Scientists have found that only great apes and humans share a particular type of brain cell. We have more of the cells than do our primate cousins; their relative abundance may shed light on the evolution of speech, self-awareness, and emotional control. The findings appear in the current Proceedings of the National Academy of Sciences.
Up to now, scientists interested in the evolution of human intelligence have examined fossil skulls and the brains of great apes, but they have never had much to go on besides size. From humans to hamsters, the basic cell types and the architecture of the brain appeared the same. But a team of neurobiologists, led by Patrick Hof of the Mount Sinai School of Medicine in New York City and John Allman of the California Institute of Technology in Pasadena, recently decided to look more closely at a particular brain region. The frontal lobe's area 24, as it's called, has been implicated in higher thought processes regarded as unique to humans--including complex problem-solving and fine control of the mouth, throat, and vocal cords.
The researchers compared brains from 28 primate species, including lemurs, monkeys, gibbons, great apes, and humans; and from 20 nonprimate mammals as diverse as mice and whales. (All humans and most animals had died of disease or natural causes.) They found that humans and great apes share a sparse population of large brain cells, shaped like rolling pins, that other species lack. The researchers couldn't find such cells outside area 24. The cells, which appear to be a variety of pyramidal neuron known as spindle cells, were distributed most densely in the brains of humans, followed by bonobo, chimpanzee, gorilla, and orangutan.
"This is the first description of a cellular specialization in the brains of great apes and humans," says neuroscientist Tom Insel of Emory University in Atlanta. By pursuing this finding, he says, scientists may learn what aspects of brain evolution led to the sophisticated mental abilities that make us human. For example, only the great apes and humans appear self-aware, Insel says, so spindle cells may point scientists to the origin of self-awareness.