Mammoths: Made in America
The iconic species of woolly mammoth immortalized in cave paintings and uncovered in the permafrost of Siberia is thought to have arisen in Europe or Asia. But it actually hailed from North America, according to a study of ancient DNA.
Mammoths trace their origins back more than 6 million years to an ancestor shared with African elephants. These early beasts gave rise to new species and subspecies that came and went across Europe, Asia, and Africa, with at least one migration heading from Asia to America about 1.5 million years ago (Science, 2 November 2001, p. 1094). Woolly mammoths appeared late with the origin of the species Mammuthus primigenius about 400,000 years ago. Researchers believe that they came from Europe or Asia because the fossils there are older than those found in North America.
But a study of the DNA preserved in those bones tells another story. Researchers led by molecular evolutionary geneticist Hendrik Poinar of McMaster University in Hamilton, Canada, have sequenced mitochondrial DNA from 108 woolly mammoths from America, Europe, and Asia that were alive more than 44,000 years ago to 11,000 years ago. Combined with published data from 52 other mammoths, they found that these mammoths sorted into three main groups: one that was exclusively Asian, one that was exclusively American, and one that lived in both places.
When the researchers used statistical methods to trace the evolution of the three groups, they found that the exclusively American group probably arose in America about 464,000 years ago (with a range of 781,000 to 281,000 years). This suggests that these American mammoths headed west across the Bering Strait and replaced or succeeded the other two groups of more primitive mammoths native to Asia sometime between 200,000 and 300,000 years ago, when these two groups disappeared. The offspring of the American migrants, however, continued to thrive on both continents, where their members remained genetically diverse until the end of the Ice Age. Paleolithic hunters and climate change finally drove them extinct 10,000 to 5000 years ago, the team reports online today in Current Biology.
Paleontologist Adrian Lister of University College London says that the new data make "a convincing case that North American populations of woolly mammoth largely replaced indigenous Siberian ones sometime around 300,000 years ago," although it is still possible that the first members of their species arose in Asia before quickly migrating to America--and then back to Asia. The study also challenges the notion that animals living in northeastern Siberia and northern Alaska were isolated from each other for tens of thousands or millions of years. If the mammoths came and went over the Bering Strait land bridge several times, Poinar says, other species may also have made the trek back and forth between the continents when the land bridge was in place.