The Washington Post reports:

Life on Earth used to look a lot more impressive. Just a little more than 100,000 years ago, there were sloths as long as a giraffe is tall, monstrous bears whose shoulders were six feet off the ground, and Bunyanesque beavers that weighed as much as an NFL linebacker. But over time, all of these creatures disappeared in a manner so rapid and so mysterious that scientists still can’t fully explain what went down.

Did an asteroid discharge the mega-beasts, similar to the one thought to have snuffed out the dinosaurs? Or was it widespread climatic change or a plague of new diseases? Did our penchant for hunting play a role?

It’s likely that a combination of factors led to a planet-wide demise in sizable mammals as the Ice Age came to a close. But a study published Thursday in the journal Science provides evidence that the major drivers were humans and other hominids.

“We looked at the entire fossil record for 65 million years, in million-year increments, and we asked the question, ‘Is it ever bad to be big?’ ” said lead author Felisa Smith, a paleoecologist at the University of New Mexico. For most of evolutionary history, the answer was no — larger body mass did not make an animal more likely to go extinct, she said. “For 65 million years, it didn’t matter what size you were.”

That is, until a new kind of predator arrived on the scene: Homo erectus. Around 1.8 million years ago, hominids that had long been dependent on plants became hominids that were “heavily and increasingly dependent on meat as a food source,” Smith said.

As these tool-wielding team hunters spread out from Africa, large-mammal extinctions followed. If you’re going to spend time and energy on a hunt, these early humans and their ancestors probably believed, it’s go big or go home.

“You hunt a rabbit, you have food for a small family for a day,” Smith said. “You hunt a mammoth, you feed the village.” [Continue reading…]

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