If 2,000 neurons are sufficient for simple learning, imagine the explosion of complex behavior that accompanied the growth of neural complexity about 530 million years ago. Larger clumps of neurons in the diverse animal population that seemingly emerged overnight encouraged the flourishing of new animal species. The variety of new species could better react to, and survive, changes in their environments. Ocean life diversified into the ancestors of today’s worms, mollusks, and crustaceans.

The forward tip of the neural cords in the first proto-vertebrates began swelling and folding to create primitive brains. Neural networks in those early brains began to diversifY. Some connections began to specialize in vision. Some took on the function of hearing. Among the sharks, neural connections specializing in smell became hypersensitive, empowering them to detect blood in concentrations as small as 1 part per 25 million of water. That allowed them to smell bloody prey a third of a mile away (and, not coincidentally, strengthened their chances for survival in the constant interspecies combat of evolution).

A developing spinal cord is already visible in a three-day-old chicken embryo developing inside its eggshell.
A developing spinal cord is already visible in a three-day-old chicken embryo developing inside its eggshell.

As animals began crawling out of the ocean onto the shore, around 360 million years ago, their brain didn’t begin anew. Instead, new experiences and new evolutionary developments were laid down atop their existing neural networks. Birds and reptiles added new levels of behavior, and new brain matter developed as well. Mammals put their own layers on top of their evolutionary predecessors. And finally, humans with their gigantic brain added the newest and most complex layers in the wrinkly pink walnut of the cerebral cortex.

Darwin explicitly put humans in the crosshairs of his theory with the 1871 publication of The Descent of Man. Human bodies and brains evolved and continue to do so.

The human brain differs physically from those of other mammals in its size, complexity, and dominance of its cerebral cortex. Just like speed and strength, early advantages in the brain such as analytical power (“How can I trap that animal?”) and capacity for speech (“How can I get others to help me trap that animal?”) improved the odds of early humans’ survival. Advantages spread to new generations and became common.

Networks of synapses constantly compete with each other; roughly like animal species fighting for limited food. Networks that get steady stimulation grow stronger; while others atrophy. Nobel laureate Gerald Edelman calls the process neural Darwinism.

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