A、The reason why early men were so intelligent. B、The tools that belonged to early men. C、The language development of early men.

Good afternoon, and welcome once again to "The World We Never Knew". We left off last week talking about how the tool-making industry of early men also stimulated the growth of the brain. The possession of a good brain has become one of the factors that enabled humans to make tools at the start. But the use of tools became, in turn, a driving force towards the evolution of an even better brain. The characteristics of good memory, a long head, and innovativeness that were needed for tool-making varied in strength from one individual to another. Again, these circumstances trimmed the human stock, expanding the centers of the brain in which past experiences were recorded, future actions were expected, and new ideas were considered. As a result, from generation to generation the brain grew larger. The evolution of speech may have been the most important factor of all. When early man mastered the loom of language, his progress accelerated dramatically. Through the spoken word a new invention in tool-making, for example, could be communicated to everyone; in this way the innovativeness of the individual enhanced the survival prospects of his fellows, and the creative strength of one became the strength of all. More importantly, through language the ideas of one generation could be passed onto the next, so that each generation not only inherited the genes of its ancestors but also their collective wisdom.
16. What is the speaker mainly talking about?
17. According to the speaker, how did tool-making affect the brain?
18. Why could language improve the human mind?