Human brains have expanded enormously in particular areas since the species began to diverge from more apelike progenitors. There is no consensus as to what factor or combination of factors contributed to this runaway brain growth — theories range from meat eating to the invention of cooking to enhanced cooperation skills to evolution by sexual selection as well as things like brain pills — but more research will have to be done before any definitive answers can be found.
However, in a related and intriguing study, published on the website of the Proceedings of the National Academy of Sciences, researchers have noted the similarities between the developing brain of a child and the brain growth that characterized the emergence of Homo sapiens.
What Makes a Human Brain Human?
Though humans share almost the entirety of their genome with great apes like chimpanzees and bonobos, it is the monumental changes in the brain that characterize the human species. The greatest difference between the brains of humans and their primate cousins is the cerebral cortex, which has vastly expanded and become more complex over humans’ evolutionary history and is largely responsible for what humans think of as their quintessential traits, like abstract reasoning, complex language, foresight and memory.
A Study of Infant Brains
Research conducted at the Washington University School of Medicine in St. Louis demonstrated that the parts of the human brain that have enlarged the most since the divergence from the rest of the primates are the same brain areas that expand the fastest in the brains of infants and children.
The study itself looked at brain scans from twelve infants and compared them both with scans of the brains of twelve young adults aged eighteen to twenty-four, and with brain scans of macaque monkeys. From the scans of the monkey brains, the researchers could pinpoint the areas where human brains had changed the most throughout evolutionary history.
In comparing the infant brains with the young adult brains, the scientists noted that areas of the cerebral cortex — specifically the lateral temporal, parietal, and frontal cortex — expanded twice as much as other brain areas in the transition from childhood to adulthood. The area of the brain responsible for vision processing also expands quickly, perhaps because vision is such an important sense for humans.
Reasons for Uneven Brain Growth
Researcher David Van Essen and others have speculated on reasons why the brain might develop in the order in which it does. An infant’s brain is still largely undeveloped at birth because more development in the womb would lead to a head too large to pass through the mother’s birth canal.
Van Essen believes evolution has molded the brain to “prioritize” certain key brain areas, like visual processing, that will be needed immediately by the infant. Other areas can grow more slowly if they are not as crucial to the infant’s survival, or if slower development might encourage more plasticity, making the brain more receptive to learning and experience as it matures.
Van Essen is quick to report that the correlation between infant brain growth and brain expansion over the evolutionary history of Homo sapiens is not completely perfect, but is close enough to suggest links that could be further studied. Similar research — some with premature babies — is being conducted by others, like professor of pediatrics Dr. Terrie Inder, also of the Washington University School of Medicine, who was also a researcher on this particular study.