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SALT LAKE CITY — A mistake in DNA copying may have led to the drastic changes that made humans capable of language and other civilized behavior that separates us from our primate ancestors.
Two studies published Monday in Cell suggest a cell mutation over the course of a single generation enabled more connections between the brain's neurons, giving humans more computational power than their ancestors.
The mutations resulted during cell division, when cells make copies of their entire genome. Errors during this process that go unfixed are mutations — sometimes hurtful and sometimes helpful, but usually harmless. In this case, a section of the genome was copied twice, an error known as duplication.
These duplications are thought to make up about 5 percent of the human genome, and many of them seem to have impacted how the human brain develops.
"There are approximately 30 genes that were selectively duplicated in humans," Franck Polleux, a study participant and expert in brain development at The Scripps Research Institute, said in a statement. "These are some of our most recent genomic innovations."
Polleux and his team studied one gene specifically: SRGAP2. The gene is believed to have been duplicated at at least two points in human history: 3.5 million years ago, and again 2.5 million years ago.
It is the second duplication, SRGAP2C, that is of interest: only part of the gene was duplicated, potentially causing it to interfere with the original SRGAP2. The partial copy of the gene was shown in mice to speed the migration of brain cells during development, making brain organization more efficient and potentially allowing the brain to become more powerful.
It's got to play some important function.
–Franck Polleux
The copy also caused the neurons of the mice to form dendritic spines in the brain cell structure, leading the neurons to forge more connections with neighboring neurons.
The emergence of SRGAP2C could also have boosted the brain power of human ancestors, according to the research, although the cause of the brain's change in size is still unknown.
"It's got to play some important function," Polleux said. "If you're increasing the total number of connections, you're probably increasing the ability of this network to handle information. It's like increasing the number of processors in a computer."
SRGAP2C appeared around the same time as the large-brained Homo evolved from the Australopithecines. It was around this time that stone tools first appeared in the fossil record, and it was from this species that Homo erectus — the first human ancestors to leave Africa — evolved. Researchers have long tried to find just what makes humans human, and they said this discovery brings them a bit closer to their goal.
"We may have been looking at the wrong types of mutations to explain human and great ape differences," said lead researcher Evan Eichler, of the University of Washington. "These episodic and large duplication events could have allowed for radical — potentially Earth-shattering — changes in brain development and brain function."
