美国科学60秒 SSS 2013-05-24（在线收听）

 　　What had the legs of agator and the jaws of fish? Why the earliest land animals? Because a new studies shows that animals evolved weight-bearing limbs long before they had the chambers to really take advantage of a terrestrial diet. The research is in the journal Integrative and Comparative Biology. Scientists had suspected that the first four-legged creatures to haul their carcasses out the ocean didn't belly up to the salad bar straight away. But they lacked definitive proof. Now, researchers have carefully examined the fossil like faces of 89 beasties lived on land and sea, some 300 to 400 millions years ago. They probed the jaws for a range of biomechanical features, such as how much force they can give to their bite. The result: seems it took tens of millions of years after setting food on land to come up with the mouth that could munch on the greenery. Why the leg? Could be the critters had stop being mouth breathers and shift from using gills to using lungs, which freed their jaws to develop in new ways. And which left no more excuses to not eat their veggies.