Where did dogs come from? That simple question is the subject of a scientific debate right now. In May, a team of scientists published a study pointing to East Asia as the place where dogs evolved from wolves. Now, another group of researchers has announced that dogs evolved several thousand miles to the west, in Europe.
This controversy illuminates the challenges that scientists face as they excavate the history of any species from its DNA.
Scientists have long agreed that the closest living relatives of dogs are wolves, their link confirmed by anatomy and DNA. Somewhere, at some point, some wolves became domesticated. They evolved not only a different body shape but also a different behavior. Instead of traveling in a pack to hunt down prey, dogs began lingering around humans. Eventually, those humans bred them into their many forms, from shar-peis to Newfoundlands.
A few fossils supply some tantalizing clues to that transformation. Dating back as far as 36,000 years, they look like wolfish dogs or doggish wolves. The oldest of these fossils have mostly turned up in Europe.
In the 1990s, scientists started using new techniques to explore the origin of dogs. They sequenced bits of DNA from living dog breeds and wolves from various parts of the world to see how they were related. And the DNA told a different story than the bones. In fact, it told different stories.
In a 2002 study, for example, Peter Savolainen, now at the Royal Institute of Technology in Sweden, and his colleagues concluded that dogs had evolved in East Asia. Eight years later, however, Robert Wayne, a geneticist at the University of California, Los Angeles, and his colleagues analyzed some new dog breeds and concluded that the Middle East was where dogs had got their start.
Savolainen and his colleagues continued to sequence DNA from more dogs, and they published more evidence for an East Asian origin of dogs — narrowing it down to southern China.
While early studies of canine origins were limited to fragments of DNA, scientists are now starting to sequence entire genomes of dogs and wolves. In May, for example, Savolainen and Chinese colleagues reported that Chinese native dogs had the most wolflike genomes. By tallying up the mutations in the different dog and wolf genomes, they estimated that the ancestors of Chinese village dogs and wolves split about 32,000 years ago.
If this were true, then the first dogs would have become domesticated not by farmers but by Chinese hunter-gatherers more than 20,000 years before the dawn of agriculture.
Wayne and his colleagues think that is wrong.
A dog may have wolflike DNA because it is a dog-wolf hybrid. In a paper that is not yet published, they analyze wolf and dog genomes to look for signs of ancient interbreeding. They cite evidence that, indeed, some of the DNA in dogs in East Asia comes from wolf interbreeding.
Now Wayne and his colleagues are introducing a new line of evidence to the dog debate: ancient DNA.
On Thursday in the journal Science, Wayne, Shapiro, and their colleagues report on the first large-scale comparison of DNA from living and fossil dogs and wolves.
They extracted DNA from 18 fossils found in Europe, Russia and the New World. They compared those genes to those from 49 wolves, 77 dogs, and four coyotes.
The scientists examined a special kind of DNA found in a structure in the cell called the mitochondrion. Mitochondrial DNA comes only from mothers. Because each cell may have thousands of mitochondria, it is easier to gather enough genetic fragments to reconstruct its DNA.
The scientists did not find that living dogs were closely related to wolves from the Middle East or China. Instead, their closest relatives were ancient dogs and wolves from Europe.