Anxious? Blame the Predators in Your Primate Family History.

What makes humans human.
Oct. 15 2012 1:54 PM

What Are You So Scared of? Saber-Toothed Cats, Snakes, and Carnivorous Kangaroos.

The evolutionary legacy of having been prey.

The world’s remaining tigers are clustered in just a few dozen small areas that could serve as source populations for a recovery, but only if resources are in place to keep poachers out.
People. We're what's for dinner. Or rather, we used to be.

Julie Larsen Maher/Wildlife Conservation Society/Oak Ridge Nat. Laboratory.

In the developed world, we live in the most peaceful, healthful time in history. The murder and violent crime rate is dropping; we are vaccinated against the most deadly diseases of previous generations; our houses protect us from most storms; relatively few people go hungry. The average lifespan is longer than it has ever been. Then why do we walk around so anxious, so full of fear? The answer is not terrorists, TV, Republicans, or Democrats. The answer is our legacy of ancient fears, the result of having spent millions of years running from predators. Our fear response is more influenced by the ancient species we struggled to escape than any modern challenges. We live in a demon-haunted world.

Until not that many generations ago, Homo sapiens and our primate ancestors found shelter under lean-tos, in caves, and up among branches. Exposed and relatively defenseless, our predecessors stood a good chance of being eaten by bigger, badder, species. For most of our evolutionary history as primates, we were far more likely to play the role of Big Mac than Big Man on Campus. Our ancestors evolved many traits to help them escape that fate—if not forever, at least long enough to reproduce and pass their genes along. These responses still frame how our bodies work today, which would be great if we were still being stalked by large cats. But Vegas performers notwithstanding, most of us are not.

Nor was it just cats. Humans were eaten by giant hyenas, cave bears, cave lions, eagles, snakes, other primates, wolves, saber-toothed cats, false saber-toothed cats, and maybe even—bless their hearts—giant, predatory kangaroos. Amazingly, these are just the predators that consumed our ancestors during relatively recent history, the past 100,000 years or so. Go further back in time, and the diversity of things that ate our kin goes up (particularly given that our earlier, pre-hominin ancestors were progressively smaller).  Some predators, such as leopards, ate many of our ancestors. Others, like crocodiles, komodo dragons, or sharks, took their bites, but more opportunistically, savoring the occasional human or proto-human the way one might enjoy some special holiday treat. We were, in other words, their thanksgiving turkey.

In those few places where large predators are still common, primates, especially cute baby ones, are eaten with great frequency and alacrity. When our species evolved, human children were special only in as much as their hairlessness made them slightly easier to digest. Even today, where humans live alongside predators, both children and adults get eaten. Harry Greene, a herpetologist at Cornell University and one of a handful of my colleagues more likely to be eaten by a wild animal than to die of old age, and Thomas Headland, an anthropologist, recently conducted a study of Agta hunter-gatherers in the Philippines. Harry was excited to find that the Agta lived among a high density of pythons. The Agta tend to be not quite so excited; Greene and Headland found that one in four Agta men had been attacked by a reticulated python. Of the 120 men whose stories were considered for the study, six had been killed by a python. That’s a death-by-python rate of 1 in 20. Those are lousy odds, but most of us have escaped such risks by living in houses and cities and living where our ancestors killed off the most dangerous predators, be they tigers, cave bears, or giant, carnivorous kangaroos. We should be grateful for having escaped—and yet we haven’t really escaped, because our bodies are burdened by our long history of trying to get away.

When our hamburger-size ancestors lived in trees, it was extraordinarily valuable to be able to respond immediately to the potential presence of a predator. Many primate species have alarm calls that are specific for different predators. The first primate nouns were almost certainly those embedded in calls that meant, “Oh shit, big cat!” “Oh shit, giant eagle!” or “For the love of god, did you see the size of that snake?”  In this way, predators may have had a positive impact on who we are now, having given us the precursors of language, or at the very least, cussing.

In addition to inventing words for these predators, we also responded in other ways. When we saw or heard a sign of danger—a movement in the grass, a strange shadow—hormonal reactions screamed out inside our bodies. These fight-or-flight responses sped up the heart, increased blood flow to muscles, caused hyperventilation (to get more oxygen for quick reaction), and made us more likely to respond quickly to a predator by searching for it, hiding, running away, or for the truly brave, throwing a stick and then running away.

