Tucked deep in the brain, toward the back, there’s a tiny ridge. It’s easy to miss, frankly — a “physically unimpressive” structure, in the words of one contemporary neuroscientist. But for a few years in the middle of the 19th century, this structure — known as the hippocampus minor — played a major role in debates about what makes us special .
According to one of Britain’s leading anatomists at the time, Richard Owen, only humans possessed a hippocampus minor. Gorillas and other apes simply lacked one. The idea was certainly appealing: if Owen was to be believed, here was concrete evidence — evidence in the actual flesh — for human uniqueness. Owen first made these claims prominently in an 1858 paper. And, when ideas about natural selection began making waves in the years that followed, he doubled down.
Things got a bit heated, especially between Owen and Thomas Huxley, a ferocious defender of evolutionary theory nicknamed “Darwin’s bulldog.” Huxley seems to have made it his personal mission to refute Owen’s observation about the hippocampus minor. He started by challenging Owen at public meetings, and soon took the fight to the ultimate scientific arena: the pages of a leading academic journal. All apes have a hippocampus minor, Huxley declared, not just humans. In support of this rebuttal, he marshalled a range of data — some from other experts, some from his own dissections — and presented anatomical drawings showing the relevant folds and fissures with new clarity. With this detailed documentation now available, the scientific community started to line up with Huxley. The matter was finally settled when Charles Lyell, one of the most venerated thinkers of the day, effectively declared Huxley the winner in 1863. Whatever it was that made us unique, it was not some unassuming ridge toward the back of our brains.
The “Great Hippocampus” debate, as this scuffle became known, was hardly the first time thinkers had made pronouncements about human uniqueness. Among Greek philosophers, the “man alone among animals” idea was a favorite trope . Aristotle noted that only humans can count and that we’re the only species to distinguish good from bad. Galen trumpeted our unusual upright posture and uniquely deft hands. Other classical authors ventured that only humans have religion and exercise free will; that we’re the only creatures that laugh or blush or feel shame; that we, uniquely, can draw analogies and think through logical consequences. Some of their proposals were bizarrely specific. We have the most peculiar hair, observed one thinker. We’re the only ones that can sit with ease, noted another.
It was a time-honored tradition then, that Richard Owen carried forward — a long-standing genre of speculative self-congratulation. And it’s also a tradition that, since Owen, has only gathered steam. Over the next 150 years, dozens and dozens of uniqueness claims would be proposed, so many it’s difficult to keep track. Only humans make art, appreciate beauty, and control fire. Only humans wear clothing, carry out rituals, or engage in warfare. Only humans understand the nature of time. Only humans cry emotional tears. Only humans understand death.
Clearly, we need a term for these putative hallmarks of humanness. A few possibilities have been floated but none, as far as I know, have really caught on. One author calls them “human differentiae”; another, borrowing a term from taxonomy, offers “human autapomorphies.” We could of course opt for something plain, perhaps just “human peculiarities.” But I think we need something different, less serious — something with a touch of whimsy, like the tradition itself. I suggest “human uniquals.” Much as “human universals” are characteristics that are common to all humans, across all cultures, “human uniquals” are characteristics that are unique to us. Or, rather, characteristics proposed to be unique to us. After all, uniquals, like universals, are notoriously difficult to prove.
Uniquals come in different forms. There are anatomical ones and behavioral ones and cognitive ones. Very often uniquals concern, not whether a trait is unique to the entire animal kingdom, but whether it is unique to the primate lineage. In this vein, it’s been claimed that we’re the only primate without body hair, the so-called “naked ape”; we’re the only ones with cream-white sclera and protuberant chins; the ones with strange sleeping habits. A website devoted to the study of human origins catalogues such “man alone among primates” features quite voluminously. It includes accurate overhand throwing, acne, assisted childbirth, Alzheimer’s, aquatic food use — and that’s just a sampling of ones that start with ‘A’.
Not all uniquals are absolute, of course. There are uniquals of degree, and these get appropriately qualified. We’re the only primate that habitually walks upright. The only animal that systematically gets high . We’re clearly not the only species that communicates, but we may be the only one that communicates ostensively. Our thumbs may not be the only opposable thumbs in the animal kingdom, but they are, can’t we all agree, quite opposable indeed.
Uniqueness claims get qualified in other ways, too. Often, it’s admitted, we may not be the only species to exhibit this or that notable feature, but we’re one of very few. Not many animals, for example, live past menopause . Only a handful of species have Von Economo neurons. Deception is probably very rare in the natural world, and there really aren’t so many vocal learners around, at least not among mammals. We might think of these as “almost but not quite” uniquals, or maybe just quasi-uniquals.
Zooming out to consider this rich genre, there are a few questions we might want to ask. A first, our focus so far, is simply a question about the history of ideas: Which characteristics and behaviors, however fanciful or far-fetched, have been proposed to be unique to humans? A second question, of course, is: Which of these proposals are actually right?
