Although the existing races of man differ in many respects, as in colour, hair, shape of skull, proportions of the body, &c., yet if their whole structure be taken into consideration they are found to resemble each other closely in a multitude of points. Many of these are of so unimportant or of so singular a nature, that it is extremely improbable that they should have been independently acquired by aboriginally distinct species or races. The same remark holds good with equal or greater force with respect to the numerous points of mental similarity between the most distinct races of man. The American aborigines, Negroes and Europeans are as different from each other in mind as any three races that can be named; yet I was incessantly struck, whilst living with the Fuegians on board the Beagle, with the many little traits of character, shewing how similar their minds were to ours; and so it was with a full-blooded negro with whom I happened once to be intimate. CHARLES DARWIN, THE DESCENT OF MAN
AFTER THE ANCESTRAL PEOPLE had dispersed from their homeland in northeast Africa, there was no longer a single human population but many. Across the far-flung reaches of the globe, human evolution continued independently. Over the course of many generations the peoples of each continent emerged as different races.
Such an outcome is not so surprising. An array of influences would have pushed each population along a separate evolutionary path. And the one force that could have kept the population the same—a thorough mixing of genes, through intermarriage—could no longer operate once people lived vast distances from each other, and probably in warring tribes who killed as spies anyone found traveling through their territory. The genealogies of the Y chromosome and mitochondrial DNA, whose major branches are still largely confined to different continents, are evidence that throughout the world people have tended overwhelmingly to live, marry and die in the places they were born, at least until modern times.
The genetic differentiation of the human population, into races and ethnicities within races, has long been a matter of both controversy and ignorance. Because of the many evils that racism has caused, from discrimination to genocide, researchers have generally sought to minimize the existence of race. Many social scientists even assert that race is a social concept without biological basis.
Race is not well understood, in part because it has not been regarded as a fit subject for academic study. In many respects, this has been a prudent position. The matter of race seemed of no great scientific interest, was inherently divisive, and had been seriously polluted by a history of racial classifications designed with an agenda of proving one race superior to another. But two valid scientific reasons for considering the question of race have begun to emerge, and at the same time technical advances in sequencing DNA have at last made it possible to study the still somewhat mysterious nature of race on a scientific basis.
One valid reason for reconsidering race is historical; people of different races may hold in their genetics essential clues to human history since the fragmentation of the ancestral human population 50,000 years ago. Races presumably developed in part in response to the pressures experienced by each population, and the genetic changes involved in race may allow those pressures to be identified. The different branches of the human family have their own histories, which cannot be explored or told until the branches are recognized and their genetics examined.
A second and more practical reason for defining race is medical. Many diseases have a genetic component, which often varies with race or ethnicity. Hemochromatosis, a genetic condition thought to have been spread by the Vikings, affects mostly Europeans. The Pima Indians are particularly susceptible to diabetes, Pacific Islanders to obesity. Crohn's disease occurs in both Europeans and Japanese but the three genetic variants known to be the cause of the disease in Europeans are not found in Japan, where presumably a different mutation leads to the same symptoms.
People of different races may also differ in their response to drugs. This is sometimes because the enzymes that break down the drugs are being lost at different rates in different races. (The enzymes' original role was to break down the natural toxins in wild plant foods; since they are no longer needed for that purpose, they are being randomly inactivated by mutations that natural selection no longer sweeps away.) People may also possess different versions of the protein on which a drug is meant to act. The heart drug enalapril reduces blood pressure and the risk of being hospitalized for heart failure in white
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