Humans Have Spread Globally, and Evolved Locally
By NICHOLAS WADE
Historians often assume that they need pay no attention to human evolution because the process ground to a halt in the distant past. That assumption is looking less and less secure in light of new findings based on decoding human DNA
People have continued to evolve since leaving the ancestral homeland in northeastern Africa some 50,000 years ago, both through the random process known as genetic drift and through natural selection. The genome bears many fingerprints in places where natural selection has recently remolded the human clay, researchers have found, as people in the various continents adapted to new diseases, climates, diets and, perhaps, behavioral demands
A striking feature of many of these changes is that they are local. The genes under selective pressure found in one continent-based population or race are mostly different from those that occur in the others. These genes so far make up a small fraction of all human genes. ..
Even more strikingly, Dr. Williamsonâ€™s group reported that a version of a gene called DAB1 had become universal in Chinese but not in other populations. DAB1 is involved in organizing the layers of cells in the cerebral cortex, the site of higher cognitive functions
Variants of two genes involved in hearing have become universal, one in Chinese, the other in Europeans
The emerging lists of selected human genes may open new insights into the interactions between history and genetics. â€?If we ask what are the most important evolutionary events of the last 5,000 years, they are cultural, like the spread of agriculture, or extinctions of populations through war or disease,â€? said Marcus Feldman, a population geneticist at Stanford. These cultural events are likely to have left deep marks in the human genome
A genomic survey of world populations by Dr. Feldman, Noah Rosenberg and colleagues in 2002 showed that people clustered genetically on the basis of small differences in DNA into five groups that correspond to the five continent-based populations: Africans, Australian aborigines, East Asians, American Indians and Caucasians, a group that includes Europeans, Middle Easterners and people of the Indian subcontinent. The clusterings reflect â€?serial founder effects,â€? Dr. Feldman said, meaning that as people migrated around the world, each new population carried away just part of the genetic variation in the one it was derived from
The new scans for selection show so far that the populations on each continent have evolved independently in some ways as they responded to local climates, diseases and, perhaps, behavioral situations
The concept of race as having a biological basis is controversial, and most geneticists are reluctant to describe it that way. But some say the genetic clustering into continent-based groups does correspond roughly to the popular conception of racial groups
â€?There are difficulties in where you put boundaries on the globe, but we know now there are enough genetic differences between people from different parts of the world that you can classify people in groups that correspond to popular notions of race,â€? Dr. Pritchard said
David Reich, a population geneticist at the Harvard Medical School, said that the term â€?raceâ€? was scientifically inexact and that he preferred â€?ancestry.â€? Genetic tests of ancestry are now so precise, he said, that they can identify not just Europeans but can distinguish between northern and southern Europeans. Ancestry tests are used in trying to identify genes for disease risk by comparing patients with healthy people. People of different races are excluded in such studies. Their genetic differences would obscure the genetic difference between patients and unaffected people. [More
For almost a decade, I've been pointing out that we know with tautological certainty that extended families exist, extended families of whatever size you wish from the tiny to the vast. And we know with as close to absolute certainty as anything can be in the human empirical world that some big extended families have a certain degree of coherence and endurance because their ancestors were not randomly outbreeding but tended to inbreed so some degree — if you go back 1,000 years, there are 1 trillion openings in your family tree but there weren't 1 trillion people alive. So a lot of your ancestors did double duty, to say the least. So, nobody is randomly descended from the entire human race. Even if you are Tiger Woods' daughter, you can still divvy up your ancestry into Thai, Swedish, etc
Now, what you want to call these partly inbred extended families is a subjective semantic issue. "Racial groups" strikes me as the most obvious, but if Dr. Reich wants to call them "ancestral groups," well, swell, that's a useful term too
The point is that partly inbred extended families exist and they are important.