Small-minded! Pinhead! Peabrain! The insults people hurl reveal our assumptions about what makes us intelligent. But two new studies in the 10 May issue of Nature suggest that it's not just brain size that makes us smart. Instead, we higher primates have brains with a distinctive design that allows us to think big.
Most human intelligence resides in our overgrown neocortex, the convoluted outer layer of the brain that helps us make sense of the world. Scientists have argued for years about how our neocortex got so big. Some say that higher primates evolved a new brain plan, perhaps in response to evolutionary pressure to develop complex social skills. Others contend that the basic primate brain plan hasn't changed that much compared to other mammals; our big neocortex developed incidentally when our brain had to grow big enough to control a big body.
To track how brain plans differ in primates and other mammals, neuroscientist Sam Wang of Princeton University and colleagues scrutinized the relative volumes of 11 brain regions. First, they calculated the fraction of the brain volume each region occupies in 75 species. Then the researchers turned the 11 relative volumes into an overall measure of brain composition called a cerebrotype. Groups of closely related species, such as lemurs or higher apes, had similar cerebrotypes. This means that brain structures, like bones or other tissue, can be used to deduce evolutionary relationships, the researchers suggest.
The second new study could help explain why the neocortex grew disproportionately large in humans. By examining data on brain size and brain cell density from 23 higher primates, neurobiologist Chuck Stevens of the Salk Institute in San Diego showed that the sharper a primate's vision, the more cortex it devotes to processing images. That is, the number of cells in the visual cortex is proportional to the number of cells lower down in the visual system raised to the power of 3/2. A similarly unbalanced increase in cortex size relative to simple sensations could account for enlarged language, hearing, and motor cortex as well, Stevens says.
The cerebrotype study is "important work" that suggests parts of the brain have in part evolved independently, says neuroscientist Jon Kaas of Vanderbilt University. And he adds that the visual system study provides a "good argument" that explains why the human neocortex evolved to grow so big.