Why animals left the sea for land

Losing legs
Not only is it more efficient to swim without limbs, burrowing without legs is also better, and this raises questions about the twists and turns of reptilian evolution.

"If you're a good headfirst burrower, arms and legs just get in the way," Conrad said. "You have to make bigger holes to fit your arms and legs, which create drag as you move through burrows."

For instance, although scientists know snakes can burrow more efficiently without arms and legs, exactly how they lost their limbs remains a mystery.

Just as the earliest whales may have taken advantage of the unexploited resource of crocodile-free water, the earliest snakes must have taken advantage of transitional traits expressed by their relatives.

But which four-legged relatives and their family traits led the transition for legless snakes? Conrad explains that scientists have come up with three candidates — iguanas, monitor lizards, and skinks — as the most likely relatives to have made way for the slithering snake.

DNA evidence has pinned iguanas and chameleons as the closest relatives to snakes. But some scientists point to the elongated bodies and snake-like tongues of monitor lizards as signs that, over time, snakes evolved from some monitor lizards. Still others suggest the short or sometimes nonexistent legs of the 800 species of skinks are clues to where limbless snakes originated.

Between the holes in the fossil and molecular evidence, scientists can only make educated guesses for now as to where and when legs of snakes walked off into evolutionary history.

"It's like putting together a really big puzzle with only a quarter of the pieces and trying to figure it all out," Conrad told LiveScience. "You find tantalizing tidbits that lead you in one direction and others that send you in the other direction."

Standing upright
The puzzle pieces fit together recently for a team of scientists who studied why our closest ancestors stopped walking on all fours.

Biological anthropologist Herman Pontzer from Washington University in St.Louis and his colleagues found that walking on two legs costs humans only one-quarter the energy used by chimpanzees that knuckle-walk on four legs. The group measured the oxygen burned by five chimpanzees and four people as they walked on a treadmill. The findings were detailed in the July 16 issue of the journal Proceedings of the National Academy of Sciences.

In general, chimps exert more energy than people. But one chimp, with a longer stride than his fellow chimps, was more efficient at walking upright. Pontzer assumes the variation he saw among the five chimps in his study is similar to what exists in the wild. Some chimps are born with longer legs than others.

In the fossil record, the research team found evidence of changes in leg length and pelvic structure that may have made it easier for some chimps, like the one in their study, to stand upright.

"Variation is a foot in the door with which evolution can select for bipedalism," Pontzer told LiveScience. "Not only do we have system we can understand here, but it shows us how evolution could have tinkered with variation."

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