NOVA | Ape Genius | What Makes Us Human?

Current research is shedding new light on this age-old question

Congratulations: You are an ape. A "great ape," technically. Alongside us in this brainy family of animals are four other living species: chimpanzees, gorillas, orangutans, and bonobos (formerly called "pygmy chimpanzees").

The biological gap between us and our great ape cousins is small. At last count, only 1.23 percent of our genes differ from those of chimpanzees. But mentally, the gap between us and them is a Grand Canyon.

On an average day in the life of the human species, we file thousands of patents, post tens of thousands of Internet videos, and think countless thoughts that have never been thought before. On a good day, chimpanzees are lucky to exploit rudimentary tried-and-true techniques, such as using stone tools to crack nuts.

Not only do we innovate more than the other great apes, we are vastly better at sharing ideas with one another. The majority of recent behavioral studies focus on information-transmission rather than invention. All of the great apes can learn new tricks by imitating a human or another ape. But only humans go one step further and routinely teach each other. Teaching may be the signature skill of our species, and researchers are now zeroing in on three particular mental talents that make it possible.

Our unique talents

Mind-reading. Humans are exceptionally skilled at thinking about what's on other people's minds. A teacher, for example, needs to understand what a student knows and doesn't know. Researchers used to believe that chimpanzees lacked this talent entirely. Although recent experiments at the Max Planck Institute for Evolutionary Anthropology in Germany are showing that chimps share at least a bit of this skill, humans are clearly head and shoulders above the great apes in mind-reading savvy.

The Triangle. Watch a human parent building a block tower with a child and you'll see a special skill at work. Let's call it the Triangle; its three points are the adult, the youngster, and the tower. Both adult and child are not only focused on the same object, they know the other is focused on it too. The Triangle is the foundation for teaching—a mentor and pupil must jointly pay attention to the lesson at hand. Amazingly, humans seem to be the only great apes that possess this mental skill.

Impulse control. Whereas mind-reading and the Triangle are cognitive skills, the third mental talent that sets us apart from our kin is emotional. We seem to have much greater control over our emotions, and being less reactive and impulsive is a good way to get to the head of the class.

Researchers discovered all three of these distinguishing human talents by observing human and ape behavior, sometimes with solid, carefully controlled experiments. But what's going on under the biological hood? What brain mechanisms are responsible for the mental and behavioral differences between them and us? Biologically inclined researchers are starting to answer these questions, and the clues they are finding, while still patchy and nascent, are tantalizing.

A place for mind-reading

The mind-reading skill, it turns out, appears to have its own particular region of the human brain. In 2003, Rebecca Saxe of MIT ran studies using functional magnetic resonance imaging, or fMRI, a non-invasive technology that creates a kind of movie of brain activity. The studies revealed an area perhaps half the volume of a sugar cube above and behind the right ear.

This brain region appears to have a remarkably specific function. When I am thinking about who a friend believes will be the next American president, this area in my brain is highly active. But when I am thinking about whether my friend is thirsty—another internal state, but not a belief—this brain region is quiet. Saxe's discovery—one of the most surprising in cognitive science in the last decade—begs a question: Do the other great apes have their own version of this brain area, and if so, what is it doing?

Clearly, for anyone interested in the gap between us and our nearest living relatives, these are exciting times.

With fMRI, a conscious person lies still inside a scanner. Why not use the same technique with apes? Apes, not surprisingly, are strong, impulsive animals with little inclination to hold still inside a big, noisy cylinder. So the prospect of scanning apes to see what's on their minds seemed dim, until primatologist Lisa Parr and colleagues at the Yerkes National Primate Center at Emory University solved the problem in 2007.

Using a different kind of scanning—positron emission tomography, or PET—Parr's team showed chimps pictures after injecting them with a fast-decaying radioactive agent. When the chimps were anesthetized for a PET scan, the brain areas selectively activated by looking at the pictures remained "lit up." Parr is currently investigating the area of chimp brains responsible for recognizing faces. Functional brain imaging of apes is just beginning, but this new technique holds huge promise.

Where other talents lie

The Triangle skill has also been suggestively linked to a particular region of the human brain. In a study conducted by Andrew Whiten and colleagues at the University of St. Andrews in Scotland, a person's brain is scanned using fMRI. When the human subject and an animated character on a TV screen are both looking at the same moving object, a small area in the human's brain—located an inch behind the middle of forehead—is highly active. But when the person and cartoon character are looking in separate directions, this brain region is much less active. In a separate study by other researchers, this same brain region is more active when a person is watching two cartoon characters collaborating on a joint project compared to when the characters are working independently.

Yet another region of the brain—the frontal lobes, the part of the brain behind your forehead—is linked, by many lines of evidence, to impulse control. Anthropologist Katerina Semendeferi of the University of California, San Diego compared the size of this brain area across the great apes. To her surprise, she discovered that humans do not possess proportionately bigger frontal lobes. Semendeferi is now investigating more subtle differences in the wiring of this region: Humans have far more neural connections in their frontal lobes than do other apes.

Other researchers are exploring different compelling avenues. Neuroscientist John Allman at Caltech and colleagues are investigating a particular kind of brain cell, for example. "Spindle neurons" are found in humans, orangutans, gorillas, bonobos, and chimpanzees—but not in any of the roughly 350 other primate species. In a rare evolutionary event, spindle cells seem to have arisen abruptly 15 to 20 million years ago with the emergence of the great apes. The function and significance of these intriguing cells, like so much else in this field, remain to be worked out.

Clearly, for anyone interested in the gap between us and our nearest living relatives, these are exciting times. Comparative studies of our own minds and brains and those of the other great apes are finally getting traction on one of biggest questions in science: What makes us human?

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