Gene Study Traces Cattle Herding in Africa
for National Geographic News
|April 11, 2002|
African herders rely on cattle for food and other basic needs, and as
beasts of burden. But how cattle domestication occurred in Africa has
been obscured by long-ago migrations and trade.
Now, by studying
the DNA of cattle in 23 countries, an international team of scientists
is filling in the picture.
Evidence suggests that sheep and goats, first domesticated in the Near East, were imported into Africa through colonization and ocean-going trade. Scientists have long speculated that the domestication of cattle also occurred first in the Near East and that the practice of herding cattle was similarly imported.
But new evidence, reported in the April 12 issue of the journal Science, suggests that Africans independently domesticated cattle.
Belgian geneticist Olivier Hanotte, who headed the new study, said the research "reconciles the two schools of thought" about how cattle domestication occurred in Africa.
"There were Near Eastern influences" on African herds, he said, "but they came after local domestication."
Since then, there has been considerable mixing of African and Asian breeds.
In general, the domestication of cattle and other livestock has followed the establishment of agriculture. But archaeological research has shown that the domestication of cattle unfolded differently in Africa than elsewhere in the world.
In many parts of Africa, people herded cattle long before agriculture was introduced from the Near East and south Asia. Some African groups that have herded cattle for centuries have never adopted agriculture at all, or have done so only recently. One example is the Masai of eastern Africa, who rarely slaughter cattle but instead mix the milk and blood of the animals to create a staple of their diet.
Intrigued by the uncommon pattern of cattle domestication in Africa, Hanotte moved to Kenya in 1995 in an effort to explain the development. He and other researchers in Europe began untangling layers of genetic information in cattle DNA to help answer major questions about the history of herding in Africa.
Their findings offer scientists and herders a virtual history book describing how cattle, crucial to so many Africans, came to be so genetically diverse. The research also underscores why preserving that variety is essential.
Hanotte and his colleagues analyzed more than a dozen segments of the cattle genome. Because the sections they looked at don't affect how "fit" an animal is evolutionarily, they aren't subject to the effects of natural selection.
As a result, those genetic segments record the genetic twists and turns of different cattle lineages and, in the language of DNA, serve as scribes of bovine history.
The researchers compared this DNA material among many individual cattle belonging to 50 different herds in 23 African nations.
Herders, scientists, and government officials in those countries aided the study by tracking down sometimes-remote herds, testing them, and transmitting the data to Hanotte and his team.
When Hanotte and his colleagues analyzed the samples of cattle DNA, they found that the variation associated with certain segments of genetic code reveal a telling geographic pattern across Africa.
The nature of genetic variation changed like the colors of a rainbow as the researchers looked at cattle from West Africa, Central Africa, and southern Africa. The greatest amount of genetic diversity was found among herds in Central Africa.
Based on the data, Hanotte and his colleagues concluded that people living in Central Africa developed cattle domestication on their own, and that the techniquesor the herders themselvesgradually migrated toward the west and the south, spreading domestication across the continent.
In looking at the wide genetic variation among African cattle, the researchers found evidence of interbreeding between cattle native to Africa and an imported breed.
Most modern African herds represent mixtures of two breeds: Africa's native cattle, called taurines (Bos taurus), and a slightly larger Asian breed, known as zebu (Bos indicus), which was domesticated before it arrived in Africa.
Long-distance trade across the Indian Ocean brought many domesticated plants and animals to Africa, including the chicken and camel and cereals such as finger millet and sorghum. Presumably, Hanotte said, trade also brought zebu bulls that farmers interbred with domesticated taurine cows, producing the mixed herds of today.
Some variation in the African herds is also attributable to European influences, Hanotte said. These genetic contributions came in the past few hundred years, during Europe's colonial influence in Africa.
For thousands of years, animal farmers have gradually improved their livestock by selectively breeding animals with different desired traits to endow the offspring with valuable combinations of traits.
Resistance to sleeping sickness is one trait that potentially could spread through selective breeding. Taurine cattle in one region of western Africa, unlike most livestock, are resistant to the parasite that causes the deadly disease.
But the number of animals with the protective adaptation is dwindling, as local farmers give up their taurine herds for large zebu animals.
Hanotte, along with other people, is worried by this trend. "The starting material for selective breeding is diversity," he said. "We can't afford to lose it."
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