Emmanuel Foudrot / Reuters

There used to be a type of elephant called Palaeoloxodon that could have rested its chin on the head of a modern African elephant. There was a hornless rhino called Paraceratherium, which was at least 10 times heavier than living rhinos. There was once a giant wombat that could have looked you level in the eye, a ground sloth the size of an elephant, a short-faced bear that would have loomed over a grizzly, and car-sized armadillos with maces on their tails. After most of the dinosaurs went extinct at the end of the Cretaceous period, 66 million years ago, mammals took over as the largest creatures on land—and they became really big.

But during the late Pleistocene, from around 125,000 years ago, these megafauna started disappearing. Today, they’re all gone. The reasons for their extinctions have been thoroughly studied and intensely debated, but a new study by Felisa Smith from the University of New Mexico puts the blame squarely on humans and our hominin relatives.

By looking at how mammals have changed in size over time, Smith and her colleagues have shown that whenever humans are around, the mammals that disappear tend to be 100 to 1000 times bigger than those that survive. This isn’t entirely new: Many scientists, Smith included, have found the same trends in Australia and the Americas. But the new analysis shows that this pattern occurred in every continent except Antarctica, and throughout at least the last 125,000 years.

“Size-selective extinction is a hallmark of human activity,” Smith says. In other words, when we’re around, big animals die.

“It doesn’t take a lot to make a species go extinct,” says Advait Jukar from George Mason University. “Humans didn’t need to go out and kill every last individual; all you need is a stressed population and just enough hunting pressure to keep the fertility rate [below replacement levels]. Eventually, the population will collapse.”

The distribution of body size is generally related to the size of a land mass. Africa is smaller than Eurasia but bigger than the Americas, so you’d expect its animals to weigh in somewhere in the middle. But by the time hominins left Africa, the average mammals there were about 50 percent smaller than the average ones in either Eurasia or the Americas. For that reason, Smith thinks these size-specific collapses started well before the rise of Homo sapiens, and probably dates back to the origins of Homo erectus, roughly 1.8 million years ago. That was the species that marked the shift from hominins that depend heavily on plants to ones that depend more on meat,” says Smith. “Being a good predator is a general feature of our genus.”

When hominins like Neanderthals, Denisovans, and modern humans spread through Europe and Asia, the average mass of mammals there halved. When Homo sapiens later entered Australia, the mammals there became 10 times smaller on average. And when they finally entered the Americas, with effective long-range weapons in hand, they downsized the mammals there to an even steeper degree. By around 15,000 years ago, the average mass of North America’s mammals had fallen from 216 pounds to just 17.

This is not a general feature of mammal evolution. Smith’s colleague, Kathleen Lyons from the University of Nebraska-Lincoln, has been collecting data on mammalian body size over the last 65 million years. Her data show that the biggest beasts only became disproportionately vulnerable to extinction in the last few million. “People make this assumption that large animals are more at risk,” says Smith. “But large animals also have larger geographic ranges, which buffers them against extinction. For most animals across most time, being large was a good thing.”

Even during huge changes in climate, including several ice ages and warm spells, large mammals weren’t especially vulnerable. To her, that should settle the long-running and often acrimonious debate about whether humans were actually responsible for the loss of the megafauna. “When it got warmer or colder, it didn’t select for bigger or smaller mammals,” says Smith. “It’s only when humans got involved that being large enhanced your extinction risk.”

But “it’s not a slam dunk that humans are responsible for the entire [megafaunal] extinction,” says Jessica Theodor from the University of Calgary. As other studies have shown, it can be hard to parse out the effects of human hunting, climate change, and the big changes that ecosystems undergo when big mammals start to disappear. All of these things often occurred simultaneously, and compounded each other. Still, as Kaitlin Maguire from the Orma J. Smith Museum of Natural History puts it, “while it’s thought that the megafaunal extinctions were a result of a one-two punch from shifting climate and human influences, this work demonstrates that the human punch was strong.”

Even if climate change wasn’t primarily responsible for killing off large mammals in the past, three things are very different now: The climate is changing at an extraordinary rate; that change is now our doing; and humans have shrunk the space available to wild animals. It used to be that large mammals could cope with rising temperatures or shifting rainfall by moving. Now, cities, farmland, and roads are in the way.

These changes mean that modern humans have also become adept at killing medium-sized and smaller mammals, weakening the size-specific trends that held for tens of thousands of years. Our ancestors killed mammals by hunting them. Now, we can indirectly usher them into extinction by shrinking their habitats or introducing unfamiliar predators.

Even now, the biggest mammals are still in most severe danger. Most live in countries with the fewest resources, many of which are rife with conflict. African elephants are heavily poached. The northern white rhino is down to just two females. Even the giraffe is coming up short—its population has fallen by 40 percent in just 30 years, and it moved onto the Red List of Threatened Species last year. “It was frightening to think that this iconic animal that everyone thinks is fine is actually vulnerable,” says Smith. “That took people by surprise.”

The unmaking of the megafauna is a tragedy in itself, but it would also unmake much of the world. Elephants and their kin, for example, are very good at keeping trees from encroaching onto grasslands, and as mammoths disappeared from the Northern hemisphere, the grass-dominated worlds that they maintained also went extinct. “Our assumption that modern ecosystems are “normal” is flawed,” says Theodor. “They’re not necessarily functioning in the way that they did even 11,000 years ago.”

Smith calculates that if all the species that are currently threatened eventually go extinct, the largest mammal left on land will be the domestic cow. In terms of body size, our order will be back to where we were around 45 million years ago. “And that’s largely because of humans,” Smith says.

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