Dogs and Wolves: They're Only a Tail Wag Apart
BOSTON — Yapping at the drop of a hat, a carefully coiffed toy poodle hardly inspires the awe of a howling timber wolf.
Based on a study by an international team of biologists, however, it may be time to show Fifi a little more respect. By comparing genetic material from dogs with that of wolves, coyotes, and jackals, the researchers have proved what zoologists have long suspected: Domestic dogs descended from wolves.
What's more, the scientists say, their work suggests that canines were in the process of becoming man's best friend between 60,000 and 135,000 years ago - as much as 121,000 years earlier than the archaeological record suggests.
Up to now, the case for the wolf as the pooch's forerunner has largely rested on physical and behavioral similarities, as well as archaeological records, according to Robert Wayne, associate professor of biology at the University of California at Los Angeles and a member of the research team. He and his colleagues, however, hunted at the molecular level for the biological equivalent of the "smoking gun."
To do so, "we wanted to sample as many breeds as possible," Dr. Wayne says. These ranged from greyhounds and Australian dingos to more modern schnauzers, setters, and spaniels.
Overall, the team's study involved 162 wolves from 27 locations worldwide and 140 dogs from 67 breeds. Five coyotes and a dozen jackals were also included because all wild species of canines can interbreed, and these species seemed the most likely alternative candidates as forebears to the modern dog.
The evidence the team sought is locked up in mitochondria, tiny organs inside cells responsible for supplying the energy the cell needs. These tiny powerhouses, which lie outside the cell's nucleus, are governed by their own self-contained DNA, the molecule that holds the basic chemical building blocks of organic life.
Mitochondrial DNA has two things going for it as evolutionary historian. It evolves faster than the DNA in the cell's nucleus, where an organism's entire genetic blueprint resides.
These changes take place over thousands of years, instead of millions of years for nuclear DNA, so researchers have more points of comparison. And mitochondrial DNA is passed only from mother to offspring, so it is less confusing to decipher than nuclear DNA, which gets jumbled as it's shuffled between males and females and is a larger molecule to untangle.
After taking DNA samples from their subjects and comparing images of a common region in the mitochondrial DNA, "the only evidence was for wolf ancestry, with two or three episodes of interbreeding" between wolves and dogs after dogs began running with humans or their ancestors, Wayne says.
If the team's DNA work confirmed a long-held view of dog ancestry, however, it is challenging notions about when domestic dogs first emerged.
According to the archaeological record, dogs were first domesticated about the time the first human agricultural communities formed some 14,000 years ago.
Not so, according to the team. According to the fossil record, wolves and coyotes split from a common ancestor about 1 million years ago.
Using this as a reference point for the age of the wolf lineage, the researchers crunched the numbers. The DNA evidence pointed to a much earlier starting time for dog domestication - as far back as 135,000 years ago.
The team, whose results appear in the current issue of the journal Science, speculates that archaeologists haven't been able to pick up earlier domestication because dogs' physical characteristics didn't shift until humans became settled and began to breed them. Earlier hounds looked like wolves, they suggest. Moreover, the researchers note that fossil remains of wolves have been associated with those of hominids as far back as 400,000 years ago.
At home on the hearth
But finding wolves in the same caves with hominids doesn't mean they were there as part of the family, warns Marion Schwartz, a research associate in the anthropology department at Yale University in New Haven, Conn., and the author of a new book on the history of dogs in the Americas.
Hominids would stay for a week or so; the wolves may have entered to pick up hearth scraps. Moreover, Ms. Schwartz says, the physical changes in dogs constitute domestication.
"Morphological change is the process they undergo as people weed out the ferociousness of wolves," she says.
Mitochondrial DNA doesn't mark time with Rolex precision, adds Charles Aquadro, professor of population genetics at Cornell University in Ithaca, N.Y. "It's a ballpark estimate."
Whatever the outcome, the study reinforces a larger point about the value of genetic diversity and the need to preserve progenitor species, the researchers say.
It was periodic interbreeding with wolves, Wayne notes, that gave dogs a sufficiently enriched gene pool to permit the selective breeding that has led to "an amazing amount of diversity in dogs. This is an extended lab for evolution studies."