Driving Questions

The study of the human mind, or psychology, has been humanity’s preoccupation for thousands of years. By the time Darwin proposed his theory of evolution by means of natural selection, he predicted that the field of psychology would change. “Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history,” he wrote (3).

With 6 million years separating us from our last common ancestor with chimpanzees, studying those gradations means considering 12 million years of evolution, 6 million years for each species. That leads to some tension, says de Waal, because after 12 million years of evolution, Darwin’s gradations appear rather discontinuous: our closest living relatives appear quite different from us.

That tension affects people differently. “There are two kinds of people in the Western world,” says de Waal. “The people who don’t mind at all being compared to animals and the people who are offended by the comparison, who think it is terrible to be compared to a chimpanzee,” he says. That tension also drives research, says Tomasello: “People say, ‘geez, if we have a common ancestor with apes, then we want to know what is the difference’.”

Derek Wildman, a biological anthropologist at Wayne State University in Detroit, MI, says he wants to study human uniqueness to focus on “what it is that unites all the members of the human species” so that “we can gain a sense of their common humanity” rather than identifying with ethnic differences that may lead to racially motivated actions.

Redefining Cognitive Capacities

Decades after Darwin, scientific consensus began to coalesce around the idea that human uniqueness was defined by our ability to make tools. Then, in 1960, Jane Goodall reported her observation that chimpanzees in the wild were making tools to get at termites. By that time, the idea that tool making defined humanness was so ingrained in the scientific community that Louis Leakey famously wrote in a telegram to Goodall: “Now we must redefine tool, redefine Man, or accept chimpanzees as humans (4).”

Scientists chose to try to redefine tool by deciding exactly what constituted “making” a tool. They also tried redefining self-recognition, deception, and any of the other characteristics researchers subsequently postulated as being uniquely human. Then, in gathering evidence to test those definitions, scientists often had to redefine them again before gradually accepting that many animals were using tools, recognizing themselves, deceiving others, and much more.

“Even with major capacities like morality, culture, and language, as soon as you take sub-components of them, you’re going to find those capacities in other species,” says de Waal. Language, for example, has been redefined several times after captive apes demonstrated the capacity to use symbolic communication and grammar and to organize items into categories. More recently, scientists experimenting with wild bottlenose dolphins observed them to be demonstrating a language ability that, until recently, was still considered uniquely human. Called “learned vocal labeling,” the dolphins showed that they were pairing sounds with specific objects by making and responding to signature whistles and therefore, in a sense, calling one another by name (5).

Experimental testing of subcomponents of cognitive capacities has refined our understanding of the cognitive capacities themselves. Tomasello’s test for apes sharing food is a subcomponent of cooperation and, in turn, morality. Santos recently tested whether rhesus macaques reason about others’ beliefs, a subcomponent of the cognitive capacity to understand another’s desires, needs, knowledge, or mental states, an ability called “theory of mind.” The macaques failed. Previously, Tomasello had tested chimpanzees. They also failed. However, in many other experiments, scientists have observed animals demonstrating other subcomponents of theory of mind, such as knowing what members of their species have or have not seen. Animals also show a capacity for deception, which necessitates having some sense of others’ thoughts and intentions (6).

The persistent challenge in doing these kinds of studies, says Preston Foerder of the University of Tennessee at Chattanooga, is that “you’re always trying to figure out good ways to ask animals questions.” Before Foerder’s work, for example, no one properly asked Asian elephants to demonstrate insightful thinking or the sort of problem-solving that does not require prior trial-and-error behavior.

In his experiments, Foerder placed food above but just out of reach of three Asian elephants (7). The elephants had previously been trained to stand with their two front feet on a large plastic block, but not trained to do so to reach food. On just his seventh trial, Kandula, a 7-year-old male elephant, suddenly left the location of the hanging food, went over to one of the plastic blocks, rolled it beneath the food, stood on it, and got the food. “In people, this is what we would call the ‘Aha!’ moment,” says Foerder. Kandula continues to exhibit the behavior even though he had never demonstrated it before that experiment.

