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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. 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. 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. 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. 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). 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. 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. 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. Those ancestors who lived in areas where malaria was and is most problematic evolved responses to escape Plasmodium’s deadly wrath. Among the deadliest parasites in human history are those carried body-to-body by mosquitoes, such as the malaria parasite. But once we are attacked by parasites, we often still stand a chance. 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. Parasites and pathogens have also shaped our bodies in ways that are affecting you now. 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.īut it wasn’t just predators that influenced our evolution. When we saw or heard a sign of danger-a movement in the grass, a strange shadow-hormonal reactions screamed out inside our bodies. In addition to inventing words for these predators, we also responded in other ways. 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. Many primate species have alarm calls that are specific for different predators. 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. 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.
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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. That’s a death-by-python rate of 1 in 20. Of the 120 men whose stories were considered for the study, six had been killed by a python. 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. Harry was excited to find that the Agta lived among a high density of pythons. 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. Even today, where humans live alongside predators, both children and adults get eaten. When our species evolved, human children were special only in as much as their hairlessness made them slightly easier to digest. In those few places where large predators are still common, primates, especially cute baby ones, are eaten with great frequency and alacrity.