送交者: xj 于 2005-3-16, 01:52:39:
N.Y. Times
March 14, 2005
OP-ED CONTRIBUTOR
A Family Tree in Every Gene
By ARMAND MARIE LEROI
London -- Shortly after last year's tsunami devastated the lands on the
Indian Ocean, The Times of India ran an article with this headline:
"Tsunami May Have Rendered Threatened Tribes Extinct." The tribes in
question were the Onge, Jarawa, Great Andamanese and Sentinelese - all
living on the Andaman Islands - and they numbered some 400 people in all.
The article, noting that several of the archipelago's islands were
low-lying, in the direct path of the wave, and that casualties were
expected to be high, said, "Some beads may have just gone missing from the
Emerald Necklace of India."
The metaphor is as colorful as it is well intentioned. But what exactly
does it mean? After all, in a catastrophe that cost more than 150,000
lives, why should the survival of a few hundred tribal people have any
special claim on our attention? There are several possible answers to this
question. The people of the Andamans have a unique way of life. True,
their material culture does not extend beyond a few simple tools, and
their visual art is confined to a few geometrical motifs, but they are
hunter-gatherers and so a rarity in the modern world. Linguists, too, find
them interesting since they collectively speak three languages seemingly
unrelated to any others. But the Times of India took a slightly different
tack. These tribes are special, it said, because they are of "Negrito
racial stocks" that are "remnants of the oldest human populations of Asia
and Australia."
It's an old-fashioned, even Victorian, sentiment. Who speaks of "racial
stocks" anymore? After all, to do so would be to speak of something that
many scientists and scholars say does not exist. If modern anthropologists
mention the concept of race, it is invariably only to warn against and
dismiss it. Likewise many geneticists. "Race is social concept, not a
scientific one," according to Dr. Craig Venter - and he should know, since
he was first to sequence the human genome. The idea that human races are
only social constructs has been the consensus for at least 30 years.
But now, perhaps, that is about to change. Last fall, the prestigious
journal Nature Genetics devoted a large supplement to the question of
whether human races exist and, if so, what they mean. The journal did this
in part because various American health agencies are making race an
important part of their policies to best protect the public - often over
the protests of scientists. In the supplement, some two dozen geneticists
offered their views. Beneath the jargon, cautious phrases and academic
courtesies, one thing was clear: the consensus about social constructs was
unraveling. Some even argued that, looked at the right way, genetic data
show that races clearly do exist.
The dominance of the social construct theory can be traced to a 1972
article by Dr. Richard Lewontin, a Harvard geneticist, who wrote that most
human genetic variation can be found within any given "race." If one
looked at genes rather than faces, he claimed, the difference between an
African and a European would be scarcely greater than the difference
between any two Europeans. A few years later he wrote that the continued
popularity of race as an idea was an "indication of the power of
socioeconomically based ideology over the supposed objectivity of
knowledge." Most scientists are thoughtful, liberal-minded and socially
aware people. It was just what they wanted to hear.
Three decades later, it seems that Dr. Lewontin's facts were correct, and
have been abundantly confirmed by ever better techniques of detecting
genetic variety. His reasoning, however, was wrong. His error was an
elementary one, but such was the appeal of his argument that it was only a
couple of years ago that a Cambridge University statistician, A. W. F.
Edwards, put his finger on it.
The error is easily illustrated. If one were asked to judge the ancestry
of 100 New Yorkers, one could look at the color of their skin. That would
do much to single out the Europeans, but little to distinguish the
Senegalese from the Solomon Islanders. The same is true for any other
feature of our bodies. The shapes of our eyes, noses and skulls; the color
of our eyes and our hair; the heaviness, height and hairiness of our
bodies are all, individually, poor guides to ancestry.
But this is not true when the features are taken together. Certain skin
colors tend to go with certain kinds of eyes, noses, skulls and bodies.
When we glance at a stranger's face we use those associations to infer
what continent, or even what country, he or his ancestors came from - and
we usually get it right. To put it more abstractly, human physical
variation is correlated; and correlations contain information.
Genetic variants that aren't written on our faces, but that can be
detected only in the genome, show similar correlations. It is these
correlations that Dr. Lewontin seems to have ignored. In essence, he
looked at one gene at a time and failed to see races. But if many - a few
hundred - variable genes are considered simultaneously, then it is very
easy to do so. Indeed, a 2002 study by scientists at the University of
Southern California and Stanford showed that if a sample of people from
around the world are sorted by computer into five groups on the basis of
genetic similarity, the groups that emerge are native to Europe, East
Asia, Africa, America and Australasia - more or less the major races of
traditional anthropology.
One of the minor pleasures of this discovery is a new kind of genealogy.
Today it is easy to find out where your ancestors came from - or even when
they came, as with so many of us, from several different places. If you
want to know what fraction of your genes are African, European or East
Asian, all it takes is a mouth swab, a postage stamp and $400 - though
prices will certainly fall.
Yet there is nothing very fundamental about the concept of the major
continental races; they're just the easiest way to divide things up. Study
enough genes in enough people and one could sort the world's population
into 10, 100, perhaps 1,000 groups, each located somewhere on the map.
This has not yet been done with any precision, but it will be. Soon it may
be possible to identify your ancestors not merely as African or European,
but Ibo or Yoruba, perhaps even Celt or Castilian, or all of the above.
The identification of racial origins is not a search for purity. The human
species is irredeemably promiscuous. We have always seduced or coerced our
neighbors even when they have a foreign look about them and we don't
understand a word. If Hispanics, for example, are composed of a recent and
evolving blend of European, American Indian and African genes, then the
Uighurs of Central Asia can be seen as a 3,000-year-old mix of West
European and East Asian genes. Even homogenous groups like native Swedes
bear the genetic imprint of successive nameless migrations.
