Intelligence is highly overrated. Most species do not have very much of it. Eighty percent of animal species are arthropods, which do not have high levels of intelligence and show no evolutionary trend toward increasing it. High levels of intelligence evolved in the few species that do have it as a result of special circumstances that are still not understood.

Intelligence has proven impossible to define in a manner that all scientists can accept. If intelligence requires only a coordinated response to complex environmental factors, then plants have intelligence, as explained in the article by biologist Anthony Trewavas. Most scientists restrict the concept of intelligence to animals, which have nervous systems. Human intelligence includes consciousness, self-awareness, and the ability to form abstract concepts. Scientists recognize a whole range of levels of animal intelligence.

There are evolutionary reasons why intelligence has evolved only in animals. Microbes and protists could not have evolved intelligence, since they are primarily single-celled. Plants and fungi could not evolve intelligence, which requires a nervous system for the acquisition, processing, and storage of information.

In some groups of animals, such as the arthropods, a large body size and therefore a brain as large as that of a human is unlikely to evolve. But in many other animal lineages, intelligence could have evolved if it had provided a significant fitness advantage. The fact that high levels of intelligence have evolved so seldom during the history of animal life on Earth suggests that the circumstances that favor intelligence are rare. Therefore, as evolutionary biologist Stephen Jay Gould pointed out, if complex life were to evolve all over again, it is unlikely that a human level of intelligence would evolve. Furthermore, science fiction notwithstanding, scientists should not expect to find highly intelligent life on every planet that is capable of supporting complex life. Gould strongly rejected the idea that structurally simpler organisms are evolving toward higher intelligence.

High levels of intelligence require big brains, and big brains are very expensive. The human brain consumes a tremendous share of the food and oxygen that is available in the blood. Humans have an encephalization quotient, which compares human brain size with that of an average mammal of the same size, of about 6. The encephalization quotient of humans compared to other primates is about 3. Big brains, and high intelligence, are simply not worth the cost for most animal species. . .

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