Question 23 is based on the recording you have just heard.

Recording Three

听力原文

    Skin may seem like a superficial human attribute, but it’s the first thing we notice about anyone we meet. As a zoologist focusing on the studies of apes and monkeys, (22) I’ve been studying why humans evolved to become “the naked ape”, and why skin comes in so many different shades around the world.   

   (23) We can make a very good estimate from the fossil record that humans probably evolved naked skin around a million and a half years ago. And meanwhile, they mostly lost their coat of fur. Today, we have a few patches of hair remaining on various parts of our bodies, but compared with apes and monkeys, we have very little. Basically, we turned our skin darker to serve as a natural sun protector in the place of the hair we lost.

    (23) We think we lost this hair because of the need to keep ourselves cool when we were moving around vigorously in a hot environment. We can’t really lose heat by breathing quickly and loudly like dogs. We have to do it by sweating, so we evolved the ability to sweat plentifully and lost most of our fur. (24) Most animals protect themselves from the sun with fur. What we did in our ancestry was to produce more permanent natural coloring in our skin cells.

    This was really an important revolution in human history, because it allowed us to continue to evolve in equatorial environments. It really made it possible for us to continue along the path toward modern humans in Africa. For most of human history, we all had dark skin. What we see today is the product of evolutionary events resulting from the dispersal of a few human populations out of Africa around 60,000 to 70,000 years ago. Our species originated around 200,000 years ago and underwent tremendous diversification—culturally, technologically, linguistically, artistically—for 130,000 years. (25) After that, a few small populations left Africa to populate the rest of the world. These early ancestors of modern Eurasians dispersed into parts of the world that had more seasonal sunshine and much lower levels of sun radiation. It’s in these populations that we begin to see real changes in the genetic makeup of natural coloring.

    Today, skin color is evolving via new mixtures of people coming together and having children with new mixtures of skin color genes. We can see this in almost every large city worldwide. Not only the coloring genes, but lots of other genes are getting mixed up, too.

23. What had probably caused humans to lose most of their hair 1.5 million years ago?

解析：B。录音开头指出，人类在约150万年前进化出了露在外面的皮肤，同时覆盖全身的毛发消失了。随后又指出，人类的毛发消失是因为需要活动并且在炎热的环境下保持身体凉爽，因此B项正确。