Forget the insult “fathead.” We may actually owe our extraordinary smarts to the fat in our brain. A study published in Neuron in February revealed that the variety of fat molecules found in the human neocortex, the brain region responsible for advanced cognitive functions such as language, evolved at an exceptionally fast rate after the human-ape split.
The researchers analyzed the concentrations of 5,713 different lipids, or fat molecules and their derivatives, present in samples of brain, kidney and muscle tissues taken from humans, chimpanzees, macaques and mice. Lipids have a variety of critical functions in all cells, including their role as the primary component of a cell's membrane. They are particularly important in the brain because they enable electrical signal transmission among neurons. Yet until this study, it was unknown whether the lipids in the human brain differed significantly from lipids in other mammals.
The team discovered that the levels of various lipids found in human brain samples, especially from the neocortex, stood out. Humans and chimps diverged from their common ancestor around the same time, according to much evolutionary evidence. Because the two species have had about the same amount of time to rack up changes to their lipid profiles, the investigators expected them to have roughly the same number of species-specific lipid concentrations, explains computational biologist and study leader Kasia Bozek of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Indeed, lipid changes in the cerebellum, a primitive part of the brain similar in all vertebrates, were comparable between humans and chimps. But the human neocortex has accumulated about three times more lipid changes than the chimpanzee cortex has since we split from our common ancestor.
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The results suggest that as human cognition evolved, the types and amounts of fat in key brain areas were rapidly shifting and mutating—and this growth was crucial to the development of our complex abilities. Genes tend to get the most attention, but they are only part of the story, the researchers explain. An enzyme encoded by a single gene, for example, can regulate the synthesis of many different lipids. “The significance of this study is that we're going to see more comparative studies of macromolecular concentrations—such as differences in proteins and lipids—which reveal things that can't be read out directly” from the genome, says Todd Preuss, a neuroscientist at Emory University who specializes in the evolution of the human brain and who was not involved in the study. Learning that lipids played an essential role in the evolution of human intelligence, he says, is “the tip of a very big iceberg.”
