I am an evolutionary biologist who studies cognition and behavior in other (non-human) primate species in order to understand the evolution of our own behavior and cognition, particularly in the context of decision-making and cooperation. Perhaps the question I get asked more often than any other is how humans are different from other animals. What people generally want is a quick rundown of the trait or traits that differ between us and other species, which is exactly what I can’t give them.

Obviously we’re different—but all species are different from each other, and the (meaningful) ways in which humans differ from other species tend to be more of degree than of kind. For example, in just the last few years, we’ve learned that humans are not the only species that hunt, kill our own kind, make tools, coordinate and cooperate (both within our species and with other species), show evidence of language and morality, or cooperatively care for young. Indeed, reports of these “human” behaviors in other species turn up regularly, and these findings are sufficiently interesting to the general public, as well as the scientific community, that they typically get picked up by the popular press. But while some other species do some of these things, there are differences; they may show some aspects of these traits but not others, or show them in different contexts, or in a different or simpler form.

In other words, there isn’t a quick answer.

On the one hand, as I said, we’re obviously different. After all, I, a human, am writing an essay about what makes us unique; but a chimpanzee or a meerkat can’t do the same for their species. But beyond this, we have a suite of characteristics, many linked to our very large brains, either obviously (language, problem solving, complex sociality) or not so obviously (our infants are born particularly altricial because our large brains need very large skulls and so must be born earlier in development), that are different from other species. While there are many generalist species, we are the “uber-generalist,” having spread to nearly every ecological niche on the planet, and even showing that we can survive for up to a year, continuously, off the planet. While there are many smart species, we are smart in a variety of contexts, at a variety of tasks, and have developed systems of thought that are passed down for centuries. While there are many species with sophisticated communication systems, nothing is as complex as human language, and no other species has developed written communication. While there are many species that use tools, and some that make tools, none rival the complexity and variety of human tools.

On the other hand, as also mentioned previously, all species are unique, so being different doesn’t make us special. Each species is, by definition, different from every other, and we are not alone in having distinctive traits. After all, giraffes have their long necks, beavers have the ability to completely alter a landscape, and termites are symbiotic with microbes that let them digest wood. What we each have is a suite of behaviors and traits that are adapted to our environmental niche. Indeed, an important—and often overlooked—point is that the traits that a species has evolved for their specific environmental niche wouldn’t be particularly well adapted for another niche. That is, just because a long neck benefits a giraffe doesn’t mean it will help another species, and our cognitive skills might not be all that useful for other species. Our superior intelligence and associated skills look particularly meaningful to us because to us they are meaningful; without them we wouldn’t be so well adapted to our niche. However, that doesn’t mean that these same skills are necessary or even desirable for other species to be well adapted to their niches.

The comparative approach

One of the most powerful tools we have to study how traits have evolved and how they have adapted to their environments is the comparative approach. This approach allows us to determine where in the phylogenetic tree a trait emerged (in the case of homology, or traits shared by related species) or what shared environmental traits selected for it across different species (in the case of convergence, or traits shared among unrelated species). To do so, scientists not only look for shared traits, but also consider whether the shared traits are manifesting in similar or different ways, or whether there are precursors that exist that may help to indicate how a trait evolved.

To give you an example from my own work, I have spent nearly twenty years studying how other primates respond when they are treated unfairly as compared to a social partner. In our studies, we seat subjects from the same social group next to one another and have them complete a simple task, such as trading a token, to earn a reward. Sometimes we “pay” them the same, and sometimes we give one a more preferred reward than the other. What we find is that subjects are perfectly happy to work for the less preferred food as long as their partner also gets it, but when their partner gets the better one, they are more likely to refuse to participate, either refusing to do the work or completing the task and then refusing the reward.

Fascinatingly, it gets even more interesting: we only see this response in species that routinely cooperate with one another, which includes humans. Indeed, economists had previously hypothesized that humans’ tendency to get upset when they were treated unfairly could be because they use unequal outcomes as a proxy to determine the value of a cooperative partner; if I am constantly getting less than you, it’s time for me to go find someone new to work with! The fact that only species that routinely cooperate seemed to care what their partners received, relative to them, supported this conclusion.

Moreover, individuals are also sensitive to the equity of a specific interaction. Indeed, it turned out that among capuchin monkeys, it wasn’t unequal rewards that were the problem per se, but the behavior of the partner. If, in a cooperative task with unequal outcomes, subjects took turns receiving the less good reward, cooperation continued, but if one partner always claimed the better reward, the less benefited partner quit participating. Thus, it seems like species that cooperate pay attention to outcomes, but that individuals are also sensitive to their partner’s behavior and, possibly, intentions (we can’t easily measure intentions in nonverbal species, so that must be inferred).

