From the tundra of Alaska to the plains of central Mexico, from islands off California to the Atlantic coastline, mammoths trumpeted and bellowed across North America. Paleontologists traditionally have divided all these Ice Age pachyderms into at least three species—and perhaps as many as four or five. This division was based on differences in teeth and bone. But these aren’t the only clues; mammoths also left remnants of their genes, and this DNA tells a different story. Where there were once multiple mammoths, there may have only been one
The traditional picture looked something like this: The first mammoth thought to have arrived in North America was the “southern mammoth,” notes Illinois State Museum paleontologist Chris Widga. It was followed by the northern woolly mammoth, the southern Columbian mammoth and the dwarfed Channel Islands mammoth, all contemporaries. A fifth, and controversial, species—a Columbian mammoth variant called Mammuthus jeffersonii—rounded out the roster
The key to telling these behemoths apart was molar shape. “A mammoth molar is made up of a series of enamel ridge-plates,” Widga says, and the patterns of these ridges seemed to outline two lines of mammoths. In the first, populations of southern mammoths evolved into Columbian mammoths, which spun off the dwarfed species on the Channel Islands. Woolly mammoths, meanwhile, have been thought to be later immigrants from Eurasia. These mammoth species appeared to be separated from each other geographically, with the cold-loving woollies staying in the north and Columbian mammoths trundling over southern grasslands.
But the remains of mammoth genes tell a different story. In 2011, McMaster University paleogeneticist Jacob Enk and colleagues reported that the mitochondrial DNA of some Columbian mammoths contained signs that these herbivores interbred with woolly mammoths. This was unexpected—and a conclusion about which bones alone were mute—but the connection rested on sequences from just two non-woolly mammoths. That was too small a pool of genetic data to tell if such mammoth matings were commonplace and resulted in viable offspring. Also unclear was whether all of the various mammoth “species” were simply variants of a single one.
Now Widga, Enk and an entire squad of experts on ancient DNA and mammoth biology have examined an even wider sample of 67 complete mitochondrial genomes to suss out what was going on with North America’s greatest beasts. They found that “shaggy” could be used to describe mammoth behavior as well as their coats.
The new view, published this month in Frontiers in Ecology and Evolution, reshuffles what had been thought about mammoth populations. “We can still sort out different groups morphologically,” Widga says, but the new data show that interbreeding between those species was no rare event. There was frequent genetic interchange between mammoth populations throughout North America. “Mammoths will breed with other mammoths,” he says.
“In the western U.S., those mammoths look a certain way, have a different diet and operate on the landscape differently than mammoths in the eastern U.S.,” Widga says, but genetically these distinct groups undoubtedly interbred. It’s similar to what paleontologists have discovered with our own species and Neanderthals—skeletally different but linked through swapped genetic material.
So what’s the story of North America’s mammoths now? The new study, combined with other recent research, has dramatically rewritten the tale of North America’s great elephants. Citing a paper published last year in Science, Widga says that the southern mammoth, Mammuthus meridionalis, never made it to North America from Eurasia, after all, and so they couldn’t have been the ancestor of the Columbian mammoth.
What the genetic research adds are two possibilities. One is that “a single lineage of North American mammoths diversified into many different ‘species,’” Wigda says, that included the Columbian mammoth, some unusual specimens sometimes called Mammuthus jeffersonii and a population of Alaskan woolly mammoths that recolonized Siberia. Rather than being genetically isolated species, these were all just variations of one enormous mammoth population. Then again, Widga adds, the genetic profile could be explained by new woolly mammoth populations from Eurasia coming in and washing out the genetic signal as they mingled with North America’s native mammoths. Either way, though, it is clear that mammoths were interbreeding all across the continent.
“I’m probably weird in this respect, but I find the fact that these are all a single population, and not multiple biological species, somewhat liberating,” Widga says. It changes our perception of what a mammoth is. “The idea that pygmies and woollies are simply a single animal exhibiting evolutionary responses to different landscapes means that we’ve greatly increased our sample size for looking at how animal populations become tuned to different ecological systems,” he says, and this new view may held bring clarity to two new vistas: not only where mammoths came from, but also how such a variable, flexible species disappeared forever.