They came. They saw. They hunted to extinction.
That's the story of what humans did to most species of plant-eating mammals living in the United States after first arriving 13,400 years ago, according to a report published today in the journal Science.
The study by John Alroy, a paleobiologist at the University of California in Santa Barbara, concludes through an examination of population dynamics that not only was a human-induced extinction plausible, it was "unavoidable," even though humans were inept hunters.
Among the 32 species driven extinct over about 1,500 years in what is now the United States were three types of mammoths and mastodons, three kinds of ground sloths, two varieties of tapirs, about six species of horse, three types of camels, four classes of pronghorns, two kinds of wild pigs and the glyptodont, an armoured armadillo-like beast the size of a Volkswagen Beetle.
Each of these used to roam the continental United States, according to the fossil record. But after humans arrived, even in small numbers, and began to hunt for food, most of the animals disappeared. Just nine species survived, including the modern bison.
"The survivors are scraps," Dr. Alroy said in an interview from California. His paper, and a second article in the same Science issue on the possibility that humans had a hand in mass extinctions in Australia 46,000 years ago, are fodder for the fiery debate over humanity's place in the clusters of die-offs of other species over millennia.
While some scientists have suspected that these extinctions were a result of humans' arrival on the scene, others have argued that continents such as Australia and North America were too massive and the populations of bizarre creatures too large for humans to have had an effect. Instead, they point to climate change or disease.
The research published today argues that humans could -- and did -- cause widespread extinctions, not only of plant-eaters, but also of other organisms on the food chain, including big meat-eaters.
"It's significant because it very clearly and finally refutes a commonly made argument that an extinction of this scale could not have happened with the number of humans who arrived [in the United States]" Dr. Alroy said.
The reason any of this matters is that it points to a far more modern dilemma. Humans are capable of unintentionally pushing other species into extinction, Dr. Alroy said.
We tend to meet our needs and plan for them over extremely short time frames, even a few years or a single generation, but the fossil record shows that our actions have repercussions lasting for hundreds and even thousands of years.
"The overkill model thus serves as a parable of resource exploitation," Dr. Alroy's paper states. ". . . [It]was too gradual to be perceived by the people who unleashed it."
The kicker is that the humans who began populating the United States 13,400 years ago were simple hunters and gatherers. They didn't destroy the ecosystems animals needed to exist. They didn't hunt animals for a single body part only to leave the rest rotting on the ground. They were not orchestrating profound climate change by their emissions of carbon dioxide.
Humans today are doing all those things, and are already thought to be driving extinctions on a bigger scale than the one that killed off the dinosaurs. Dr. Alroy hopes scientists can use his mathematical models to predict future extinctions.