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Giant ground sloths, wooly rhinoceros, glyptodonts and short-faced kangaroos: All have gone the way of the dodo, vanished from the face of the Earth. And that’s just a sampling of the large mammals that are no longer with us. Of the 57 species of megaherbivores that are known to have existed 50,000 years ago, only 11 of them survive—a grim 81 percent extinction rate.
Defined as large-bodied terrestrial mammals with a mean adult body mass of 2,200 pounds or more, today’s remaining megaherbivores comprise the likes of elephants, rhinoceroses, giraffes, and hippopotamuses. These animals play critical roles in their ecosystems, from seed dispersal to landscape management.
Elephants, for example, reduce the density of trees and shrubs through their movement and feeding habits. These open spaces make way for plains animals like antelope and zebra, while the hollows and crevices formed by broken branches and felled trees create habitats for small mammals, insects, and fungi.
But these large creatures—who can’t hide under a log or move with gazelle-like agility—were particularly susceptible to early humans looking to make the most of each hunt. After all, if you’re looking for meat to fill your belly and fur to keep your family warm, a wooly mammoth has far more to offer than a hare.
“We know that prehistoric humans were very focused on hunting big species,” says Jens-Christian Svenning, director of the Danish National Research Foundation’s Center for Ecological Dynamics in a Novel Biosphere at Aarhus University. He’s also the lead author on a recent paper published in Cambridge Prisms: Extinction that argues it was not climate change but rather human hunting that caused the extinction of most megaherbivores over the past 50,000 years.
A wooly mammoth has far more to offer than a hare.
To draw their conclusions, Svenning and his team analyzed ancient extinction, climate, and human migration data collected over the past six decades. The work continues a conversation that began in earnest in 1966 when an American paleontologist named Paul Schultz Martin first posited his overkill hypothesis—wherein he suggested that migrating humans hunted North American megafauna from the Pleistocene epoch to extinction. A few years ago, researchers published a paper in Nature that likewise found that distributions of megafauna across time and space in prehistoric South America align closely with human demographic data as well as findings of spear points called fishtails in the archaeological record.
“It’s a long-term discussion,” says Svenning, who argues that improvements in research techniques and data quality over recent decades have helped to get us closer to a definitive answer to how things went down for the megafauna. “We have a much better understanding now than in the 1960s,” he says. “What we have done is reassessed all this data and it allows us to say that altogether, we can really rule out that climate has played a major role in this kind of extinction.”
The where’s and when’s of extinctions just don’t line up with global patterns of climate change, according to the data the researchers collected and analyzed, but they do correspond closely with patterns of human colonization—occurring at or after our arrival in many distinct times and places around the globe.
“We conclude that it’s one of the strongest, most consistent patterns we have in ecology,” says Svenning. His team’s findings indicate that these patterns of megafauna extinction began when humans first migrated out of Africa some 100,000 years ago. The extinctions accelerated approximately 50,000 years ago as Eurasia and Australia were colonized by large game-hunting humans.
Ending up on the business end of a spear had such a significant impact on large mammals because they have a naturally slow replacement rate. Gestation periods are long, and so is the process of maturation. The 46 species of megaherbivores that have been lost to history simply could not have reproduced fast enough to offset human kills.
Felisa Smith, a conservation paleoecologist and professor at the University of New Mexico, believes that the human impact on megafauna extinction is no longer up for debate. “I think work over the past few decades has rather convincingly demonstrated that humans had a pretty substantial part in the extinction,” says Smith.
This is not about assigning blame, says Svenning. “People who lived thousands of years ago never had access to the full picture. These things took place across long time scales and big spatial scales over which no one had an overview; whatever people did, it was difficult to see the consequences. Plus, of course, people just had to survive the best they could.”
Svenning hopes readers take away an increased understanding of the relationships between humans and megafauna and the natural world. Large mammals remain highly vulnerable to disappearance today, with over half of existing species weighing over 22 pounds listed as vulnerable, endangered, or critically endangered by the International Union for the Conservation of Nature. “When we restore forests, we can’t just think about the trees,” he says. “We must think about the animals that belong there.” 
*An earlier version of this story stated that muskoxen are extinct. In fact, they were only extirpated from Eurasia and have since been reintroduced. The story has been revised accordingly.
Lead image: maradon 333 / Shutterstock
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