By Carl Zimmer
For millions of years, North America was home to a zoo of giants: mammoths and mastodons, camels and dire wolves, sloths the size of elephants and beavers as big as bears. And then, at the end of the Pleistocene Epoch about 12,000 years ago, most of them vanished.
Scientists have argued for decades about the cause of their extinction. Now, a study analyzing the ancient bones of a young child who lived in Montana suggests that early Americans hunted mammoths and other giant mammals to oblivion.
“I was surprised to see things fit so nicely,” said Ben Potter, an archaeologist at the University of Alaska Fairbanks and an author of the new study, which was published last Wednesday in the journal Science Advances.
For decades, most paleontologists blamed the climate for the disappearance of North America’s megafauna. Their extinction coincided with the end of the last ice age, a time when the planet quickly warmed and glaciers retreated northward. The large mammals appeared unable to adapt.
But in the 1960s, American geoscientist Paul Martin challenged that hypothesis. The last ice age was part of a cycle of warming and cooling that had lasted for millions of years. Why had the megafauna survived earlier periods of warming, but not this one?
Martin believed that the difference was people. At the time, researchers were discovering some crucial clues about how humans had spread from Asia across the Bering Land Bridge into North America. They discovered that the oldest known archaeological remains in North America — stone spearheads known as Clovis points — dated to the end of the ice age, suggesting that their arrival coincided with the extinctions.
Martin argued that people had made their way into North America as the glaciers receded, and they began hunting the continent’s big game. The large mammals had never encountered our species before, leaving them with few defenses.
His “overkill hypothesis” gained traction over the years. Some researchers have even argued that the same wave of people who hunted down giant mammals in North America caused a similar wave of extinction when it got to South America.
But some scientists were skeptical. Critics argued that there was little clear-cut evidence from archaeological sites of humans killing and eating giant mammals. Instead, they said that early Americans ate small mammals, fish and plants. Some claimed that Clovis points weren’t even strong enough to pierce a mammoth’s hide.
Potter and his colleagues recently tackled the debate from a new direction: by investigating the diet of the Clovis people from the chemical composition of their bones.
The researchers took advantage of the fact that the atoms of each element in our bodies are not all identical. A tiny fraction of our carbon atoms, for example, carry an extra subatomic particle, making them a little heavier than the common form.
The heavy and light forms of carbon — known as carbon isotopes — behave differently in chemical reactions that take place inside of plants. The same is true for nitrogen. As a result, plants end up with a distinctive isotopic signature. Animals that eat plants end up with a signature of their own, as do the predators that eat them.
Potter and his colleagues set out to determine the isotopic signature in a 12,800-year-old skull found at a Clovis site in Montana. It belonged to an 18-month-old boy known as the Anzick child, named after the family who owned the ranch where his remains were discovered.
Researchers analyzed samples of the Anzick child skull before it was reburied in 2014. Among other things, they calculated the levels of different isotopes of carbon and nitrogen. Potter and his colleagues gained the permission of local tribes to analyze that data to infer the Anzick child’s diet.
The researchers determined that two-thirds of the boy’s diet came from breast milk and the remaining third was solid food, mostly meat. His mother’s diet, too, was composed almost entirely of meat.
To determine what sort of animals the family ate, Potter and his colleagues surveyed the isotopes in both large and small mammals that lived in the region. They concluded that more than 40% of the Anzick diet came from mammoths. The second most common meal was elk or bison. Small mammals made up only 4% of their diet, at most.
David Meltzer, an archaeologist at Southern Methodist University who was not involved in the work, questioned whether the Anzick family might have eaten small mammals that didn’t leave fossils behind and thus weren’t part of the study. What’s more, he questioned how much could be made from a single skull’s isotopic signature.
“The most problematic aspect of the paper is the speed with which it races from a single data point in Montana to humans playing a role in megafaunal extinctions hemisphere-wide,” he said.
But Todd Surovell, an archaeologist at the University of Wyoming, disagreed, saying that the results supported the theory that early Americans were skilled big-mammal hunters.
“It’s a really nice confirmation of a long-recognized pattern in the archaeological record,” Surovell said. “This is exactly the diet we would expect to see if humans were the main drivers of Pleistocene extinctions.”
Shane Doyle, a member of the Crow Tribe who coordinated the consultation with local Indigenous groups, said that the results offered a striking glimpse at what life was like for early Americans. “These folks faced down some of the steepest odds of all time, and they not only survived, but they thrived,” Doyle said.
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