DNA suggests North American mammoth species interbred

Supposedly separate types may really have been one

LAS VEGAS — The two major species of North American mammoth may actually have been one. DNA analysis of the Ice Age beasts’ remains suggests that the woolly mammoth (Mammuthus primigenius) interbred with what has been considered a separate, more southerly species — the Columbian mammoth (Mammuthus columbi).

Although separate animal species do interbreed now and then, mammoth mixing may have been more than an occasional fluke. Two Columbian specimens turn out to carry woollylike DNA inherited from their mothers, said Ross MacPhee of the American Museum of Natural History in New York City, who presented the findings November 3 at a meeting of the Society of Vertebrate Paleontology.

“Woolly and Columbian mammoths may be so close that they should really be regarded as the same thing,” he said. “One extraordinarily variable species.”

Both types of mammoths roamed the North American tundra tens of thousands of years ago. The smaller woolly mammoth was thought to have emigrated from Eurasia, while the larger Columbian was considered native to North America.

The new findings come from one well-preserved Columbian mammoth from Utah, and a second, less well-preserved one from Wyoming. DNA analysis placed both on the same branch of the genetic family tree as a cluster of woollies.

The results have left MacPhee “gobsmacked,” he said. “There will be resistance to this conclusion because it is so unexpected.”

Unexpected indeed. “Whoa!” said vertebrate paleontologist Russell Graham of Pennsylvania State University, as he hunched over and clutched his temples at the thought of combining mammoth species. He wants to see a lot more DNA evidence from more specimens, he said, but he relishes the chance that something so seemingly established still holds surprises.

The study’s authors themselves urge caution. They’re interpreting leftover DNA from animals that died more than 10,000 years ago, and in some cases are poorly preserved. Even MacPhee considers himself “a bit agnostic” until geneticists recover more mammoth DNA.

So far the team has decoded only DNA from cells’ little energy-generating structures called mitochondria, which mothers pass along to offspring. This DNA doesn’t always tell the same story as that extracted from the cell nucleus, noted study co-author Jacob Enk of McMaster University in Hamilton, Canada. Determining patterns in the nuclear DNA is a much bigger task. “That is the next step,” Enk said.

The mitochondrial DNA results were published May 31 in Genome Biology, and the research team has now moved on to other mammoth specimens from North America, some classified as woolly, some as intermediate Columbian-woolly. Very early results, Enk said, hint that these other animals might end up on the same branch as previously examined specimens. If so, then animals may have taken a variety of forms in a sort of woolly-Columbian mashup.

Some researchers have argued that North America had a third Ice Age mammoth, M. jeffersoni. Shaking up the mammoth family tree may show that M. jeffersoni is just a woolly-Columbian hybrid after all, as some scientists have suspected, said study collaborator Daniel Fisher of the University of Michigan.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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