A few years back, he developed a theory about a fundamental difference between chimpanzees and us that might be at the root of human language. The story starts with a simple gaze. Monkeys rarely look into each other’s eyes; what is known as a “mutual” gaze is usually read as a sign of hostility. Many scientists thought that affectionate eye-to-eye contact was uniquely human—until Matsuzawa and his colleagues put it to the test with chimps.
The birth of Ayumu and two other babies at the Primate Research Institute in a five-month span in 2000 gave Matsuzawa the opportunity to observe mother-infant pairs intensely. The scientists learned that new mothers looked into the eyes of their babies 22 times per hour. And chimp babies, like human infants, communicate by imitation, sticking out their tongues or opening their mouths in response to similar adult human gestures. Because chimps and humans engage in this distinctive behavior but monkeys do not, Matsuzawa asserts that the common ancestor of chimps and humans must also have made eye-to-eye contact between mother and infant, thereby setting the stage for humans to develop our unique language skills.
Humans introduced a twist in this evolutionary tale. Chimpanzee babies cling to their mother. But our babies do not cling; mothers must hold their infants or they will fall. It may seem like a small difference, but it changes the way adults and infants interact.
Chimpanzees have babies once every four or five years, and those babies are constantly physically close to their mothers. But humans can reproduce more frequently and take care of multiple offspring at once. And a human mother is not the only possible caregiver. “We changed the system of rearing children and giving birth,” Matsuzawa says. “With the assistance of spouse, and grandparents, we are collaborating together to raise children.”
Because human mothers separate themselves from their babies, human babies have to cry to get attention. “Not many people recognize the importance,” he says. “Human babies cry in the night, but chimpanzee babies never do because mother is always there.” This crying is a proto-language of sorts.
Add to this the fact that humans are the only primates that can lie on their backs without having to stabilize themselves. Chimpanzee and orangutan infants have to raise one arm and a leg on the opposite sides of their bodies to lie on their backs. They must grasp something. Human babies can stably lie in the supine position, allowing easy face-to-face and hand-gesturing communications to let others know what they’re thinking or feeling.
“All of these things are interconnected, and from the beginning,” Matsuzawa says. “The underlying mechanism of communication is completely different between humans and chimpanzees because of the mother-infant relationship.” Though Matsuzawa’s theory is difficult to test, it’s logical and alluring. “What is the definition of humans?” he asks. “Many people say bipedal locomotion. Decades ago, they said it’s language, tools, family. No. Everything is incorrect. My understanding is the stabile supine posture, that is completely unique to humans.” Muscles, he says, shaped our minds.
The list of differences between humans and chimpanzees is long, and the most obvious ones have received intense attention from researchers. We have bigger and more complex brains, full-fledged language and writing, sophisticated tools, the control of fire, cultures that become increasingly complex, permanent structures in which to live and work, and the ability to walk upright and travel far and wide. Matsuzawa and his colleagues are clarifying more subtle—but no less profound—distinctions that often are as simple as how a chimpanzee learns to crack a nut or how a human mother cradles her infant, rocks it to sleep and lays it down for a night’s rest.
Jon Cohen wrote about stem cells for Smithsonian in 2005. Jensen Walker is a photographer based in Tokyo.
Adapted from the book Almost Chimpanzee: Searching for What Makes Us Human, in Rainforests, Labs, Sanctuaries, and Zoos by Jon Cohen, published this month by Times Books, an imprint of Henry Holt and Company, LLC. Copyright © 2010 by Jon Cohen. All rights reserved.