Sunday, December 10, 2006

Latose tolerance and evolution

NY Times, December 10, 2006
Study Detects Recent Instance of Human Evolution


A surprisingly recent instance of human evolution has been detected
among the peoples of East Africa. It is the ability to digest milk in
adulthood, conferred by genetic changes that occurred as recently as
3,000 years ago, a team of geneticists has found.

The finding is a striking example of a cultural practice — the raising
of dairy cattle — feeding back into the human genome. It also seems to
be one of the first instances of convergent human evolution to be
documented at the genetic level. Convergent evolution refers to two or
more populations acquiring the same trait independently.

Throughout most of human history, the ability to digest lactose, the
principal sugar of milk, has been switched off after weaning because
there is no further need for the lactase enzyme that breaks the sugar
apart. But when cattle were first domesticated 9,000 years ago and
people later started to consume their milk as well as their meat,
natural selection would have favored anyone with a mutation that kept
the lactase gene switched on.

Such a mutation is known to have arisen among an early cattle-raising
people, the Funnel Beaker culture, which flourished some 5,000 to
6,000 years ago in north-central Europe. People with a persistently
active lactase gene have no problem digesting milk and are said to be
lactose tolerant.

Almost all Dutch people and 99 percent of Swedes are lactose-tolerant,
but the mutation becomes progressively less common in Europeans who
live at increasing distance from the ancient Funnel Beaker region.

Geneticists wondered if the lactose tolerance mutation in Europeans,
first identified in 2002, had arisen among pastoral peoples elsewhere.
But it seemed to be largely absent from Africa, even though pastoral
peoples there generally have some degree of tolerance.

A research team led by Sarah Tishkoff of the University of Maryland
has now resolved much of the puzzle. After testing for lactose
tolerance and genetic makeup among 43 ethnic groups of East Africa,
she and her colleagues have found three new mutations, all independent
of each other and of the European mutation, which keep the lactase
gene permanently switched on.

The principal mutation, found among Nilo-Saharan-speaking ethnic
groups of Kenya and Tanzania, arose 2,700 to 6,800 years ago,
according to genetic estimates, Dr. Tishkoff's group is to report in
the journal Nature Genetics on Monday. This fits well with
archaeological evidence suggesting that pastoral peoples from the
north reached northern Kenya about 4,500 years ago and southern Kenya
and Tanzania 3,300 years ago.

Two other mutations were found, among the Beja people of northeastern
Sudan and tribes of the same language family, Afro-Asiatic, in
northern Kenya.

Genetic evidence shows that the mutations conferred an enormous
selective advantage on their owners, enabling them to leave almost 10
times as many descendants as people without them. The mutations have
created "one of the strongest genetic signatures of natural selection
yet reported in humans," the researchers write.

The survival advantage was so powerful perhaps because those with the
mutations not only gained extra energy from lactose but also, in
drought conditions, would have benefited from the water in milk.
People who were lactose-intolerant could have risked losing water from
diarrhea, Dr. Tishkoff said.

Diane Gifford-Gonzalez, an archaeologist at the University of
California, Santa Cruz, said the new findings were "very exciting"
because they "showed the speed with which a genetic mutation can be
favored under conditions of strong natural selection, demonstrating
the possible rate of evolutionary change in humans."

The genetic data fitted in well, she said, with archaeological and
linguistic evidence about the spread of pastoralism in Africa. The
first clear evidence of cattle in Africa is from a site 8,000 years
old in northwestern Sudan. Cattle there were domesticated
independently from two other domestications, in the Near East and the
Indus valley of India.

Both Nilo-Saharan speakers in Sudan and their Cushitic-speaking
neighbors in the Red Sea hills probably domesticated cattle at the
same time, since each has an independent vocabulary for cattle items,
said Dr. Christopher Ehret, an expert on African languages and history
at the University of California, Los Angeles. Descendants of each
group moved southward and would have met again in Kenya, Dr. Ehret

Dr. Tishkoff detected lactose tolerance among both Cushitic speakers
and Nilo-Saharan groups in Kenya. Cushitic is a branch of
Afro-Asiatic, the language family that includes Arabic, Hebrew and
ancient Egyptian.

Dr. Jonathan Pritchard, a statistical geneticist at the University of
Chicago and the co-author of the new article, said that there were
many signals of natural selection in the human genome, but that it was
usually hard to know what was being selected for. In this case Dr.
Tishkoff had clearly defined the driving force, he said.

The mutations Dr. Tishkoff detected are not in the lactase gene itself
but a nearby region of the DNA that controls the activation of the
gene. The finding that different ethnic groups in East Africa have
different mutations is one instance of their varied evolutionary
history and their exposure to many different selective pressures, Dr.
Tishkoff said.

"There is a lot of genetic variation between groups in Africa,
reflecting the different environments in which they live, from deserts
to tropics, and their exposure to very different selective forces,"
she said.

People in different regions of the world have evolved independently
since dispersing from the ancestral human population in northeast
Africa 50,000 years ago, a process that has led to the emergence of
different races. But much of this differentiation at the level of DNA
may have led to the same physical result.

As Dr. Tishkoff has found in the case of lactose tolerance, evolution
may use the different mutations available to it in each population to
reach the same goal when each is subjected to the same selective
pressure. "I think it's reasonable to assume this will be a more
general paradigm," Dr. Pritchard said.

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