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.

(Continued from Page 1)

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.

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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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