Threescore and Ten

Natural History, Dec, 2000 by Jared Diamond

Our exceptionally long life span may have influenced the evolution of how we learn and think.

In the course of my nineteen ornithological expeditions to the island of New Guinea, I've been in many unfamiliar situations that initially seemed risky to me. But because my New Guinean field associates are intimately acquainted with the jungle, they're usually unfazed by things that strike me as dangerous. Hence I was very surprised, one night in 1966, to see their terror when I picked a campsite next to a big tree in the Bewani Mountains. My associates feared that the tree might fall on us as we slept. During all my years in the field, though I have heard trees falling far away, I have never been close enough to witness such an event. For a New Guinean, however, falling trees rank as a leading hazard of life in the jungle: while you may see a tree fall only a couple of times a decade, if you're hoping to last seventy years but are not careful, you may well end up crushed under a falling trunk or branch before having lived out your allotted time.

Of all the lessons I've learned in New Guinea, that one about the cumulative odds of rare but recurring dangers is the one that has most affected my outlook. Out there, the penalty for forgetting is severe--you can't call 911 in a life-threatening emergency. But disasters that have recently befallen friends of mine remind me of this lesson's importance for every American who drives a car, climbs a ladder to change a light bulb, or hesitates to call the doctor when feeling faint. Our potential for learning to recognize the riskiness of such mundane acts derives from a trait distinguishing us humans from other animals: our unusually large capacity to modify our behavior in response to acquired information rather than relying solely on instinct. Contrary to assumptions cherished by modern literate societies, I suspect that we still learn best in the way we did during most of our evolutionary history--not by reading but through direct experience. Some limitations on our thinking skills, I believe, stem from our evolutionary history. And although these limitations are not insuperable, we do need to be more aware of them and work harder to overcome them.

Our exceptionally long life span makes us humans unusual among animal species. The Bible tells us that "the days of our years are threescore and ten" or, "by reason of strength," sometimes fourscore (Psalms 90:10). Nothing about the biology of human aging has changed since biblical times. Even in New Guinean villages, I usually find some people who have reached that biblical age limit. Half of us living in modern Western societies reach it, and a handful of us reach age 100 (the current record is 121). But not even the best medical care, the healthiest lifestyle, or the wealth of Howard Hughes will let you survive longer. Still, by animal standards that's impressive. Few other warm-blooded vertebrates (the category that includes all mammals and birds), with the notable exception of killer whales, elephants, and albatrosses, are known to rival us in length of life.

Our long lives and our capacity for learning have combined, in traditional nonliterate human societies, to produce many cultural practices that help their members survive rare dangers. A Western visitor who spends only a few years in such a society is very unlikely to appreciate such dangers and hence to understand why some seemingly senseless practices really make sense for a long-lived animal like us. For instance, New Guinean farmers don't build villages or plant crops above an altitude of about 8,000 feet, even though sweet potatoes grow perfectly well up there in most years. A lifetime of hard experience has taught New Guineans that planting at 9,000 feet involves a serious risk of starvation, because about once per decade a frost at that elevation will destroy the crop. Another example is the land tenure system of medieval peasants, with its numerous small strips of land scattered in different directions from the village. Historians thought the system utterly nonsensical--until ecologists pointed out that climatic ups and downs can affect fields at different locations differently and that scattering the strips provided excellent insurance against the risk that, once in a decade, all of one's strips might fail simultaneously.

We take for granted our human outlook, geared to a long life span. Most shorter-lived animals have had to evolve different "outlooks." Unlike us, such animals don't have memories that warn them against settling in habitats where climate fluctuations may doom them to die within a few years. Instead, they settle there, breed in great numbers, and end up suffering a population crash when drought or frost strikes. The Carolina wren of the southeastern United States, for example, extends its breeding range northward in years of mild winters, until a harsh winter wipes out all the wrens for hundreds of miles at the northern edge of the range.

We moderns also take for granted the availability of written information, which extends our experience far beyond a single human life span. But that's a relatively new source of information for us. From the origins of the human lineage about 5 million years ago until about 3400 B.C., nobody wrote anything. And mass literacy arose only within the past thousand years. Hence, for almost the whole of our evolutionary history, our repository of knowledge lay in the memories of people, not in books. Therein lies a sad but potent reason for the status of old people being so much lower in many modern, literate societies than in traditional, nonliterate societies: many individuals in literate societies no longer believe that their elders are important repositories of knowledge and thus do not perceive them as valuable. In addition, technology is now changing so quickly that certain kinds of experience are soon outdated, as is made clear to me whenever my eighteen-year-old students watch my sixty-three-year-old eyes glaze over at their efforts to teach me how to use computers.