Scientists believe that the appearance of a partial, duplicate copy of a gene, occurring around the time that the Australopithecus and the Homo lineages separated, was responsible for the sudden increase in brain complexity that led to language and modern humankind. Writing about their findings in Cell, the researchers say the momentous gene duplication event occurred about two or three million years ago, at a critical transition in the evolution of the human lineage.
The work is the first to explore the evolutionary history and function of any uniquely human gene duplicate. These “extra” genes are of special interest as they provide likely sources of raw material for adaptive evolutionary change. “There are approximately 30 genes that were selectively duplicated in humans,” said researcher Franck Polleux, at The Scripps Research Institute. “These are some of our most recent genomic innovations.”
Many of these genes appear to play some role in the developing brain. Polleux and Evan Eichler, a genome scientist at the University of Washington, focused their attention on one of the genes known as SRGAP2. This gene has been duplicated at least twice during the course of human evolution, first about 3.5 million years ago and then again about 2.5 million years ago.
Polleux explains that SRGAP2 helps control the development of the neocortex – the area of the brain believed to control higher functions like language and conscious thought. Having an extra copy of the gene “slowed” the development of the brain, allowing it to forge more connections between nerve cells, growing bigger and more complex.
If this gene duplication did indeed produce an immediate effect during our evolution as Eichler and Polleux suspect, they expect there must have been a fascinating period in human history characterized by “huge variations” in human cognition and behavior. SRGAP2 and other human-specific gene duplicates might also help to explain the big differences between humans and other primates, despite few apparent differences in our genome sequences.
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