英語(yǔ)時(shí)差：食肉鯨和食素鯨

Everyone is familiar with the riddle: Which came first, the chicken or the egg? This sometimes leads to discussions about bird evolution. But most people don’t realize this type of question is common in paleontology. For example: Which came first, the meat-eating whale ancestor or the plant-eating one? Answering that question has not been easy. Early fossil finds suggested whales might be related to the mesonychids, an extinct group of wolf sized, hoof toed carnivores. These mammals had triangular teeth similar to whales. But when genetic analysis began, that idea was put to the test. You see, scientists comparing DNA of whales to other mammals found that whales were most closely related to plant eating ungulates. This group includes sheep, antelopes and pigs. As a matter of fact, the closest living relative to whales is the hippopotamus. So, how is it that modern whales are meat eaters?