News: Darwin's children.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Fri, 14 Dec 2007 14:16:18 GMT
Darwin's children
Dec 13th 2007
From The Economist print editionhttp://www.economist.com/science/displaystory.cfm?story_id=10283306
Human evolution has speeded up over the past 80,000 years. That raises
awkward questions about the concept of ?race?
PROBABLY, more bad science has been conducted on the concept of human
race than on any other field of biology. The reason is that an awful
lot of research into race has been motivated by preconceived ideas
that one lot of people are somehow ?better? than another lot, rather
than being a disinterested investigation of regional variations in a
single species and the evolutionary pressures that have created them.
Contrariwise, even well constructed studies, if they do find racial
differences, risk opposition from those who deny that people from
different parts of the world could ever differ genetically from one
another in important ways. As a result, only the foolish or the daring
rush in to add to the carnage. It remains to be seen which category
the authors of two papers in this week's Proceedings of the National
Academy of Sciences fall into.
One of the papers, written by Andrea Migliano and her colleagues at
Cambridge University, looks at a local outcome of human evolution?the
short stature often known as pygmyism?and tries to explain the
evolutionary circumstances that cause it. The other, by Robert Moyzis
of the University of California, Irvine, and his colleagues, asks a
broader question: how much evolutionary change has happened since Homo
sapiens climbed out of his African cradle and began to colonise the
world? The answer is, quite a lot?and the rate of change seems to have
speeded up.
Breed early and breed often
The best known pygmies are the Aka, Efe and Mbuti of central Africa.
However, by the standard definition?which is that a group's men have
an average height below 1.55 metres?there are also pygmies in the
Andaman Islands, Malaysia, Thailand, Indonesia, the Philippines, Papua
New Guinea, Brazil and Bolivia.
African pygmies usually live in forests, and the conventional
explanation for their stature has been that it makes it easier for
them to move through dense vegetation. There are also two competing
explanations: that small bodies keep cool more readily than large ones
(pygmyism tends to be a tropical phenomenon) or that pygmies live in
places with unreliable food supplies, and their size means they can
make do with smaller meals.
Dr Migliano and her team reject all these explanations. Some
non-African pygmies live outside forests and many live in cool, dry
areas. Moreover, some of the world's tallest people, such as the
Turkana and Maasai of East Africa, suffer periodic interruptions to
their food supply. Instead, the researchers suggest that short stature
is not a desirable feature in itself, but is rather a consequence of
something else, namely a need to reproduce early.
By adding pre-existing data for African pygmies to new information
they have collected about the Aeta and the Batak of the Philippines,
they show that at the beginnings of their lives all these pygmy
populations follow the same growth curves as taller people, including
Turkana and Americans. This demonstrates that pygmyism is not a result
of early malnutrition, as another hypothesis has it. At the age of
about 12, however, pygmies stop growing. That is also the age at which
they become sexually mature?about three years earlier than taller
people.
The other part of the argument is that all observed pygmy populations
have a short life expectancy. Indeed, this, according to Dr Migliano's
hypothesis, is the crucial evolutionary pressure. Of the six groups of
pygmies for whom data exist, two have a life expectancy of 24 years
and the other four about 16 years.
Exactly why that is so probably varies from place to place (in the
case of the Filipino groups, Dr Migliano cites tuberculosis, measles
and malaria as the most likely causes), but the actual reasons do not
affect the evolutionary argument. This is that a short life exerts
pressure to mature early, and thus switch resources from growth to
reproduction. A mathematical model used by the team confirms that,
given pygmy life expectancies, their growth and reproduction patterns
have indeed been optimised by natural selection. The various pygmy
groups are thus the products of harsh circumstances.
Local heroes
Such alteration of stature is a particularly clear example of what Dr
Moyzis's paper suggests is a wider phenomenon?that Homo sapiens is
continuing to undergo local evolution. He and his colleagues reckon
they can both estimate the rate of evolution and identify many of the
evolving genes, by using a trick with the clumsy name of linkage
disequilibrium.