Some critics believe that these ambiguities render the very notion of race
worthless. I disagree. The physical topography of our world cannot be
accurately described in words. To navigate it, you need a map with
elevations, contour lines and reference grids. But it is hard to talk in
numbers, and so we give the world's more prominent features - the mountain
ranges and plateaus and plains - names. We do so despite the inherent
ambiguity of words. The Pennines of northern England are about one-tenth
as high and long as the Himalayas, yet both are intelligibly described as
mountain ranges.
So, too, it is with the genetic topography of our species. The billion or
so of the world's people of largely European descent have a set of genetic
variants in common that are collectively rare in everyone else; they are a
race. At a smaller scale, three million Basques do as well; so they are a
race as well. Race is merely a shorthand that enables us to speak
sensibly, though with no great precision, about genetic rather than
cultural or political differences.
But it is a shorthand that seems to be needed. One of the more painful
spectacles of modern science is that of human geneticists piously
disavowing the existence of races even as they investigate the genetic
relationships between "ethnic groups." Given the problematic, even
vicious, history of the word "race," the use of euphemisms is
understandable. But it hardly aids understanding, for the term "ethnic
group" conflates all the possible ways in which people differ from each
other.
Indeed, the recognition that races are real should have several benefits.
To begin with, it would remove the disjunction in which the government and
public alike defiantly embrace categories that many, perhaps most,
scholars and scientists say do not exist.
Second, the recognition of race may improve medical care. Different races
are prone to different diseases. The risk that an African-American man
will be afflicted with hypertensive heart disease or prostate cancer is
nearly three times greater than that for a European-American man. On the
other hand, the former's risk of multiple sclerosis is only half as great.
Such differences could be due to socioeconomic factors. Even so,
geneticists have started searching for racial differences in the
frequencies of genetic variants that cause diseases. They seem to be
finding them.
Race can also affect treatment. African-Americans respond poorly to some
of the main drugs used to treat heart conditions - notably beta blockers
and angiotensin-converting enzyme inhibitors. Pharmaceutical corporations
are paying attention. Many new drugs now come labeled with warnings that
they may not work in some ethnic or racial groups. Here, as so often, the
mere prospect of litigation has concentrated minds.
Such differences are, of course, just differences in average. Everyone
agrees that race is a crude way of predicting who gets some disease or
responds to some treatment. Ideally, we would all have our genomes
sequenced before swallowing so much as an aspirin. Yet until that is
technically feasible, we can expect racial classifications to play an
increasing part in health care.
The argument for the importance of race, however, does not rest purely on
utilitarian grounds. There is also an aesthetic factor. We are a
physically variable species. Yet for all the triumphs of modern genetics,
we know next to nothing about what makes us so. We do not know why some
people have prominent rather than flat noses, round rather than pointed
skulls, wide rather than narrow faces, straight rather than curly hair. We
do not know what makes blue eyes blue.
One way to find out would be to study people of mixed race ancestry. In
part, this is because racial differences in looks are the most striking
that we see. But there is also a more subtle technical reason. When
geneticists map genes, they rely on the fact that they can follow our
ancestors' chromosomes as they get passed from one generation to the next,
dividing and mixing in unpredictable combinations. That, it turns out, is
much easier to do in people whose ancestors came from very different
places.
The technique is called admixture mapping. Developed to find the genes
responsible for racial differences in inherited disease, it is only just
moving from theory to application. But through it, we may be able to write
the genetic recipe for the fair hair of a Norwegian, the
black-verging-on-purple skin of a Solomon Islander, the flat face of an
Inuit, and the curved eyelid of a Han Chinese. We shall no longer gawp
ignorantly at the gallery; we shall be able to name the painters.
There is a final reason race matters. It gives us reason - if there were
not reason enough already - to value and protect some of the world's most
obscure and marginalized people. When the Times of India article referred
to the Andaman Islanders as being of ancient Negrito racial stock, the
terminology was correct. Negrito is the name given by anthropologists to a
people who once lived throughout Southeast Asia. They are very small, very
dark, and have peppercorn hair. They look like African pygmies who have
wandered away from Congo's jungles to take up life on a tropical isle. But
they are not.
The latest genetic data suggest that the Negritos are descended from the
first modern humans to have invaded Asia, some 100,000 years ago. In time
they were overrun or absorbed by waves of Neolithic agriculturalists, and
later nearly wiped out by British, Spanish and Indian colonialists. Now
they are confined to the Malay Peninsula, a few islands in the Philippines
and the Andamans.
Happily, most of the Andamans' Negritos seem to have survived December's
tsunami. The fate of one tribe, the Sentinelese, remains uncertain, but an
Indian coast guard helicopter sent to check up on them came under bow and
arrow attack, which is heartening. Even so, Negrito populations, wherever
they are, are so small, isolated and impoverished that it seems certain
that they will eventually disappear.
Yet even after they have gone, the genetic variants that defined the
Negritos will remain, albeit scattered, in the people who inhabit the
littoral of the Bay of Bengal and the South China Sea. They will remain
visible in the unusually dark skin of some Indonesians, the unusually
curly hair of some Sri Lankans, the unusually slight frames of some
Filipinos. But the unique combination of genes that makes the Negritos so
distinctive, and that took tens of thousands of years to evolve, will have
disappeared. A human race will have gone extinct, and the human species
will be the poorer for it.
---
Armand Marie Leroi, an evolutionary developmental biologist at Imperial
College in London, is the author of "Mutants: On Genetic Variety and the
Human Body."
Copyright 2005 The New York Times Company