Study Detects Recent Instance of Human Evolution
Published: December 10, 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.
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 said.
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.