These fight-or-flight signals and associated jumpiness and anxiety are part of the problem in modern urban life, part of our discontent. They are triggered by all sorts of ordinary activities. Thinking of taxes causes our hearts to beat fast. So does being late for a meeting, forgetting your homework, or contemplating how to pay for a home improvement. In none of these cases does our fight-or-flight response serve a purpose. It gets us agitated. It makes our heart beat faster. It prepares us to run, but to where? To what end?

Right now there are anxious people all around you, ready to run from predators that aren’t there. Our misplaced anxiety can seem silly (the other day I just about melted down while trying to find my keys so that I could drive to the gym, where I was planning to run in place). But it is still a serious problem that can cost money and can cost lives. We have resolved this plague in part by medicating. Xanax, Valium. and other drugs help. “There are no more leopards,” the little pills say, and we rest more easily.

Anxiety is just one lingering result of predators’ influence. Another is, well, tingly. When you are spooked by a sound or a testy boss, the hair on your arms stands up. Along with the upright hair come chills. But why does this happen? Once upon a time, when we still had fur, when our hair stood on end it made us look bigger, less like a Big Mac and more like, well, something slightly larger than a Big Mac.  But now that we have lost most of our hair, the chills that run through our body and the contraction of the tiny muscles in our skin that pull our hair erect simply make us look ridiculous, more cowardly than bold.

Many traits that influenced our ability to spot predators or flee from them have been under strong natural selection for much of the past 40 million years of primate evolution and even before then. (We have been prey essentially since the beginning.) Researchers are just beginning to explore these possibilities. Lynne Isbell at the University of California-Davis has argued that the range of our color vision evolved in part because those of our ancestors who could see more colors were more likely to spot snakes. A study this year found that children spot snakes more quickly than they do flowers . They also spot snakes when using color vision more quickly than in gray scale. Our interactions with other species (be they snakes, or as some have argued, fruits) shaped our eyesight. Our screams, those preverbal (and universal) utterances, are alarm calls signaling, simultaneously, both a threat and the need for help.

But it wasn’t just predators that influenced our evolution. Parasites and pathogens have also shaped our bodies in ways that are affecting you now. With predators, most of our adaptations relate to avoiding encounters in the first place, for the simple reason that by the time we encounter a predator, it tends to be too late. But once we are attacked by parasites, we often still stand a chance. Among the deadliest parasites in human history are those carried body-to-body by mosquitoes, such as the malaria parasite. Those ancestors who lived in areas where malaria was and is most problematic evolved responses to escape Plasmodium’s deadly wrath. One adaptation puts people at an increased risk of sickle-cell anemia, and another raises the risk of favism, a condition in which consuming fava beans causes anemia. Even the relative prevalence of different blood types has been argued to be a consequence of Plasmodium’s influence; some blood types appear to be more resistant to malaria than others.

Lice and other parasites and the diseases they carry may have played a role in our loss of hair; parasites now have fewer places to hide. Parasites might have played a role in our original sociality, too, having brought us together to pick lice off one another’s backs (and feel the endorphin release and social appeasement that rewards such behavior). Then there are the parasitic worms whose presence may have shaped our immune systems to such an extent that some of us miss their absence; autoimmune disorders—including Crohn’s disease and asthma—have been linked to now-obsolete adaptations to keep these worms in check.

How our bodies work (or fail to work) in modern environments relates not to the species we confront now, but the collective effect of the species we confronted over millions of years. We are left with the bodies that were best able to survive despite the daily threat of being eaten by a predator, sickened by a parasite or pathogen, or otherwise assaulted by Mother Nature’s well-armed hordes. Of course, we are still evolving. In every generation, some genes are favored relative to others, and yet the rate of our evolution is slow relative to how much we have changed. And so we go on getting anxious when our football team loses and letting our hair stand on end, ridiculously, when we are scared. We could bemoan these legacies, but it makes more sense to confront them head on, to consider just how we should live not in light of the bodies we wish we had but instead with the ones we are born with, bodies that evolved in the wild, thanks to ancestors who only just barely got away.

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