May 31, 2018

Summary

Three brain development genes are found only in humans and may have helped drive the rapid expansion of the brain starting roughly three million years ago.

A newly discovered set of genes could help explain how humans diverged from other apes.

Three genes involved in nerve cell generation in the brain emerged roughly 3.5 million years ago and may have contributed to the rapid evolution of the large human brain, Howard Hughes Medical Institute (HHMI) Investigator David Haussler and colleagues report on May 31, 2018, in the journal Cell. That work – and a paper from an independent research team published in the same issue – identifies genes that help build the neocortex, the wrinkly outer layer of the brain that gives humans our ability to think, plan, and reason.

Until now, the genes had been unexplored, masked by an error in the published version of the human genome. These genes, and others unique to humans, offer clues about what separates us from chimpanzees, says Haussler, of the University of California, Santa Cruz (UCSC).

“Understanding ourselves and how we’re different from other species has been a scientific quest for millennia,” he says. “These studies help address the age-old question of what makes us human.”

Ancient history

About 6.5 million years ago, humans and chimpanzees diverged from a common ancestor. A few million years after that, human brain size began to grow. “If you look at craniums from fossils of our ancestors, you can see this expansion,” Haussler says.

Today, the human neocortex is about three times larger than that of a chimp, but researchers haven’t identified all the genetic factors responsible for the difference. Since humans and chimps split, the human genome has undergone roughly 15 million changes – tweaks to the DNA letters that make up our genetic instruction book.

Roughly three million years ago, human brains began to expand. Today, the human neocortex is larger than that of an orangutan, gorilla, or chimpanzee, our closest living primate relative. Scale bar equals 5 centimeters. Credit: I. Fiddes et al./Cell 2018. Gary Mantalas took the photos of skulls that were provided by Richard Baldwin of the Anthropology Department at UCSC.

Haussler is well-versed in the particulars of people’s genes. His team was part of the Human Genome Project, the multiyear quest to spell out all of the DNA letters that make up humans’ roughly 20,000 genes. Haussler and colleagues posted the first draft of the human genome on the Internet on July 7, 2000­. An official version came out in 2003, but it’s still a work in progress, he says. “We keep filling in little gaps and clarifying places that we were uncertain about.”

Researchers also have to decide which variants of genes to include. Humans all have the same basic blueprints, but genes can be rearranged, deleted, and come in different “flavors.” The official genome is “mostly some anonymous guy from Buffalo,” Haussler says, but scientists have swapped out some of his altered genes to build a more complete version of the genome.

They’re now on version 38, and each new iteration can unearth valuable nuggets of information. For Haussler’s team, version 38 struck gold.

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