Cows Were in the Air

By Carl Zimmer|Thursday, September 01, 1994
The idea of a large, docile animal that would pull your plow and make a good stew occurred to more than one person 8,000 years ago.

Civilization rode into the Old World on the backs of cows. Not only were they a richer source of protein than the pigs, goats, and sheep that had been domesticated earlier, they were also the first draft animals. With cattle pulling plows, farmers were suddenly able to cultivate large areas of previously untillable soil. It was something like the invention of the combustion engine, says Caroline Grigson, a zooarcheologist at the Royal College of Surgeons in London. In fact, the domestication of cattle was so important that it apparently happened more than once.

Today’s tame cattle all descend from the larger, wild aurochs, an animal that once lived across Asia, northern Africa, and Europe (the last known aurochs died in Poland in 1627). They fall into three groups: zebu, the large-humped, droopy-eared cows common to southern Asia; taurine cattle, the humpless types found all over the Americas and Europe and across northern Asia to Japan; and African sangas, sometimes humped and sometimes not, and thought by some to be a hybrid of zebu and taurine breeds.

Based on archeological remains, many researchers have long believed that cattle were domesticated only once, 8,000 years ago, in southwestern Turkey. From there, it was thought, domestic cattle were brought to Africa, Asia, and Europe. The first cattle were taurine, the thinking went; then, 7,000 years ago, people living in the arid parts of Iran developed the drought-resistant zebu, with its water-storing hump. Only later were zebu breeds brought to India, where they are now ubiquitous.

This story, if true, ought to be preserved in the DNA of living cattle. DNA is found both in the nucleus of a cell and in its mitochondria- -the rod-shaped organelles that supply the cell with energy. Nuclear DNA is a mix of genetic material from both parents, but mitochondrial DNA passes intact from the mother alone to her descendants. As a result, while nuclear DNA is shuffled drastically with every generation, the only changes that occur in mitochondrial DNA are spontaneous mutations, which occur with clocklike regularity. By measuring how many differences have accumulated in the mitochondrial DNA of different lineages, researchers can gauge how long ago they diverged from a common ancestor.

A group of geneticists at Trinity College in Dublin recently collected blood samples from 13 breeds of African, Asian, and European cattle to construct the first mitochondrial-DNA genealogy for the Bos genus. If the single-domestication-site hypothesis were true, explains Ronan Loftus of Trinity, then all the differences between zebu and taurine breeds would have accumulated in the last few thousand years--that is, after cattle were supposedly first domesticated in Turkey. Yet the DNA record suggests that the last common ancestors of zebu and taurine cattle lived at least 500,000 years ago, and probably closer to 750,000 years ago- -long before modern humans evolved. Since our hominid ancestors weren’t cattle ranchers, zebu and taurine cattle must have evolved in the wild from two populations of aurochs that had become isolated at least 500,000 years ago. Much later, humans must have domesticated the two kinds of cattle separately.

That’s not news to Grigson, who, flouting the standard view among archeologists, has long argued that zebu cattle were domesticated independently in India. The genetic evidence also favors, and may yet confirm, an even more controversial belief of Grigson’s, namely that North Africans domesticated the aurochs as well. Most researchers believe that African sangas are a cross between taurine cattle, which were brought to Africa 7,000 years ago, and zebu cattle that were imported to Africa by Arabs around 600 A.D. But Grigson points out that African cattle have characteristics that are neither zebu nor taurine, such as a narrow face and long horns. She thinks Africans domesticated their own aurochs--which then mixed over the centuries with immigrant cattle--and thus took this important step toward civilization on their own.
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