By Ewen Callaway Long before humans first arrived in North America, some of the continent’s hulking woolly mammoths had had enough of the place, and headed back to Asia. A new analysis of DNA from more than 100 mammoth fossils suggests that returning American populations replaced Asian mammoths, before going extinct themselves. “For some reason the North American guys went back over and became kings,” says Hendrik Poinar, an evolutionary geneticist at McMaster University in Hamilton, Ontario, Canada, who led the study. His team’s analysis of mammoth DNA spanning tens of thousands of years and two continents also hints that the ancient pachyderms weathered climate changes that eradicated other beasts. Previous work had suggested that woolly mammoths, who first reached the New World roughly 200,000 years ago, returned to their ancestral stomping grounds. To firm up that conclusion, Poinar and his colleagues collected DNA samples from 108 mammoths, and decoded a swathe of DNA in their mitochondria – the cell’s power generators. They added 52 previously published samples, giving Poinar’s team a total of 160 samples from North America and Siberia. When the researchers sorted the samples according to similarities in their DNA sequences, mammoths clumped into five distinct groups, Poinar says. One was found exclusively in Asian mammoths and another in North American, while three spanned both continents, which were connected by an intermittent land bridge across the Bering Straight. Yet by 22,000 years ago, the Asian population had disappeared, replaced by transcontinental and North American populations. More detailed analysis of theses sequences suggests that the North American mammoths gave rise to the transcontinental herds, which moved back and forth, from Alaska to Siberia, beginning between 200,000 and 50,000 years ago. “It’s a two way rough road,” Poinar says. The analysis also indicates that while North American mammoths eventually went extinct around 12,000 years ago, they lived through previous eras of climate change that spelled the doom of other large animals, including bears, bison and horses. “They saw the ebb and flow of climate for many generations,” Poinar says. “I think it shows they were probably fairly resilient to the effects of climate change.” That is not to say that mammoths emerged unfazed by global weather shifts that occurred after the end of the last ice age. Dense forests replaced open grasslands – the mammoth’s preferred habitat, and the Bering land bridge vanished. The appearance of humans may have been a tipping point. “I certainly think that humans probably played a role and to some effect contributed to the ultimate demise of these guys,” Poinar says. Ian Barnes, a molecular paleobiologist at the University of London who also proposed the out-of-America hypothesis for Asian mammoths, says the new report adds weight to this conclusion by including far more DNA samples – especially from North American mammoths. “The story is really interesting,” says Webb Miller, a genomicist at Pennsylvania State University in University Park. Although he adds: “I’m concerned about the robustness of the data.” He and colleague Stephan Schuster who is sequencing the woolly mammoth genome, argue that explaining hundreds of thousands of years of evolutionary history with a small stretch of mitochondrial DNA is nearly impossible. A fuller picture, Schuster says, will soon emerge with more comprehensive sequence information, particularly from the cell’s nucleus, which holds many thousands times more DNA letters than a mitochondrion. “More data is always better,” agrees Poinar. “But the mitochondrial DNA turns out to generally be a good marker of population history, migrations and population size estimates.” Human migrations out of Africa, he says, have been accurately mapped using the same kinds of sequences, and nuclear DNA has only buttressed this picture. He expects the same will be true of woolly mammoths. Journal reference: Current Biology (DOI: 10.1016/j.cub/2008/07.061) Climate Change – Want to know more about global warming – the science, impacts and political debate?