In the Brain

Given studies such as Foerder’s, it’s no wonder that Santos says, “the jury is still out on what makes humans unique.” Observational studies and testing animals for cognitive behaviors haven’t unequivocally established the uniqueness of humans—and they may never do so. That’s prompting some scientists to look for evidence elsewhere, such as in the human brain itself. “Humans have an extended juvenile period that isn’t seen in other mammals, and during that period, we see brain growth continuing and also plasticity,” says Wildman. “We think that’s an important aspect of humanity that distinguishes us from other species,” he says.

In one study, Wildman is investigating gene activity in postnatal brain growth. He has found a wider array of gene expression in children’s brains compared with adults (8). Wildman is not sure of its significance, but his team is gathering comparative data in great apes and macaque monkeys. If both the apes and monkeys lack that flexibility in gene expression, Wildman’s team may have discovered the genetic clues to the plasticity of human brains and our capacity to learn from experience and produce culture.

Joanne Altman of High Point University in High Point, NC, hypothesizes that if a qualitative difference is to be found between humans and other animals, it will be in the behavior associated with the prefrontal cortex. “The prefrontal cortex is the most evolved part of the brain,” she says, noting that it doesn't fully develop in humans until they’re about 25 years old. Therefore, Altman is testing whether animals have the cognitive capacities considered to be located in the prefrontal cortex; together they are called “executive function” and include such capacities as making decisions, correcting errors, responding to novel sequences of actions, and overcoming habitual responses (9). If Altman can find evidence that other animals have executive function, she will have ruled it out as a uniquely human trait.

Currently, she is training lemurs to take an executive function test normally administered to human patients. The test, called the Wisconsin Card Sorting Test, involves sorting cards, each with a figure, according to a shifting set of rules that the subjects have to decipher based only on feedback from the proctor. People who do poorly on the test often either have physical damage to their prefrontal cortex or suffer from conditions such as schizophrenia (10). If none of the lemurs pass the test, Altman next has plans to take on the potentially more costly and challenging task of testing apes and elephants. She also hopes to distribute a tablet app she is developing so zoos around the world can test their animals.

Persistent Critiques, Obvious Answers

A common critique of the search for what makes us uniquely human is our own intraspecies variation. Some people do poorly on tests for executive function without brain damage or diagnosed mental disorders. Some 3-year-old children simply do not cooperate. Some people lack intelligence, morality, or other social-cognitive behaviors for reasons we do not yet understand. Despite the difficulty of uncovering a behavior that’s clearly common to all humans, some researchers still seek out uniquely human traits.

Indeed, the apparent vast gaps between ourselves and our closest living animal relatives would be much narrower if scientists were studying human uniqueness 40,000 years ago, says Thomas Suddendorf of the University of Queensland, Australia. At that time, “there were several smart, upright-walking, stone tool-carrying cousins, including Neanderthals,” says Suddendorf. “All of our closest surviving animal relatives today are endangered,” he adds. If apes don’t exist 40,000 years from now, then scientists studying human uniqueness at that time would find the gap even wider than we do today, says Suddendorf.

Tomasello argues that our edge was in collaborating; it’s how our ancestors eliminated or outcompeted other species. “Evolutionarily, something happened in the ecology so that humans had to collaborate to get food,” he proposes. “Natural selection was that anyone who wasn’t a good collaborator would die,” he says. Of course, many other species, from birds to ants to bats to apes, “collaborate” in some sense. Tomasello continues to experiment and document how humans’ cooperative abilities compare with those of nonhuman great apes such as chimpanzees, bonobos, orangutans, and gorillas; recently, he’s attempted to distinguish how finger-pointing behavior among human babies indicates cooperative communication and differs from apes’ behavior (11).

However, zeroing in on any one characteristic that makes humans unique is unlikely to tell the whole story. The answer to the larger question may be just the sui generis blend of traits and abilities that allow us to occupy our particular ecological niche—making humans different from all other species and, in a sense, just like them.