"There was once this amazing menagerie of large mammals living in South America that included sloths, saber-toothed cats, horses, camels, giant bears, and a panther as big as today's tigers," Jessica Metcalf, an evolutionary biologist and ecologist at the University of Colorado, Boulder, tells The Christian Science Monitor.
But, about 12,000 years ago, these massive animals went extinct.
Megafauna across the Americas mysteriously disappear from the fossil record toward the end of the last ice age. Scientists have long debated what may have happened and largely point to two possible culprits.
The timing of that extinction seems to coincide with two major events: the peopling of the Americas and the end of the last ice age. So did humans hunt the big animals to extinction or did the changing climate do them in?
Perhaps the two were partners in crime, Dr. Metcalf and her colleagues suggest in a new paper published Friday in the journal Science Advances.
The team sequenced ancient DNA from fossils found at sites across South America in order to trace the genetic history of populations of megafauna. And they found a pattern.
Humans had begun to spread across the Americas around 15,000 years ago (perhaps earlier). But the megafaunal extinctions in South America didn't occur until later, Metcalf says, about 12,000 years ago.
What happened around 12,000 years ago? The climate in South America began to heat up again. There had been a brief cold spell, the Antarctic Cold Reversal, in which the deglaciation that was happening at the time was temporarily reversed. But that ended right around the time that most of the South American megafauna were going extinct.
"What we were able to tease apart is that they were surviving in the presence of humans for 1,000 to 3,000 years, but when the climate rapidly warmed, they died off," Metcalf says.
It was a big surprise how tightly the extinction coincided with that climate warming, Metcalf says. And that suggests that it took the combination of humans and climate change to push the massive animals over the edge.
That pattern seems to match what has been found in North America, she says. Although warm periods occurred at different times on the two American continents, the extinctions clustered in those hotter times, after humans had settled the region.
"This shows us that climate warming can be a really big threat to megafauna," Metcalf says.
It isn't a new idea that humans and climate change could have both contributed to the megafaunal extinction. Scientists have often suggested that the two could have been a deadly team.
"It has become commonplace to attribute these extinctions to a combination of people and climate," Donald Grayson, an archaeologist at the University of Washington in Seattle who was not part of the study, says in an email to the Monitor.
"Although an interaction between human activities and climate change is often mentioned as a possible cause of the end-Pleistocene extinction event, this paper is one of the few that actually presents evidence for this claim," Emily Lindsey, a visiting paleoecologist at the University of California, Berkeley, who was not part of this study, tells the Monitor in an email.
The story may be one of an interplay, or synergy, between climate and human activity, says Dr. Lindsey. In addition to the potential direct impacts of the new human presence in the Americas, the rapid climate change could have made the megafauna more vulnerable.
"Most of the Pleistocene megafauna in South America (and globally), like those in Africa today, were open savannah-adapted," she explains. "The warmer wetter conditions that came with climate warming at the end of the Pleistocene caused an increase in forests and shrubby habitats in a lot of places, which shrunk and fragmented the savannah habitats that most of these animals relied on, making them potentially more vulnerable to hunting, fires, or other impacts."
Still, Lindsey warns that "While the circumstantial evidence for synergy is compelling, the possibility remains that it just took humans a few thousand years of attritional hunting to drive megafauna populations to extinction on both continents, and that climate had nothing to do with it."
Or, she adds, perhaps "the megafauna were responding primarily or exclusively to the late-Pleistocene warming, and the presence of humans was incidental."
But more, finer-scale data could help narrow in on the true culprit, or culprits.
"This is another step toward understanding the big picture of the late Ice Age die-offs around the world," Metcalf says.
And, says Lindsey, "because most of these extinctions (as is shown in this paper) occurred during a time of overlapping climate change and increasing human population, understanding how different animals responded to these pressures could be valuable in helping to preserve threatened species today."
In addition to sequencing the DNA of these ancient animals, the team also radiocarbon-dated the fossils. "The radiocarbon data allowed us to pinpoint when the megafauna were going extinct and the DNA allowed us to have a much better idea of what taxa were there," Metcalf says.
And they found a possible explanation for why some megafauna survived and others didn't.
For example, llamas and alpacas are iconic animals of South America today. And some of their wild ancestors, the guanaco and the vicuña, still roam the continent.
"It was always thought that the guanaco lived through these extinctions in the Patagonia," Metcalf says. "But our data showed, and it could only be uncovered using DNA, that no": in fact the camelids, members of the family that includes camels, llamas, alpacas, and their ancestors, went extinct in that region, too.
The DNA tells a story of a population that had retreated to a small region in the north that later spread back into Patagonia about a thousand years after the megafaunal extinction. And that was likely the ancestral population to today's South American camelids.
"It looked like it was really important to have different species of camelids at a continental scale that were able to spread out and recolonize these places where other populations had died off once the climate was more friendly," Metcalf says. And that was probably true for other animals that survived that extinction, as well.