All domestic dogs, from Chihuahuas
to Saint Bernards,
are descended from the gray
Many explorers said they could scarcely
tell the difference between a wolf and a Plains dog, which was well-known
for its ability to drag heavy loads for humans.
"There's no doubt that the dog is closest
to the wolf," says Juliet Clutton-Brock of the Natural History
Museum of London, and one of the world's foremost authorities
on the prehistory of domestic animals. Studies since the 1950s reveal many
similarities between wolf and dog morphology and behavior, and experts
have formed a consensus: the more than 400 breeds of domestic dog, from
Chihuahuas to Saint Bernards, were descended from one of the small, southern
Asian subspecies of the gray wolf-perhaps the Arabian wolf, or the Indian
wolf immortalized by Rudyard Kipling.
The oldest fossils of what are undisputedly
dogs date from about 11,000 or 12,000 years ago in Southwest Asia, making
dogs the oldest of all domesticated animals. Archaeozo-ologists-scientists
who study the remains of animals in association with humans-assumed that
the domestication process would have started much earlier, perhaps 15,000
years ago, in conjunction with the rise of permanent villages and the advent
But in 1997, a team led by evolutionary
biologists from the University of California at Los Angeles dropped a bombshell.
After analyzing DNA from wolves and wild canids around the world, as well
as from nearly 70 breeds of dogs, they concluded that dogs and wolves probably
split off from each other originally more than 100,000 years ago-almost
the same time that anatomically modern humans were first emerging, and
long before anyone suspected domestication was possible. While hailed by
some molecular biologists, the UCLA findings have been questioned by paleontologists
and archaeozoologists. Last August, at a symposium on the history
of the domestic dog at the University of Victoria in British Columbia,
sponsored by the International Council for Archaeozoology, the controversy
was a major topic.
One explanation that might reconcile the
archaeological record and the DNA findings is that the dogs ancestors were
wolves that split off from other wolf lineages 100,000 years ago, even
though dogs themselves didn't evolve until more recently.
While debate swirls around the timing of
dog domestication, some experts are taking aim at the fundamental notion
that dogs and other animals were domesticated through a human-directed
"The standard explanation of how domestication
began-that people brought in young wild animals, which they tamed and bred
over many years to produce domestic stock-is a myth," argues archaeozo-ologist
Susan Crockford, an expert on the Northwest wool dog, and the organizer
of the Victoria symposium.
For quite a while, biologists have believed
that domestication took place over a long time: a period that would cover
taming an animal, molding it from a wild form into a physically and behaviorally
different creature. That transition period should provide lots of
intermediate forms in the archaeological record-only it doesn't. Instead,
the bones of dogs suddenly pop up in archaeological sites about 12,000
years ago, at the same time humans were abandoning their hunter-gatherer
Whether you're talking about dogs, sheep,
cows, goats, pigs or water buffalo, there are consistent differences between
the wild and domesticated forms. Compared with their wild cousins, most
domestic mammals tend to be smaller, have shorter snouts, smaller brains
and are more likely to be piebald or solid in color; they are more docile,
reproduce at a younger age, have larger litters and have reproductive schedules,
such as multiple breeding seasons in a single year, that differ from those
of wild animals. Such changes also occur in domestic birds.
Interestingly, all these differences are
a consequence of changes in developmental rates, especially while the animal
is young, which result in a sexually mature adult with the size and some
of the characteristics of a juvenile of its ancestors condition known as
paedomorphosis. And those developmental rates, in turn, all appear to be
controlled directly or indirectly by a single biochemical: thyroxine, a
hormone produced by the thyroid gland, which in turn regulates a suite
of crucial growth and developmental genes. Thyroxine, Crockford believes,
was the key to domestication changes.
Crockford theorizes that in a sense, wild
canids domesticated themselves. By creating a new environment, one in which
food supplies were available to those wolves able to tolerate the presence
of people, humans set the stage for rapid evolution. Fear is controlled,
in part, by the adrenal gland, and adrenaline production, in turn, is one
of the many biological functions controlled by thyroxine.
In Crockford's view, the less fearful wolves
would thrive near settlements, scavenging garbage middens and filching
meat from drying racks, breeding among themselves and reinforcing those
attributes. Natural selection would favor canids with thyroxine levels
that produce lower adrenal response. Any pups born with a more fearful
nature would simply drift away from the villages, back into the wilderness.
After just a few generations, she believes, the wolves living near humans
would exhibit reproductive, physical and behavioral differences, triggered
by their new thyroxine patterns, that would set them apart from their wilder
counterparts. They would have become primitive dogs. Only much later, long
after primitive dogs had become genetically distinct and reproductively
isolated from wolves, did humans begin exerting artificial selection to
create distinct breeds.
Crockford cites intriguing evidence for
her hypothesis. For 2O years starting in the 1950s, researchers in Siberia,
trying to create a strain of silver fox that would be easier for fur-farm
workers to manage, began selecting breeding pairs strictly on the basis
of how calmly they behaved around people. Unintentionally, the Soviets
were selecting foxes based at least in part on their thyroxine levels,
Crockford contends. Within just 20 generations, foxes in the fearless strain
had become markedly smaller, had undergone changes in their reproductive
schedule, and had developed floppy cars, curled tails and piebald coats--precisely
the traits that often separate dogs and wolves, and all of which are under
the control of thyroxine.
"It may have taken only about 40 years,
at two years per generation, for wolves to evolve into early dogs-perhaps
more than that, but we can now look at that number as some sort of minimum.
And 40 years is almost certainly too fast to pick up intermediate stages
in the archaeological record", Crockford argues.
While some scientists look toward the dog's
past, others are casting a worried eye toward the future of primitive breeds.
Brisbin thinks the Carolina dog is relatively secure, although the expansion
of coyotes into the Southeast may be causing a reduction in their numbers.
But others, like the Tengger dog in Java and the Falkland Islands "wolf"'
are extinct, and more are threatened. Conservation organizations,
already overburdened and overextended, rarely pay attention to domestic
animals-even though many breeds represent important genetic diversity,
and an irreplaceable slice of human and natural history.
Which brings us back to the Carolina dogs.
Are they, as some people claim, a direct link to the aboriginal past, or
a recent construct of the canine melting pot?
Brisbin will keep working toward a major
genetic study of the dogs. He also hopes to track wild Carolinas by fitting
them with radio collars, to learn more about their habits, territory and
But even if further research proves the
Carolina is of modern origin, it still has much to tell us about natural
selection and how, in a relatively short time, stray dogs were molded into
an animal well suited to the wet, hot coastal plain of the Southeast. And
that lesson alone, Brisbin and other researchers believe, makes this shy,
lovely animal worthy of study and conservation.
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