02/20/2013 02:34 pm ET Updated Apr 21, 2013

Humans Are Unique Animals Because Evolution Gave Us Angst

Philosophers and scientists have long debated whether there is something unique about Homo sapiens that sets us apart from all of the other animals. It's not a question of whether we should feel like special humans - it's a question of whether in some way we are actually special animals.

The list of proposed and discarded candidates for specialness is long and varied. It turns out that we are not the only species that can make and use tools (just lately a Cockatoo joined us in that distinction). Nor are we the only animal with consciousness of our thoughts and surroundings, with language, with the ability to recognize ourselves in a mirror, or even with awareness that our companions also have thinking brains (that one would be Theory of Mind in tech speak). We are almost alone in our ability to look where someone is pointing, except that Spot and Rover can do that too. While we may be pretty smart, dolphins do come close, and even our opposable thumbs are matched by some pandas and frogs. So while we are still the only species that has domesticated fire - that really doesn't seem special enough to make us unique.

So maybe we rise above the animals by, well, being less like animals. All animals have innate biological instincts, hard won through evolutionary trial and error. All of us animals instinctively seek food, water, and air when we need it to survive, we fight to defend family and resources, and we seek sex to keep each species going (not that we mind much). The list of animal instincts is long and varied, but not always animalistic. Indeed, there is one particular set of instincts that encourages sociable herd-like behavior. Sheep, cows, chimps and others don't stick together or act social by sheer reason and choice - their biology tells them what they need to do to ensure social harmony and herd survival, even if herd instincts may put some individuals at a disadvantage.

Five are especially worth noting:
1. Don't stray from the herd: lest you get lost and die, and deprive the herd of your contribution
2. Stay in your place in the herd hierarchy: lest there be too much hostile competition within the herd
3. Take good care of the collective nest: clean to prevent infections, save resources for rainy days, control aggression, keep things tidy for safety and security
4. Don't offend others: lest you be shunned or evicted from the herd
5. Sense when you are a drain on herd resources: it may be time for you to shuffle off this mortal coil

We humans have those very same herd-like instincts, and they affect our behaviors as well. But we are the only animals with consciousness and reason enough that we can routinely ignore or over-ride those instincts. We can use our personal wishes and choices (often including the behavioral directives of our cultures) to follow through on our own personal decisions, even in the face of instincts that complain. This, indeed, is a skill which we alone fully possess.

For example, many of us try to overrule the instinct that tells us where we should be in the social hierarchy of a herd. A biologically low-ranking sheep doesn't decide to become the alpha sheep leader. But humans can, and that instinct's complaint comes in the voice of social anxiety disorder. Fear of self-embarrassment, sweaty palms, racing heart - and blushing. So, just as Mark Twain said, "Man is the only animal that blushes. Or needs to." Just this year, a new study reported that social hierarchy concerns are located in a particular part of the brain. And that's just one of these five instincts.

Our ability to defy instinct has led to much, much greater things: we developed civilizations and technologies beyond the wildest dreams of other species, or even of our own ancient ancestors. There is still that downside, though. When we fool with Mother Nature's instincts, they painfully try to coax us back to those herd roles.

The resulting Angst has five flavors. Each one comes from a specific herd instinct, and matches a particular kind of anxiety or depression. Those five instincts listed above evolved into panic anxiety, social anxiety, OCD, atypical depression and melancholic depression. These "instinctive syndromes" are a price we have paid for rational living, cultural advance and civilization.

And there is more. Finding ourselves with brain structure and wiring for herd roles, we have also repurposed some of these instincts to other advantages. Just think of all those very shy people who cope by becoming our best performers, but who suffer still. Indeed, humans even have their own genetic variations that actually increase our risk for some of the suffering. Since some of those very shy people may be built to have less serotonin activity than the rest of us, Hollywood may owe a debt to evolution. So, there is more than enough about these instinctive syndromes of anxiety and depression to fill a book!

In the past, the remedies for Angst were in societal rules, cultural reassurances and the comfort of beer. These days, we can do more. Psychotherapy and medication offer us the option of gaining humanity's benefits with a bit less pain. Even so, in the words of William Faulkner: "The past is never dead. It's not even past." The instincts from our past will forever try to coax us back to that ancient herd. And our emotional struggle to rise above them is what makes us humans human.

• Jeffrey P. Kahn, M.D. is author of "Angst: Origins of Anxiety and Depression" (Oxford University Press, 2013), and Clinical Associate Professor of Psychiatry at New York Presbyterian Hospital/Weill Cornell Medical Center, in New York City and Westchester County. "Angst" has its very own public Facebook page.