Is the Human Brain Unique?

Abstract

The human brain is not unique in terms of general structure. It exhibits a morphology typical of mammals and more specific of primates. Also, humans do not have the largest brain either in absolute or in relative terms, although they possess a brain that is seven to eight times larger than expected from general mammalian brain-body relationship. The size of the human cerebral cortex and of the prefrontal cortex as the “seat” of intelligence exhibit a slightly positive allometric growth, i.e. the cortex increases faster in size than the rest of the brain, which is again typical of mammals. Due to a relatively thick cortex and a relatively high neuronal packing density, humans have the highest number of cortical neurons (12–15 billions), which is more than the number of cortical neurons found in cetaceans (whales and dolphins) and elephants with much larger brains up to 10 kg. Furthermore, due to a higher axonal conduction velocity and shorter interneuronal distance, humans have a higher cortical information processing capacity than these large-brained mammals. The largest differences between humans on the one hand and all other mammals/vertebrates on the other consist in (1) a strongly increased growth period of the human brain exposing it to a much higher degree to education, and (2) the presence of the Broca speech center which is a necessary prerequisite of syntactical language and developed only recently, i.e., about 100,000 years ago. While these two traits appear to be minor steps in human biological evolution, they had enormous consequences for human culture and intelligence.