DOGS have been man鈥檚 best friend for much longer than most people had
thought. They were probably domesticated several times from wolves, beginning
more than 100 000 years ago, according to DNA evidence announced last
week.
The oldest evidence of a domestic partnership鈥攁rchaeological findings
showing dog-like skeletal remains in human burial sites鈥攄ates back a mere
14 000 years. Most researchers have, therefore, assumed that dogs joined human
society at about the same time humans learnt to settle down and grow crops.
Carles Vil脿, a biologist at the University of California at Los
Angeles, and his colleagues, traced the family tree of dogs by comparing DNA
sequences in a highly variable region of mitochondrial DNA. Since every pup
inherits mitochondria from its mother only, this DNA provides an especially
clear way of tracing descent through the female line. The researchers sequenced
mitochondrial DNA from 140 dogs of 67 breeds, plus 162 wolves from around the
world, 5 coyotes, and 12 jackals.
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As they expected, wolves proved to be dogs鈥� closest relatives. Indeed, dogs
can trace at least four distinct evolutionary lines back to wolves, they
reported in last week鈥檚 Science (vol 276, p 1687). The dogs鈥�
mitochondrial DNA proved to be surprisingly variable, which suggests a longer
period of evolution than biologists had thought. 鈥淚f the origin [of dogs] took
place just 14 000 years ago, we would expect the dogs to be almost identical,鈥�
says Vil脿. Based on other measures of the rate of genetic change in these
DNA sequences, Vil脿 estimates that dogs and wolves diverged roughly 135
000 years ago.
鈥淭hat is a real shock,鈥� says John Bradshaw, a zoologist at the University of
Southampton. Researchers will need to rethink their views on how people and dogs
have adapted to one another, he says.