At the heart of Crabtree’s thinking is a simple idea. In the past, intelligence was critical for survival when our ancestors had to avoid dangerous animals and hunt for food. The difference of being smart or stupid is often life or death. However, after the spread of agriculture, when our ancestors began to live in dense farming communities, the need to keep their intelligence in peak condition gradually reduced.

　　This is not hard to understand. Most of the time, pressure is what keeps us going – you need the pressure from your teachers to finish your homework; the pressure of looking pretty prompts you to lose weight when summer comes. And the same is also true of our intelligence – if we think less, we become less smart.

　　These mutations(变异) are harmful to our intelligence and they were all developed in the past 3,000 years. The other evidence that Crabtree holds is in our genes. He found that among the 2,000 to 5,000 genes that we have that determine human intelligence, there are two or more mutations in each of us.

　　However, Crabtree’s theory has been criticized by some who say that early humans may have better hunting and surviving abilities, but people today have developed a more diverse intelligence. For example, spearing a tiger doesn’t necessarily require more brainpower than playing chess or writing a poem. Moreover, the power of modern education means a lot more people have the opportunity to learn nowadays.

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