NT_015

?

綜合寫作

Madagascar, a large island about 400 kilometers off the coast of Africa,is home to many mammals that exist nowhere else on Earth Genetic evidence shows that the ancestors of these mammals arrived there from the African mainland between 60 million and 20 million years ago. According to a popular theory, the ancestor species crossed the 400 kilometers by riding debris like logs or branches floating on the ocean. However, there are serious problems with that theory.

First, land mammals normally cross the sea only by accident. For example, a few animals might be swept into the water by a big storm and then somehow stay afloat on logs or plants. That does not happen frequently. In addition, in order to reproduce and establish a population in the new location, several individuals of the same species would have to make the crossing together-an even rarer accident.It seems implausible that such accidental events could happen often enough to give rise to the large and diverse mammalian fauna of Madagascar.

Second, the sea currents near the mainland African coast where the mammals supposedly came from do not flow toward Madagascar. Even if some animals were thrown into the water by storms and were able to float on debris,the currents would carry them away from Madagascar.

Third, a 400-kilometer journey would have taken too long for animals to survive. Scientists estimate that the trip takes three weeks.It is impossible to float over the ocean for that long without any sources of food or freshwater.The animals that were washed out to sea would have died of hunger or thirst before reaching the new location

聽力文稿

見評論區(qū)。