An ancient bone found eroding out of a riverbank in the Yukon is the oldest bison fossil ever discovered in North America, researchers say.

What’s more, genetic evidence extracted from the bone reveals that all of the continent’s bison, living and extinct, descended from a single female ancestor that crossed into the Americas less than 200,000 years ago — and not millions of years ago, as some experts had thought.

“There has long been a controversy about the timing of bison arrival in North America,” said Dr. Beth Shapiro, of the UC Santa Cruz Paleogenomics Lab, in a press statement.

“Until recently, the fossil records from different parts of North America disagreed with each other, with a few fossil localities suggesting that bison arrived millions of years ago but most old fossil sites showing no evidence of bison at all.”

The newly found fossil is limited to a single foot bone, found protruding from a riverside cliff known as Ch’ijee’s Bluff in western Yukon Territory along the Porcupine River, not far from the Alaska border.

The exposed strata of the bluff allowed the researchers to date the bone to around 130,000 years old.

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