DNA diversifies domestication's roots.

Scientists generally trace the domestication of the wild ox, or aurochs, to about 10,000 years ago in ancient Turkey or nearby parts of southwest Asia. These beasts of burden then served as the founding population for modern cattle breeds throughout the world, the predominant theory holds. A new mitochondrial DNA study in living cattle breaks from the scientific herd on this issue.

Genetically discrete breeds of African, Asian, and European cattle existed 22,000 years ago or more, suggesting that domestication arose separately on each continent, assert Daniel G. Bradley, a geneticist at Trinity College in Dublin, and his colleagues.

The genetic findings lend support to the controversial proposal, advanced by Fred Wendorf of Southern Methodist University in Dallas and his coworkers, that cattle domestication emerged in northeastern Africa around 9,000 years ago, independent of any other domestications. At that time, summer rains in the eastern Sahara attracted seasonal occupations by herders, Wendorf argues.

"[Bradley's] article presents strong evidence that African domestic cattle have long been genetically separate from European and Asian cattle," Wendorf says. "Three centers of cattle domestication may have existed, with no crossbreeding of animals until sometime after 2,000 years ago."

The Irish researchers extracted mitochondrial DNA, which is inherited only through the mother, from blood samples of 90 living cattle in Africa, Europe, and India. The animals represented 13 breeds. Analyses focused on a region of mitochondrial DNA that undergoes rapid chemical alterations. Cattle from each continent displayed signature chemical sequences in their mitochondrial DNA, Bradley's group reports in the May 14 Proceedings of the National Academy of Sciences.

In a comparison of cattle mitochondrial DNA to genetic data already obtained from living bison, the scientists estimated that a common ancestor of aurochs and bison lived at least 1 million years ago. A genetic split occurred between Indian cattle and those in Africa and Europe between 117,000 and 275,000 years ago, according to a calculation based on differences in mitochondrial DNA across cattle populations.

African and European cattle parted genetically between 22,000 and 26,000 years ago, the researchers maintain. Differences in mitochondrial DNA might stem from the incorporation of local wild oxen into early African and European herds that derived from a single population, but the researchers favor separate domestication centers. The data tentatively support an early date of Saharan domestication, they add. More than 20 bones found at Saharan sites dating to 9,500 years ago probably come from cattle that belonged to herders, Wendorf argues. Several other scientists assign the same bones to wild oxen or bison.