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The monkey in the mirror

Zeena, a chimpanzee at Chimp Eden Sanctuary in South Africa. Image by AfrikaForce licensed under CC BY 2.0

Non-human primate brains offer a lens into human minds.

Bigger, slower-developing brains may distinguish humans from their non-human primate relatives, says George Washington University anthropologist Chet Sherwood, but these obvious brain differences are only the beginning of what we can learn about our evolution by studying our primate cousins.

Understanding how brain shape and function differ among primate groups could help answer many questions about the human brain, from the neurochemical controls of social behavior to the reasons people develop mental illness.

“Only by including data from great apes can we understand human brain evolution,” Sherwood said as he spoke as part of the Council for the Advancement of Science Writing’s New Horizons in Science briefing at the ScienceWriters2019 conference in State College, Pa., Oct. 28. “They’re a mirror onto ourselves.”

Sherwood’s group at the Mind-Brain Institute at GWU studies the brains of primates, from the rhesus monkey, whose evolutionary track diverged from humans’ almost 30 million years ago, to our closest cousin, the chimpanzee. Humans’ last shared common ancestor with the chimpanzee walked the Earth just six million years ago.

Size isn’t everything

Compared to our body size, humans have truly massive brains. For example, gorillas and humans have the same average total body weight, but an adult human brain weighs about 1,230 grams—more than three times the weight of a gorilla’s 370-gram brain. But size isn’t everything—the relative volumes of specific parts of the brain play a key role. In humans, the areas of the neocortex—the outermost part of the brain—responsible for complex tasks such as higher order processing, social learning, and language are disproportionately large. The area that manages motor and visual functioning is bigger in humans, but not strikingly so.

To investigate this further, Sherwood’s team examined and compared the brains of our two closest living relatives, chimpanzees and bonobos. Chimps are known for their more aggressive, dominant social interactions, whereas the sexually prodigious bonobos are more socially cooperative and peaceable.

Sherwood and his colleagues found that the bonobo brain has a proportionally larger amygdala, the part of the brain that controls social behavior and processes negative emotional stimuli such as fear. They also found twice as much of the neurotransmitter serotonin, which is partially responsible for emotional regulation, in the amygdala of bonobos as they found in chimpanzees.

“It might be the case that both chimpanzees and bonobos would receive threatening stimuli similarly,” Sherwood said. “Yet bonobos simply don’t have an amygdala that’s set up to get as worried about it to then pass on the message and provoke a more aggressive response.”

A more recent study outside Sherwood’s lab also suggests that the human amygdala appears to be an extension of the bonobo’s rather than the chimpanzee’s, giving us greater capacity for empathy and more measured social responses.

Slow and steady

Time matters, too.

Cranial volume and brain mass measurements of mountain gorillas and chimpanzees indicated that gorillas achieve adult brain mass by age three, whereas chimps take four years and humans five.

Diet may be the cause of this difference. Mountain gorillas live basically in a “salad bowl” of their preferred leaves and bamboo shoots, and so have plenty of food and energy to support fast growth. Chimps, on the other hand, need to forage far and wide for the berries and nuts they feed on, so they have to conserve energy, which constrains brain growth.

The social consequences of this difference are striking. Because chimps have a longer period of brain plasticity, they seem to have much more pronounced learned social behaviors than gorillas.

Take Julie, a chimp in Zambia who took to wearing a blade of grass in her ear.

“She just thought it looked cool,” Sherwood said. “And then it spread in the group.”

Lots of chimps in Julie’s social circle copied and passed on her fashion statement. Sherwood said this kind of “nonsensical, purely social” behavior isn’t present in gorillas, whose shared group behaviors tend to have a direct adaptive advantage — for example, using teeth as an extra limb in climbing trees.

Nurture beats nature

Chimps’ family resemblance carries through to brain structures, in the same way a distinctive facial feature might appear throughout a family line. But human brain shape is more individual, not tracking as neatly within families. This is possibly because it takes more time for human brains to mature. Basically, Sherwood said, the longer environmental and social factors have to influence brains before they are fully baked, the greater the divergence in brain shapes. The longer period of brain growth and plasticity might explain why humans are also susceptible to mental illnesses such as schizophrenia, depression, or bipolar disorder.

“These might uniquely affect our species,” Sherwood said. “And it might be the case that we are especially vulnerable to these disorders because we have this distinctive brain development trajectory.”

Sarah E. Moran is a PhD student at Johns Hopkins University, where she studies the atmospheres of planets around other stars. She tweets about space, grad student life, and her cats at @Of_FallingStars. Moran wrote this story as a participant in the ComSciCon-SciWri workshop at ScienceWriters2019.