Over the past 125,000 years, the average size of mammals on Earth has shrunk. And people are to blame.
This is the result of a recent study of fossil finds by paleobiologist Felisa Smith of the University of New Mexico.
Smith examined fossils that were 65 million years old when dinosaurs died and mammals got their money's worth. Many of the early mammals grew up. Among the huge creatures: "Llamas and camels and sloths and five types of Pronghorn [antelope] in fact," she says, "and certainly mammoths, and then many really cool predators, like Arctodus, the short-faced bear." The short-faced bear stood 1
And that was just North America.
Being tall was as successful as it was small, and had some advantages when it came to surviving large predators. "Taken as a whole, over 65 million years ago, being large did not increase the extinction risk of mammals, but it was when humans were involved," Smith said.
Looking back over the last 125,000 years of the fossil record, Smith found that the extinction rate for large mammals increased when people arrived somewhere. She says that it was basically hungry. "Surely people use big game," she says, "probably because they're tasty," and because a bigger animal makes for a bigger meal.
But humans did other things than hunting, which accelerated the disappearance of large mammals. They burned forests and meadows that used large mammals. They competed with the big carnivores for game. They brought dogs, which made them better hunters.
Over time, Smith says the downsizing of mammals affects the environment in ways you can not imagine – for example, in the erosion of the land. "When a big animal goes up a hill," explains Smith, "it moves zigzagging while a small animal rises more directly, and that has an effect because the water follows these wild paths, erosion and vegetation, and what -not-affected are. "
Smith's research appears in the journal Science. Fellow paleo-biologist Rebecca Terry at Oregon State University says the new study shows that human influence on the size of mammals in Africa began where humans first developed. The effect on mammals then followed their travels. "And finally, the spread of modern humans, Homo sapiens, (moved) to the New World," she says, "and at that point in time quite advanced weapons were in place, and the extinction in the New World in North America and South America was really extreme Outcome. "
In fact, America was the last resort for really large mammals, as they were the last people to populate.
We still have many furry little mammals on the planet. But the pattern is clear: 11,000 years ago, the average mass of a non-human mammal in North America was about 200 pounds. Now it's about 15 pounds. And the researchers say they are getting smaller and smaller.
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