Roger Bagu
Mon, Dec-11-06, 17:16
http://www.nytimes.com/2006/12/11/science/11evolve.html?ex=13-
23493200&en=a2ded9bbd0241eaf&ei=5088&partner=rssnyt&emc=rss
Lactose Tolerance in East Africa Points to Recent Evolution
By NICHOLAS WADE Published: December 11, 2006
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.
Convergent Adaptation of Human Lactase Persistence in Africa
and Europe (Nature Genetics)
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 the lactase enzyme that breaks the sugar
apart is no longer needed. 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 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 distances from the ancient
Funnel Beaker region.
Geneticists wondered if the lactose tolerance mutation in
Europeans, 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 Dr. Sarah Tishkoff of the University of
Maryland has now solved much of the puzzle. After testing for
lactose tolerance and genetic makeup among 43 ethnic groups in
East Africa, she and her colleagues have found three new
mutations, all independent of one another and of the European
mutation, that 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 reports today in the journal Nature Genetics. 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
such mutations. 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.
Nilo-Saharan speakers in Sudan and their Cushitic-speaking
neighbors in the Red Sea hills probably domesticated cattle at
the same time, because 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 south and would have
met again in Kenya, Dr. Ehret said.
Dr. Tishkoff detected lactose tolerance among 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.
Ds. Jonathan Pritchard, a statistical geneticist at the
University of Chicago and a co-author of the new article,
said there were many signals of natural selection in the
human genome but it was usually hard to know what was
being selected for. In this case Dr. Tishkoff 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.
23493200&en=a2ded9bbd0241eaf&ei=5088&partner=rssnyt&emc=rss
Lactose Tolerance in East Africa Points to Recent Evolution
By NICHOLAS WADE Published: December 11, 2006
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.
Convergent Adaptation of Human Lactase Persistence in Africa
and Europe (Nature Genetics)
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 the lactase enzyme that breaks the sugar
apart is no longer needed. 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 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 distances from the ancient
Funnel Beaker region.
Geneticists wondered if the lactose tolerance mutation in
Europeans, 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 Dr. Sarah Tishkoff of the University of
Maryland has now solved much of the puzzle. After testing for
lactose tolerance and genetic makeup among 43 ethnic groups in
East Africa, she and her colleagues have found three new
mutations, all independent of one another and of the European
mutation, that 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 reports today in the journal Nature Genetics. 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
such mutations. 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.
Nilo-Saharan speakers in Sudan and their Cushitic-speaking
neighbors in the Red Sea hills probably domesticated cattle at
the same time, because 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 south and would have
met again in Kenya, Dr. Ehret said.
Dr. Tishkoff detected lactose tolerance among 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.
Ds. Jonathan Pritchard, a statistical geneticist at the
University of Chicago and a co-author of the new article,
said there were many signals of natural selection in the
human genome but it was usually hard to know what was
being selected for. In this case Dr. Tishkoff 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.