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 cm2. Modern man has a brain averaging 1200-1400 cm2. 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. Its not easy to carry around， especially when boxed inside a massive skull（倾骨）. Its even harder to provide energy. In modern man， the brain accounts for about 2-3% of total body weight， but it consumes 25% of the bodys 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 paid for their large brains in two ways. Firstly， they spent more time in search of food. Secondly， their muscles grew smaller and weaker. Its hardly an obvious conclusion that this is a good way to survive. A chimpanzee（黑猩猩） cant 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， its 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 creatures. Thus humans who lived a million years ago， despite their big brains and sharp stone tools， lived in constant fear of meat-eating animals.