Biologically speaking, humans are pretty much just another animal, and it's actually hard to come up with any clear explanation for what sets us apart. But we have a hard time accepting this ... and the reason we're in denial about our animal status may be hardwired into our brains.
It seems as though we humans really should be exceptional somehow, but so many of the things that make us unique — complex societies, tool use, even language and self-awareness — can be found to one degree or another in other species as well. But it's hard to shake the notion that we're special somehow, and that's not exactly surprising. Partially that's because so much of the history of human philosophy has been spent conceptualizing ourselves as fundamentally different from all other life — for a quick example of that, check out the Great Chain of Being.
The roots of human exceptionalism might go even deeper than that, right down to some of the deepest functions of our brains. That's the finding of researchers from Caltech, who asked 41 epilepsy patients to look at 100 different images of animals, people, objects, and landmarks. Because the patients were about to undergo surgery, they'd had electrodes implanted in their brains, allowing the researchers to monitor the neural responses to the different images precisely.
The researchers studied the activity of nearly 1,500 different neurons, and one area of the brain in particular stuck out: the right amygdala. Some of the neurons in that section of the amygdala — the part of the brain that process emotional reaction — responded specifically to pictures of animals, and nothing else. Crucially, these neurons did not fire when photos of humans were shown, regardless of whether the photo was of a famous celebrity or a random stranger. It also didn't matter from what angle or distance the photos were taken.
Even more intriguingly, the neurons showed no preference for which animals were displayed. They didn't fire more strongly in response to, say, a potential predator or a cute lolcat. That suggests this neural response isn't a specific evolutionary response to possible threats - it's more basic than that. If anything, it suggests that we're hard-wired to respond to the presence of animals, any animals...as long as they're not humans.
So then, if you're still looking for a way to distinguish humans from all other animals, we now have a straightforward definition: humans are the only species that doesn't register a response in certain neurons of the right amygdala of the human brain. It doesn't have quite the same ring to it as "we're the only truly intelligent species" or something like that, but it does have the added value of actually meaning something concrete.
Image of Suzanne D. via the utterly recognizable CuteOverload.