A sense of fairness?

Of course, what I have just discussed pertains to how a subject responds when they get less than a partner, but a true sense of fairness requires that subjects also notice when they get more than a partner. In most of our studies, we see no evidence that this is the case; subjects refuse to participate when they get less than a partner, but not when they get more. However, chimpanzees, one of the two species of ape that are the most closely phylogenetically related to humans (the other is the bonobo), appear to notice when they are over-benefitted as well. Chimpanzees are more likely to refuse a preferred reward if their partner got a less-preferred one than in a control context in which they both get the same (preferred) reward. However, they don’t seem to rectify inequity; we virtually never see sharing of the better reward, despite the fact that chimpanzees share food in other contexts. Notably, there is an imbalance between their reaction to being over-benefitted and under-benefitted; as is the case with humans, chimpanzees do not respond as strongly to being over-benefitted as they do to receiving less than a partner.

So other species also care when they are under-benefitted, and at least chimpanzees care when they are over-benefitted. What does this mean for human uniqueness? My colleague Frans de Waal and I have proposed an evolutionary trajectory for the sense of fairness based on these data that sheds light on how we differ from other species. Specifically, we propose that the response to being under-benefitted is likely widespread among species that routinely cooperate. Becoming upset when one routinely benefits less than a partner in a cooperative task is a cognitively simple mechanism (a socially-mediated contrast effect)—one that would benefit individuals of these species directly by helping them avoid partners who take advantage of them, while also encouraging them to seek out partners who give them their relative “fair share.”

Secondarily, we propose that some cognitively advanced species that routinely cooperate, including chimpanzees, evolved a tendency to notice and respond when they were over-benefitted.  That is, even though there is a short-term cost, there is a long-term benefit to responding because it may help the over-benefitted individual maintain what are presumably beneficial social relationships and cooperative partnerships. Humans, with our atypically large brains, have well-developed abilities in planning, delay of gratification, and perspective taking that allow us to do this on a scale and in a time frame that is unprecedented among animals.

A moral system

This, then, is where I see evidence of meaningful human uniqueness: our sense of fairness did not emerge from a vacuum, and we can see precursors throughout the animal kingdom, but in us it has developed to a greater degree. Other species respond to unequal outcomes, and in a few cases may even notice when they get more than a social partner, but only humans have developed a full-blown sense of fairness. Moreover, there is a snowball effect, as this greater development has subsequently allowed for the development of other traits that are not seen in other species. Most obviously, we have developed a moral system that is largely based on reciprocity and our sense of fairness—do unto others as you would have them do unto you. This moral system is one of a suite of factors that has allowed for cooperation at a scale that far exceeds what is seen in other species, who do not have population centers at the scale of humans or economic systems that span the globe.

An obvious counterpoint is that we humans show features that are so fundamentally different from anything that is seen in other species—such as cities or our international economy—that they should be considered as without precedent. This, however, hides the underlying framework that allowed humans to develop to this degree, and actively impedes our ability to study human behavior by explicitly removing the evolutionary framework. In particular, if we lose an evolutionary perspective, we lose our understanding of what ecological features may have selected for a given behavior, which helps us understand in what contexts and in what ways a behavior may manifest. This is useful for both prediction and intervention.  Moreover, other species are important to study in their own right. The more we learn about other animals and the amazing diversity of behaviors they show, the more we appreciate species aside from our own.

So back to the original question, and my assertion that there is no simple answer. Humans are part of the animal kingdom and are adapted to our niche just like any other species. Our niche has selected for impressive cognitive abilities in concert with a complex social structure, among a suite of other behaviors and traits—a determination of which ones are the most important probably depends most heavily on the interests of the scientist you are talking to. This leads to some obvious differences between us and other species, but there do not appear to be traits that are completely unprecedented in the animal kingdom. It’s just that in humans, these traits exist in a particular combination and to a particular degree that you don’t see in other species—just like in every other species.

The interesting question, then, is not what is different or unique, but how did we get where we are and what were the environmental pressures presented when these traits were selected? A better understanding of these questions are far more important to understanding ourselves than finding a simple answer to what makes us unique.

For further reading, Brosnan recommends:

Evolution of responses to (un)fairness, by Sarah Brosnan and Frans de Waal, Science, 2014.
Behave: The Biology of Humans at Our Best and Worst, by Robert Sapolsky, Penguin Press, 2017.
Are We Smart Enough to Know How Smart Animals Are?, by Frans de Waal, W. W. Norton & Company, 2017.

Sarah F. Brosnan is Professor of Psychology, Philosophy & Neuroscience at Georgia State University.