With hundreds of billions of cells and something like one quadrillion connections among them, the human brain is a true marvel of evolution, one that led to culture and everything associated with it. How did such a complex structure evolve? Why is the human brain so much bigger than that of our closest living relatives, the great apes? And what were the forces that drove the dramatic increase in human brain size?
There’s no shortage of theories for why humans have such exceptionally large brains. Some argue that the brain expanded to accommodate our development of language, while others have suggested that social interactions, hunting and even changes in diet played an important role.
But according to Colin Blakemore, professor of neuroscience at the University of Oxford, and chief executive and vice-chairman of the European Dana Alliance for the Brain (EDAB), the answer lies in a phenomenon called neuroplasticity.
Neuroplasticity, loosely defined, refers to the various ways in which the brain remodels itself in response to new experiences, sensory loss, and brain injury or trauma. It confers great flexibility upon the developing and adult brain.
Learning, for example, is widely believed to involve changes in the strength of connections between nerve cells. If a person loses his vision, the visual parts of his brain begin to make new connections to process information from other sense organs. Similarly, the phantom limb sensations experienced by amputees are thought to occur because of a reorganization of the brain regions that have been deprived of sensory inputs.
For Blakemore, this remarkable ability of the human brain to change itself was the crucial seed for its dramatic increase in size and complexity. He presented his ideas during a special lecture at the Federation of European Neuroscience Societies meeting in Amsterdam in July. The Max Cowan Lecture, given by a prominent neuroscientist at each of the biannual meetings, is sponsored by EDAB.
There was a sudden increase in human brain size about 200,000 years ago, Blakemore said. He credits it to a genetic mutation in the brain of a single, early human—the so-called “Mitochondrial Eve,” the most recent ancestor of all human beings, from whom we have inherited a small proportion of our genetic material.
This mutation was the “species event” that led to the emergence of modern humans: It produced an approximately 30 percent increase in brain size. This rapid enlargement of the cerebral cortex generated space for additional brain areas, each of which performs specialized functions.
“We can program our brains by our own experiences, partly because our brains are so large and so flexible,” said Blakemore. “The increase in size happened not only in a very distinctive way, but also very rapidly. I’m making a speculation that a sudden speciation event happened—a spontaneous mutation, not progressive mutations selected by Darwinian evolution.”
The mutation also endowed the brain with a new capacity for plasticity, leading to the emergence of the complex and integrated neural pathways that enable us to process information more efficiently, adapt to the environment more readily, and acquire new skills and knowledge at an unprecedented rate.
Blakemore maintains that these sudden changes initially had very little effect on human behavior, but ultimately led to the emergence of human cognition and culture. He acknowledged that this idea is “anathema to conventional evolutionary biology” because it denies that the increase in size brain occurred by natural selection, the process whereby traits that improve an organism’s chances of survival and reproduction become more common in a population over successive generations.
The modern human brain is an incredibly energy-hungry organ, consuming approximately one-quarter of the body’s energy while constituting only around 2 percent of its weight. So how could the mutation that gave rise to brain enlargement have become “fixed” in the population if it did not confer immediate evolutionary advantages?
Blakemore argues that the environment of early humans was rich in food and other resources, and that such conditions could sustain a large, "greedy" brain without any major disadvantages. The enlarged brain only became advantageous later on, when the environment became less friendly, and early humans were forced to quickly adapt to radical changes.