Editor’s note: Thomas Suddendorf is a professor of psychology at the University of Queensland and a fellow of the Association for Psychological Science. He is the author of “The Gap: The Science of What Separates Us From Other Animals.”
We humans tend to think of ourselves as better than, or at least separate from, all other species on this planet. But every species is unique, and in that sense humans are no different.
Nevertheless, it seems obvious that there is something extra special about us—after all, we are the species running the zoos. In “The Gap,” I survey what we currently do and do not know about what exactly sets humans apart.
What are the physical differences that distinguish us from our closest animal relatives?
There are some notable ways in which our bodies differ from those of apes and old-world monkeys. We can lock our knees straight, have longer legs than arms, and habitually walk upright, freeing our hands to do things other than carry our weight.
We have a chin. Our body surface is covered in sweat glands that provide a more effective cooling system than those of other primates. We have lost our canines and much of our protective fur — leaving males with the apparently pointless, but persistent, growth of beards.
The iris of our eyes is relatively small and surrounded by white rather than dark sclera, making it easy for us to identify the direction of another’s gaze. Human females show no outward markers of their fertile phase, and human males lack a penis bone.
These are not exactly groundbreaking traits, compared to, say, the emergence of wings in birds, which catapulted their bearers into a new sphere of possibility. Yet despite the paltry list of distinct physical attributes, we have managed to seize control of much of the planet. It is widely assumed that the reason for this has to do with our big brains.
Do humans have the largest brains?
No. Humans have large brains, weighing between about 1.25 and 1.45 kilograms, but elephant brains can weigh more than 4 kilograms, and whale brains as much as 9 kilograms. Elephants and whales also have the biggest bodies in general, so it may be considered fairer to compare our relative brain sizes.
Our brains represent about 2% of our overall body mass (though they consume some 25% of our energy), whereas the brains of elephants and whales constitute less than 1% of their bodies. Nevertheless, the outcomes of relative size comparisons have also not been supportive of humans’ sense of superiority. Some shrews and mice, it turns out, have brains that are up to five times larger than ours relative to body size.
Since we get beaten by large mammals in terms of absolute brain size and small mammals in terms of relative brain size, another type of comparison has been proposed that takes into account that as mammals get larger, brains get absolutely larger but relatively smaller.
In this scheme, at last, humans emerge on top, with a brain more than seven times larger than that predicted for the average mammal of our size (and dolphins are runners up with five times the expected value). Yet it remains uncertain whether this or any other brain comparisons have so far unearthed some hidden truth, or whether we are primarily using statistics to support our presumptions. We are only beginning to unravel the mysteries of brains and how they generate our peculiar minds.