【正确答案】
D
【答案解析】[听力原文]
Foals are on their feet not long after being born. Chicks break through their shells and within hours are specking around for food. Snakes hatch and just slither away. Humans, on the other hand, are completely helpless at birth and remain dependent on their parents for many years.
So why does it take humans so long to develop? Why is such an intelligent species helpless for so many years? The quick answer, experts believe, is that humans are the most complex living system. And the more complex the system, the longer it takes to build.
The more involved answer has to do with evolution. It took billions of years for life to evolve from single-celled microorganisms to large warm-blooded mammals. "The ultimate cause of this evolution, the why and the how one genetic program is selected over another is not yet clear to us," says Calvin, a professor of psychiatry and behavioral sciences at the University of Washington in Seattle. But researchers have identified some of the evolutionary factors that may have affected our developmental process, he says.
One of those factors dates back to around half a billion years ago, when two strategies for protecting offspring emerged. One group of species began to operate under a more-is-better strategy, where species produced thousands of eggs so the odds increased that some of them would survive to reproduce. The second group of species, which includes humans, took the opposite approach. They began having fewer offspring and investing more energy in the development of each. The logic is that the longer the young are protected and taught the tricks of the trade, so to speak, the better chance they have of surviving to reproduce.
Another factor is called neoteny, a developmental trend where a juvenile appearance is retained well past biological adulthood (the age at which we can reproduce). Just the fact that we look young and vulnerable increases our chances of being taken care of.
Experts say it"s likely that our development also is related to the gradual increase in human brain size over millions of years. Limited by the size of the female pelvis, we evolved in a way that allows for postnatal brain growth. Because the brain plays a key role in the development of the body, this adaptation may have, in turn, forced the delay in much of our growth until we are outside the womb.
The higher intelligence and inquisitiveness of humans, our capacity for abstract thought and ability to plan ahead also play a huge role in our slow development. "Humans are capable of doing things that no other species can," says Calvin, and it simply takes us a while to master the tasks that we need to survive.
Language is a good example of this. Learning a language and the specifics of grammar, syntax and context takes years, but humans are born with an innate drive to master it.