Genes are linked together in cell nuclei on structures called
chromosomes. These come in pairs, one from each parent. However, when
sperm and egg cells are formed, the maternal and paternal chromosomes
swap bits of DNA to create a new mixture. The pieces of DNA swapped
are complementary?that is, they contain the same types of gene. But
they may contain different versions of the genes in question, and
these different versions can have different biological effects.
Over the generations this process of swapping mixes the genes up
thoroughly, and an equilibrium emerges. If a new mutation appears,
however, it will take quite a while for that thorough mixing to
happen. This means recent mutations can be spotted because they are
still linked to the same neighbouring bits of DNA as they were when
they first appeared. Moreover, the size of these neighbouring blocks
gives an indication of how long ago the mutation in question emerged;
long blocks suggest a recent mutation because the mixing process has
not had time to break them up.
All this has been known for decades, but it is only recently that
enough human DNA sequences have become available for the technique to
be used to compare people from different parts of the world. And this
is what Dr Moyzis and his colleagues have now done.
What they have found is that about 1,800 protein-coding genes, some 7%
of the total known, show signs of having been subject to recent
natural selection. By recent, they mean within the past 80,000 years.
Moreover, as the chart shows, the rate of change has speeded up over
the course of that period. (The sudden fall-off at the end is caused
because the linkage-disequilibrium method cannot easily detect very
recent mutations, rather than by a sudden reduction in the rate of
evolution.) The researchers put this acceleration down to two things.
First, the human population has expanded rapidly during that period,
which increases the size of the gene pool in which mutations can
occur. Second, the environment in which people find themselves has
also changed rapidly, creating new contexts in which those mutations
might have beneficial effects.
That environmental change itself has two causes. The past 80,000 years
is the period in which humanity has spread out of Africa to the rest
of the world, and each new place brings its own challenges. It has
also been a period of enormous cultural change, and that, too, creates
evolutionary pressures. In acknowledgment of these diverse
circumstances, the researchers looked in detail at the DNA of four
groups of people from around the planet: Yoruba from Africa, Han
Chinese and Japanese from Asia, and Europeans.
Various themes emerged. An important one was protection from disease,
suspected to be a consequence of the increased risk of infection that
living in settlements brings. In this context, for example, various
mutations of a gene called G6PD that are thought to offer protection
from malaria sprang up independently in different places.
A second theme is response to changes in diet caused by the
domestication of plants and animals. One example of this is variation
in LCT, a gene involved in the metabolism of lactose, a sugar found in
milk. All human babies can metabolise lactose, but only some adults
can manage the trick. That fact, and the gene involved, have been
known for some time. But Dr Moyzis's team have worked out the details
of the evolution of LCT. They suspect that it was responsible for the
sudden spread of the Indo-European group of humanity about 4,000 years
ago, and also for the more recent spread of the Tutsis in Africa,
whose ancestors independently evolved a tolerant version of the gene.
The pressures behind other changes are less obvious. In the past
2,000-3,000 years, for example, Europeans have undergone changes in
the gene for a protein that moves potassium ions in and out of nerve
cells and taste buds. There have also been European changes in genes
linked to cancer and Alzheimer's disease. Chinese, Japanese and
Europeans, meanwhile, have all seen changes in a serotonin
transporter. Serotonin is one of the brain's messenger molecules, and
is particularly involved in establishing mood.
The finding that may cause most controversy, however, is that in the
Asian groups there has been strong selection for one variant of a gene
that, in a different form, is responsible for Gaucher's disease. A few
years ago two of the paper's other authors, Gregory Cochran and Henry
Harpending, suggested that the Gaucher's form of the gene might be
connected with the higher than average intelligence notable among
Ashkenazi Jews. The unstated inference is that something similar might
be true in Asians, too.
The Ashkenazim paper caused quite a stir at the time. It was merely a
hypothesis, but it did suggest a programme of research that could be
conducted to test the hypothesis. So far, no one?daring or foolish?has
tried. Eventually, however, such questions will have to be faced. The
paper Dr Moyzis and his colleagues have just published is a ranging
shot, but the amount of recent human evolution it has exposed is
surprising. Others will no doubt follow, and the genetic meaning of
the term ?race?, if it has one, will be exposed for all to see.
--
Bob.
.
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