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The Cost of Thinking

Despite their many differences, all human beings share several defining characteristics, such as large brains and the ability to walk upright on two legs.

The first unique human characteristic is that humans have extraordinarily large brains compared with other animals. It seems obvious that evolution should select for larger brains. Mammals (哺乳动物) weighing sixty kilograms have an average brain size of 200 cm ². Modern man has a brain averaging 1200-1400 cm ². We are so fond of our high intelligence that we assume that when it comes to brain power, more must be better. Unfortunately, that is not the case.

The fact is that a huge brain is a huge drain-consumption of energy-on the body. I's not easy to carry around, especially when boxed inside a massive skull (倾骨). It's even harder to provides energy. In modern man, the brain accounts for about 2-3%of total body weight but it consumes 25% of the body's energy when the body is at rest. By comparison, the brains of apes (类人猿) require only 8%of rest-time energy. Early humans pad for their large brains in two ways. Firstly, they spent more time in search food. Secondly, their muscles grew smaller and weaker. It's hardly an obvious conclusion that this is a good way to survive. A chimpanzee (黑猩猩) can't win an argument with a modern man, but it can tear the man apart like a rag doll.

Another unique human characteristic is that we walk upright. Standing up, it's easier to find food or enemies. In addition, their arms that are unnecessary for moving around are freed for other purposes, like throwing stones or signaling. As a result, humans can perform very complex tasks with their hands.

Yet walking upright has its disadvantage. The bone structure of our ancestors developed for millions of years to support a creature that walked on all fours and has a relatively small head. Adjusting to an upright position was quite a challenge, especially when the bones had to support an extra-large skull. Humankind paid for its broad vision and skillful hands backaches and painful necks.

We assume that a large brain makes huge advantages. It seems obvious that these have made humankind the most powerful animal on earth. But humans enjoyed all of these advantages for a full 2 million years during which they remained weak and marginal creature. Thus humans who lived a million years ago, despite their big brains and sharp stone tools, lived in constant fear of meat-eating animals.

The   Cost of Thinking
Introduction	•Large   brains for their bodies and the ability to walk upright are two ________ of human beings.
The  ________ of large human brains	•The larger brains may not be better because of the cost. •The big brains make it harder for the body to move around and consume more  energy. •The animal brain requires less ________when  the body is at rest. •Large human brains consume more food, and weaken muscles.
The  ________ of  walking upright	•Walking  upright makes it easy to find food or ________against enemies. •Freed  hands can serve some ________   purpose and perform complex tasks.
	•Walking  upright challenges the human bone structure, and ________ the size of brains. •Walking  upright results in ________   sufferings.
Conclusion	•With a large brain, human beings ________ other beings in terms of intelligence. •Weak and marginal, human beings remained ________ of meat-eating animals.