as well.
The existence of this ancient divergence has long been known.
What is new is a subtle but important insight into what happened on a genomic level as the human species spilled across the landscape, eventually occupying every habitable part of the planet.
People adapted to what they encountered the way all living organisms do: through natural selection. A small fraction of the mutations constantly creeping into our genes happened by chance to prove beneficial in the new circumstances outside the African homeland. Those included differences in climate, altitude, latitude, food availability, parasites, infectious diseases and lots of other things.
A person who carried, by chance, a helpful mutation was more likely to survive and procreate than someone without it. The person's offspring would then probably be endowed with the same beneficial mutation. Over thousands of generations, the new variant (what geneticists call the "derived allele") could go from being rare to being common as its carriers fared better than their brethren and contributed more descendants to the population.
Scientists have long known that regardless of ancestral home or ethnic group, everyone's genes are pretty much alike. We're all Homo sapiens. Everything else is pretty much details.
Recent research has produced a surprise, however. Population geneticists expected to find dramatic differences as they got a look at the full genomes -- about 25,000 genes -- of people of widely varying ethnic and geographic backgrounds. Specifically, they expected to find that many ethnic groups would have derived alleles that their members shared but that were uncommon or nonexistent in other groups. Each regional, ethnic group or latitude was thought to have a genomic "signature" -- the record of its recent evolution through natural selection.
But as analyses of genomes from dozens of distinct populations have rolled in -- French, Bantu, Palestinian, Yakut, Japanese -- that's not what scientists have found. Dramatic genome variation among populations turns out to be extremely rare.
Instead, it is "random genetic drift" that appears to be more important in sculpting our genes. Drift describes the chance loss of genetic variation that occurred not only in the out-of-Africa migration, but through all of human history as famine, climate change or war caused populations to crash and then recover.
Despite those calamities, it appears that all contemporary populations ended up largely the same, or only crudely distinguishable from one another, on the genome level.