But, arguably, it’s more fun to start with proposals that are clearly wrong. And there’s no shortage of these. It was once thought, for instance, that only humans exhibit “categorical perception,” that is, the tendency to parse speech-like sounds — ‘pa,’ ‘ba,’ that sort of thing — as fitting into neat categories. This would make good sense, given that only humans evolved to interpret human language. But then the same perceptual tendency was found, not in a close cousin, but in chinchillas. Similarly, the descended larynx was considered a human-specific adaptation for producing speech — that is, until it was also found in red deer.
The reality though is that uniquals can be tough to either prove or disprove. The trickiest cases are those that involve inherently squishy constructs. Saying whether another species has a hippocampus minor or a descended larynx is one thing; saying whether that species has language, culture, or imitation is another. The definitions of these terms, after all, are subject to change — and change them we do. As new data comes in — from more species, using better methods — the boundaries of these lofty notions get redrawn, their essence gets further distilled. It’s a dynamic that Louis Leakey famously foresaw. In 1960, after Jane Goodall reported that chimps at Gombe were fishing for termites with blades of grass, he wrote in a telegram: “Now we must redefine tool, redefine Man, or accept chimpanzees as human.”
Probably most contentious of all are proposed cognitive uniquals — claims about how our invisible mental processes set us apart. In 2021, Kevin Laland and Amanda Seed surveyed this terrain in the Annual Review of Psychology. They noted that current debates about human cognitive uniqueness tend to cluster around a handful of topics: our ability to engage in mental time travel; to use tools; to solve problems; to engage in complex social cognition; and to flexibly communicate. Within each of these five areas, more specific proposals have been bandied about. For example, that humans alone can process complex hierarchical sequences. Or that only humans are driven to cooperate. Or that our species evolved a singular ability to teach and learn.
Taking the debate as a whole, Laland and Seed make a couple key points. For one, they argue, there does not seem to be any one human ability that underpins all our cognitive sophistication — no uniquely potent uniqual. Rather, we are, in their terms, “cognitive all-rounders,” pretty solid in a number of respects. Crucially, this well-roundedness allows for compounding effects: our abilities in one area may amplify our abilities in others. Our capacity for language, for instance, is one such amplifier. Our capacity for joint attention may be another.
There’s at least one more question we might want to ask about human uniquals: Why are we so obsessed with them? What is it that makes us so motivated to stand here, on this self-made pedestal, listing off the many ways in which find ourselves impressive? One cynical response would be to say that we’re simply the most narcissistic and self-involved of all creatures. That, as a species, we seem to be stuck in our sophomore year, still plumbing the depths of our own bellybutton. Another cynical response would be to note how much of the world order hinges on our special status. Whatever threatens that specialness threatens that order, and so we’re motivated to police our uniqueness at all costs .
To call these responses cynical is not to say their misguided — they’re almost certainly part of the answer. And yet I still take a rosier view of all our navel-gazing. Sure it’s been self-congratulatory and self-serving and occasionally goofy, but the quest to pinpoint our uniqueness has had at least one positive effect: it’s been a major force for scientific discovery. It’s driven us to look in unlikely places, to study phenomena we might have passed over. It’s led us to play nonsense syllables to chinchillas, and to stare down the throats of red deer. The quest has powered more insights than we can count — new understandings of diverse behaviors and capacities, sharper definitions of fuzzy constructs, whole new taxonomies of mind and cognition.
Also, let’s just be realistic: whatever we think of the quest for human uniqueness, we’re not giving it up any time soon. In just the past two years, one study reported that human neurons seem to have far fewer ion channels than those of other mammals. Another found that bonobos don’t show the classic “bouba-kiki” sound symbolism effect, where we associate the word “bouba” with a round shape and “kiki” with a spiky one. Yet another study, published in prestigious pages of PNAS, found that people — across cultures and beginning at a young age — are highly attuned to abstract geometry. Baboons, not so much. The paper’s subtitle trumpeted its core finding as “a putative signature of human singularity.”
For better or worse, these won’t be the last uniquals to be proposed and debated. Some will be disproven, others will pop up in their place. As a species, we’ll keep searching — probing the folds of our brain, sifting through our tendencies and capacities — for those subtle, peculiar features that, if properly defined and appropriately qualified, mark us as special, impressive, distinctive. For some reason we can’t seem to help ourselves in this pursuit. Maybe it’s only human.
– Kensy Cooperrider
1. The “great hippocampus” debate has been discussed by the neuroscientist Charles Gross here and here. Note that the so-called “hippocampus minor” is now commonly known as the calcar avis and the structure formerly known was the “hippocampus major” is now just the plain old hippocampus.
4. On the rarity of females living past menopause, see our episode with Alison Gopnik.
5. On our motivation to police our own uniqueness, see this 1990 essay by the paleoanthropologist Matt Cartmill.