It’s hard to believe that our complex human brain evolved from a simple tube. The earliest vertebrates probably had brains much like the one in the modern lancelet Amphioxus — little more than a wide spot in the hollow nerve cord running down its back. But while the lancelet’s brain looks simple, it still contains specialized regions where neurons process specific kinds of information, like the presence of light or the chemicals drifting through the water. In its early development, the human brain began as a simple tube, and even today it is divided into the same kinds of regions as the brains of our ancestors.
In early vertebrates, the “brain” end of the nerve cord developed three distinct bulges as neurons were added, improving processing in sensory and motor reflex regions. These bulges became the forebrain, the midbrain, and the hindbrain. In the forebrain, the region able to detect chemicals expanded to form the olfactory bulbs, and with the evolution of image-producing eyes, light-sensing regions expanded and began processing more complex visual signals. The cerebellum appeared as the hindbrain and expanded the regions that control escape movements and orient the body in space. Both these functions are far more important to an actively swimming fish than to a sedentary lancelet buried in sand.
Regions that could rapidly process visual and auditory information and trigger appropriate escape, feeding, or mating behaviors also expanded in vertebrates. Over time, those new types of neurons made the forebrain balloon out, forming the right and left cerebral hemispheres. In early mammals, cortical tissues in the cerebrum and the cerebellum expanded even further, packing new neurons into layers and folds generating more complex tissues with increased processing power.
Adapted from the 8th edition of Brain Facts by Diane A. Kelly.
About the Author
Diane A. Kelly studies neuroscience and anatomy at UMass Amherst, with a focus on mapping the circuits of the social behavior network. Her science writing has appeared in a variety of publications, including Gizmodo, Wired.com, FiveThirtyEight, and Muse. You can find her on Twitter @DianeAKelly.
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