NT_020

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綜合寫(xiě)作

For hundreds of thousands of years of human history, the human brain steadily grew in size. However, about 20,000 years ago, the size of the human brain began to decrease. Since then, the size of the average brain has decreased by about 10 percent. Scientists are not sure why the human brain has become smaller, but several theories have emerged to explain the phenomenon.

Climate Changes

Some scientists believe that the decrease in human brain size is a result of the decrease in the overall size of the human body, which has been caused by changes in Earth's climate. Over the past 20,000 years Earth's climate has experienced a warming trend. Generally, warmer climate favors smaller body size; therefore, as Earth's climate warmed, the size of the human body decreased, and the size of the human brain decreased with it.

Agriculture

Other researchers believe that the change to an agricultural lifestyle contributed to decreased brain size. Our ancestors’ large brains needed large quantities of proteins and vitamins to remain healthy. After the agricultural revolution, people's diets consisted mostly of grains, but grains are lower in proteins and vitamins than non-agricultural diets. People with slightly smaller brains had an advantage: they were not as affected by the low-protein diet. Over time, they became the majority.

Decrease in Muscle Mass

According to a third theory, the smaller size of the human brain reflects the fact that humans today have smaller muscles than humans did 20,000 years ago. This is important because much of the brain is used to control muscles. As humans became less muscular, the parts of the brain that controlled muscles also became smaller, resulting in an overall reduction in brain size.

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