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Evolution of the family dog

I first suspected that Boswell would have a short life when he bit my wife on our nuptial bed. Boswell was my dog, a feisty Toto-like terrier who shared my bachelor bed and resented the intrusion of a woman where he felt a dog -- Boswell -- ought to be. As it turns out, my suspicion was correct, and he did not live out the year, which was 1982. Staying with others while I and my bride were overseas, Boswell resented being denied chicken bones, ate them anyway, and died of the consequences. To this day I miss him.

This week I found myself wondering about Boswell's origins. From what creature did the domestic dog arise? Darwin suggested that wolves, coyotes, and jackals -- all of which can interbreed and produce fertile offspring-- may all have played a role, producing a complex dog ancestry that would be impossible to unravel. In the 1950s, Nobel Prize-winning behaviorist Konrad Lorenz suggested some dog breeds derive from jackals, others from wolves.

Based on anatomy, most biologists have put their money on the wolf, but until recently there was little hard evidence, and, as you might expect if you know scientists, lots of opinions.

The issue was finally settled in 1997 by an international team of scientists led by Robert Wayne of the University of California, Los Angeles. To sort out the evolutionary origin of the family dog, Wayne and his colleagues used the techniques of molecular biology to compare the genes of dogs with those of wolves, coyotes and jackals.

Wayne's team collected blood, tissue, or hair from 140 dogs of sixty-seven breeds, and 162 wolves from North America, Europe, Asia, and Arabia. From each sample they extracted DNA from the tiny organelles within cells called mitochondria.

While the chromosome DNA of an animal cell derives from both parents, the mitochondrial DNA comes entirely from the mother. Biologists love to study mitochondrial DNA because of this simple line of descent, female-to-female-to-female. As changes called mutations occur due to copying mistakes or DNA damage, the mitochondrial DNA of two diverging lines becomes more and more different. Ancestors can be clearly identified when you are studying mitochondrial DNA, because clusters of mutations are not shuffled into new combinations like the genes on chromosomes are. They remain together as a particular sequence, a signature of that line of descent.

When Wayne looked at his canine mitochondrial DNA samples, he found that wolves and coyotes differ by about 6% in their mitochondrial DNA, while wolves and dogs differ by only 1%. Already it smelled like the wolf was the ancestor.

Wayne's team then focused their attention on one small portion of the mitochondrial DNA called the control region, because it was known to vary a lot among mammals. Among the sixty seven breeds of dogs, Wayne's

team found a total of 26 different sequences in the control region, each differing from the others at one or a few sites. No one breed had a characteristic sequence -- rather, the breeds of dogs share a common pool of genetic diversity.

Wolves had 27 different sequences in the control region, none of them exactly the same as any dog sequence, but all very similar to the dog sequences, differing from them at most at 12 sites along the DNA, and usually fewer.

Coyote and jackal were a lot more different from dogs than wolves were. Every coyote and jackal sequence differed from any dog sequence by at least 20 sites, and many by far more.

That settled it. Dogs are domesticated wolves.

Using statistical methods to compare the relative similarity of the sequences, Wayne found that all the dog sequences fell into four distinct groups. The largest, containing 19 of the 26 sequences and representing 3/4 of modern dogs, resulted from a single female wolf lineage. The three smaller groups seem to represent later events when other wolves mated with the now-domesticated dogs. Domestication, it seems, didn't happen very often, and perhaps only once.

The large number of different dog sequences, and the fact that no wolf sequences are found among them, suggests that dogs must have been separated from wolves for a long time. The oldest clear fossil evidence for dogs is 12,000 - 14,000 years ago, about when farming arose. But that's not enough time to accumulate such a large amount of mitochondrial DNA difference. Perhaps dogs before then just didn't look much different from wolves, and so didn't leave dog-like fossils. Our species first developed speech and left Africa about 50,000 years ago. I bet that's when dogs came aboard, when our hunter-gatherer ancestors first encountered them. They would have been great hunting companions.

I think Boswell would be happy to know his ancestor was a wolf. I doubt, however, I will ever be able to get my wife to overlook the biting as "wolf genetic baggage" inherited from nobel ancestors. In my house, science only stretches so far.

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