Scientists studied rhesus macaques, a common ancestor of chimpanzees and humans, to see where genetic changes emerged between humans and chimps.
Human brains and chimp brains both evolved from the same ancestor millions of years ago and outwardly look anatomically similar. Wherein lies the difference that separates us from our simian cousins?
A UCLA study pinpoints uniquely human patterns of gene activity in the brain that could shed light on how we evolved differently than our closest relative. The identification of these genes could improve the understanding of human brain diseases like autism and schizophrenia, as well as learning disorders and addictions.
The research was published in the online edition of the journal Neuron.
"Scientists usually describe evolution in terms of the human brain growing bigger and adding new regions," says Daniel Geschwind, M.D., Ph.D., the Gordon and Virginia MacDonald Distinguished Professor of Human Genetics. "Our research suggests that it's not only size, but also the rising complexity within brain centers that led humans to evolve into their own species."
Examining post-mortem brain tissue, Dr. Geschwind and his colleagues used next-generation sequencing and other modern methods to study gene activity in humans, chimpanzees and rhesus macaques, a common ancestor of both chimpanzees and humans, which allowed them to see where changes emerged between humans and chimps. They zeroed in on three brain regions: the frontal cortex, the hippocampus and the striatum.
By tracking gene expression, the process by which genes manufacture the amino acids that make up cellular proteins, the scientists were able to search the genomes for regions where the DNA diverged between the species. What they saw surprised them.
"When we looked at gene expression in the frontal lobe, we saw a striking increase in molecular complexity in the human brain," Dr. Geschwind says.
"Although all three species share a frontal cortex, our analysis shows that how the human brain regulates molecules and switches genes on and off unfolds in a richer, more elaborate fashion," says Genevieve Konopka, Ph.D., a former postdoctoral researcher in Dr. Geschwind's lab. "We believe that the intricate signaling pathways and enhanced cellular function that arose within the frontal lobe created a bridge to human evolution."
The researchers took their hypothesis one step further by evaluating how the modified genes were linked to changes in function. "The biggest differences occurred in the expression of human genes involved in plasticity - the ability of the brain to process information and adapt," Dr. Konopka says. "This supports the premise that the human brain evolved to enable higher rates of learning."