Yukon children can take particular pride in those reindeer pulling Santa's sleigh. There's a fair to middling chance that caribou, the wild brothers and sisters of domesticated reindeer, evolved in this part of the world.

The evidence rests on a bone -- a very, very old bone -- found near Fort Selkirk along the Yukon River. The fossil, part of the leg bone of a caribou, is probably about 1.6 million years old. No older caribou bone has been found in North America. And, if an older one has been found elsewhere, North American experts are not aware of it.

The caribou that donated this bone to science roamed the Yukon during an early stage of the Pleistocene Epoch. Later on during the glacial ages, massive sheets of ice covered much of this area, grinding up almost everything in their path.

The bone survived -- and was found more than a million years later -- because of a series of coincidences. First, flows of magma from a volcano covered the caribou's burial spot, forming layers of basalt which protected the carcass from the ice.

Second, in 1989, a crew of geologists started studying those lava flows. While excavating beneath basalt cliffs along the Yukon River, the geologists found a number of bone fragments. The hunk of caribou was the largest of them.

The bone fragments were found in a layer of loess, or wind-blown silt, underneath the basalt. They were sent to Richard Harington, a paleobiologist with the Canadian Museum of Nature, who has worked extensively in the Yukon. The bone looked little different from the same bone found in a modern caribou, but Harington knew that it had to be older than the rocks above it, which had been dated at 1.2 million years.

In the same layer of sediment, fossil fragments were also found of a primitive type of vole. Another paleontologist pegged the age of the vole at about 1.6 million years old, which would imply that the caribou is of a similar age.

"Rodents are a good paleontological clock," says Harington, "because their teeth show marked variation through geological time which makes it possible to assess their age more accurately than species such as caribou which, evidently, have not changed much over the last million years."

Harington thinks that caribou probably evolved in the northern region we now call Beringia. He says it appears that the earliest ancestors of caribou came from a mountain deer that lived in the North American Cordillera.

Harington thinks that these deer, or their ancestors, gradually migrated north, reaching the northwest corner of the continent about two million years ago. Living there in isolation, these deer evolved into the animals that we know today as a caribou.

During the ice ages, Beringia was a refuge for caribou and many other forms of life. This cold dry grassland stretched from eastern Siberia across Alaska and the northern Yukon to the Mackenzie River in the Northwest Territories.

Harington describes caribou as one of the "Big Four" of Beringian fauna. The other members are woolly mammoths, horses and the steppe bison. These three, like most of the Pleistocene megafauna, went extinct about 10,000 years ago when the ice sheets started to disappear.

Caribou, however, survived and prospered. These animals are superbly adapted to life in the north, and are now widespread in the boreal forest and tundra regions of northern Eurasia and North America.

Trying to figure out exactly where caribou evolved, and when, is an ongoing detective story. Another caribou fossil, thought to be slightly younger than the Fort Selkirk one, was found at Cape Deceit, Alaska. A German caribou fossil has been dated at about 600,000 years old.

The dates on the German fossil fit with Harington's theory that caribou, after evolving in this corner of the continent, migrated west across the Bering Land Bridge to Eurasia, and east to the Atlantic coast of North America. This land connection between the continents has been established several times during the past when lower sea levels exposed the floor of the Bering Sea.

But regardless, Santa's reindeer must owe some credit to their hardy northern genes. What other animals could pull a loaded sleigh all night through the winter sky!

For more information on caribou fossils, you can contact Richard Harington by e-mail at rharington@mus-nature.ca.