Being big was just as successful as being small, and had some advantages when it came to surviving big predators. "Taken as a whole, over 65 million years, being large did not increase mammals' extinction risk. But it did when humans were involved," Smith found.

Looking back over the most recent 125,000 years of the fossil record, Smith found that when humans arrived someplace, the rate of extinction for big mammals rose. She says it basically came down to hunger. "Certainly humans exploit large game," she says, "probably because they are tasty"--and because a bigger animal makes for a bigger meal.

But humans did other things besides hunting that hastened the disappearance of big mammals. They burned forests and grasslands that big mammals used. They competed with the big carnivores for game. They brought dogs with them that made them better hunters.

Over time, Smith says, the downsizing of mammals affected the environment in ways you might not imagine--for example, in the erosion of the land. "When a large animal walks up a hill," Smith explains, "it zig-zags a lot, whereas a small animal walks up more directly, and that has an impact because water follows those game trails down, so erosion and vegetation and what-not are affected by that."

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 mammal size started in Africa, where humans first evolved. The effect on mammals then followed their travels. "And eventually the spread of modern humans, Homo sapiens, (moved) into the New World," she says, "and at that point pretty advanced weaponry was definitely present, and the extinctions in the New World in North America and South America were really extreme as a result."

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