Under the Skin - 20 April 2002 - New Scientist
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Under the skin - 20 April 2002 - New Scientist My New Scientist Home | In-Depth Articles |Back to article Under the skin ● 20 April 2002 by Anil Ananthaswamy ● Magazine issue 2339. Subscribe and save THOSE OF US who have felt a racist's stare know that it scalds our skin and triggers emotions ranging from anger to shame. Last year, that anger boiled over in race riots that rocked the US and Britain. Of course, those riots sprang from a complex web of causes, but it's also true that opponents on each side had one simple thing in common: the colour of their skins. It might seem a long way from the streets of Cincinnati or Bradford to the hospitals and research institutes where scientists seek to explain the diversity of human biology. But well-meaning doctors say they, too, need to pay attention to race. They argue that it's a useful indicator of people's predisposition to disease or response to drugs. Meanwhile, other scientists think that this is nonsense. Geneticists know that the racial differences most of us perceive are little more than skin-deep. They say that medical research and treatment based on visible racial characteristics is a waste of time at best and downright dangerous at worst. Even so, the latest genetic data has added a further twist to the argument. It turns out that you can group people into genetically similar clusters after all—it's just that they don't correspond to conventional racial groups. But they may provide doctors with better categories for research and treatment until the time when diagnosis and therapy can be tailored to an individual patient's genes. For most people, geneticists' talk about the non-existence of races runs up against the evidence of our own eyes. The people you meet on the street are easy to classify as being of African, South Asian, European, or some other origin. How do we explain these apparently clear differences between people of different ... well, races? Don't external features such as skin colour, hair type, eye shape and body stature separate us? They do, but only on the surface. Many of these dramatic differences are adaptations to different climates and don't imply any deeper genetic differences, says Luigi Luca Cavalli-Sforza, a population geneticist at Stanford University in California. Black people are black because of melanin, the pigment that protects their skin from the damage caused by strong sunlight. White people are white because they lack melanin, since the pigment would prevent their skin from making enough vitamin D under the weak Sun of high latitudes. The tall, thin stature that helps the Masai in eastern Africa stay cool would be a disadvantage in the Arctic where the Inuit evolved with stout torsos and short limbs. But while the Masai and the Inuit represent two extremes of external appearance, in fact there are no abrupt boundaries between human populations. Features that appear unambiguous when you see an individual on the street look a lot less clear-cut when you view a wider spread of humanity. "If you walk from Senegal to Japan and you are asked to mark a line where the African characteristics stop and the Oriental characteristics begin, that's a very hard task and one that genetics cannot help solve," says Guido Barbujani, a geneticist at the University of Ferrara in Italy. The notion of genetically meaningful races began to crumble in 1972, when Richard Lewontin, a geneticist at Harvard University, analysed variations in blood proteins taken from populations around the world. His conclusions came as a shock: humans from different "races" are not as genetically different as their appearance would suggest. He found that nearly 85 per cent of humanity's genetic diversity occurs among individuals within a single population, such as the Swedes. Another 8 per cent occurs between populations of the same race—Swedes and Spaniards, for example. Only 7 per cent was accounted for by consistent differences between races. In other words, two individuals are different because they are individuals, not because they belong to different races. At the time, some researchers doubted Lewontin's findings, pointing out that variation in proteins did not accurately reflect variation in DNA. But nearly 25 years later, Barbujani and his colleagues surveyed DNA sequence diversity directly, and their results—published in 1997—were nearly identical to Lewontin's. Other labs have also replicated these results. Lewontin and Barbujani's research provides a way of measuring our diversity that does not depend on our external appearance. And it shows that as a species we are unusually homogenous. In biology, a "race"—or subspecies—is defined as a population that is geographically isolated and genetically distinct from others of the same species. To determine if a species has races, geneticists first quantify the genetic diversity of the species on a scale of 0 to 1. A 0 means that individuals from different populations are no more different than individuals from the same population, while a 1 means that each population is made up of genetically identical individuals, and all the genetic differences exist between populations. For races to exist, a species should have a genetic diversity number of at least 0.25 to 0.3. Mammals such as coyotes, elephants, gazelles and grey wolves have genetic diversity numbers that range from 0.3 to 0.8, and geneticists recognise subspecies within them. But for humans, the number is 0.15. Compared with other animals, we don't meet the threshold. http://www.newscientist.com/article/mg17423394.200-under-the-skin.html?full=true&print=true (1 of 4)5/31/2012 3:52:09 AM Under the skin - 20 April 2002 - New Scientist Moreover, the genetic make-up of human "races" overlaps so broadly that you can't accurately predict someone's race by their genes. Barbujani and his colleagues analysed a set of 21 DNA sequences from 1330 individuals from 32 populations worldwide. Using the most sophisticated statistical software available, they asked a computer to assign each individual to his or her continent of origin. The results, to be published in the journal Genome Research, show that the computer couldn't do it. With a single DNA sequence, the computer got it wrong 80 per cent of the time. As more sequences were added, the computer got better, but even at its best the computer still failed 30 per cent of the time. So the races we think we see have little relevance to biology. But is there a better way to get at humanity's underlying genetic variations? David Goldstein of University College London thinks so. "There is quite a simple alternative to racial labels in representing the genetic structure," he says. Goldstein and his colleagues analysed DNA samples from people in eight populations from Asia, Africa and Europe, and used statistics to sort the individuals into genetically similar groups. They found that the people divided into four clusters, broadly corresponding to four geographical areas: Western Eurasia, sub-Saharan Africa, China and New Guinea. But the clusters did not follow established racial lines. For instance, 62 per cent of Ethiopians were assigned to the cluster containing most Norwegians, Ashkenazi Jews and Armenians, and 21 per cent of Afro-Caribbean individuals were grouped alongside West Eurasians. It's tempting to regard these clusters as the "real" races of humanity, but things may not be so simple. Barbujani and his colleagues performed a similar analysis using two entirely different sets of genetic markers and samples from over 30 populations—a much larger group than Goldstein's. To their surprise, they found that the two sets of markers yielded two different clustering patterns, both different from Goldstein's. One set of markers broke people down into one largely Eurasian group, plus two other groups in which individuals came from all over the world. The other set led to four groups: one made up of African and Oceanic people, one containing Asians and native Americans, and two other groups that were mainly Eurasian. The differences in the two groupings are so large that Barbujani concludes there is no obvious way of classifying humans into a few, well-defined groups. Goldstein, however, still believes there is an underlying set of clusters that represent the inherent genetic structure of humanity, and that finding it is mainly a matter of studying the right genetic markers. "We are all using too small a number of markers," he says. "My guess is that when we use a large enough set of markers and an exhaustive enough set of individuals, the results will stabilise. In fact, I'm quite sure that they will." Whatever the outcome of this dispute, one thing is clear: conventional notions of race are—or should be—dead. Even where doctors find race useful in predicting the risk of certain diseases, they would do better to abandon it in favour of the more general notion of ancestry. Ashkenazi Jews, for instance, are prone to a rare genetic mutation that causes breast cancer, a mutation not shared by other white people. A focus on race can also blind us to real groupings that cut across conventional racial lines. Sickle-cell anaemia, for example—often regarded as an African disease—occurs in people from all over the world, including those from India and the Mediterranean, because a single copy of the sickle-cell gene protects individuals against malaria, which is common in all those regions. And when it comes to the more complex diseases such as diabetes, hypertension and coronary heart disease, the situation gets even murkier.