Regular replacement also happens inside us. We constantly replace many of our cells: once every few days for the cells lining our intestines, for example, and once every four months for our red blood cells. To keep damaged molecules from building up in our bodies, our protein molecules are replaced, too. You may look the same in the mirror today as you did in a photo taken a month ago, but many ofthe individual molecules forming your body are different.
Much of an animal’s body can be repaired if necessary, or is regularly replaced. The details of how much is repairable or replaceable vary from species to species, but there is nothing inevitable about the human limits. Since starfish can regrow amputated limbs, why can’t we? To protect ourselves against arthritis, all we’d need is to regrow our joints periodically, like crabs. You might suppose that natural selection would favor the man or woman who didn’t die at eighty but lived and produced babies until at least two hundred. So why can’t we naturally repair or replace everything in our bodies?
The answer must have to do with the cost of repair. Think again about car repairs. Suppose you buy an expensive car that you expect to last for a long time, such as a Mercedes-Benz. It makes sense to invest in regular maintenance, which is cheaper than discarding your Mercedes and buying a new one every few years. But if you live in Port Moresby, New Guinea, the automobile accident capital of the world, any car is likely to be totaled within a year no matter how many oil changes and air filters you pay for. Many car owners there don’t bother with maintenance—they use the money they save on maintenance to help pay for their next car.
In the same way, how much energy an animal “should” invest in biological repairs depends on the cost of the repairs, and how long the animal can expect to live with and without them. These considerations take us into the realm of evolutionary biology. Natural selection works to increase an organism’s rate of leaving offspring that, in turn, survive to leave offspring of their own. Think of evolution as a strategy game. In evolutionary terms, the player whose strategy leaves the most descendants wins. This view helps us understand a number of biological problems, including life span.
The Problem of Life Span
If long life is good because it lets organisms leave more offspring, why don’t plants and animals— and people—live longer? If speed and intelligence are good, why didn’t we evolve to become even faster and smarter than we are now?
Natural selection acts on whole individuals, not on single parts or traits. It’s you (not your big brain or fast legs) who does or doesn’t survive and leave offspring. Increasing one part of an animal’s body might be beneficial in one respect but harmful in some other ways. That larger part might not fit well with other parts of the same animal, or it might drain off energy from other parts.
Instead, natural selection tends to mold each trait to the degree that makes the most of the survival and reproductive success of the whole animal. Each trait doesn’t increase to a possible maximum. Rather, all the traits meet at a point where they balance one another, with each trait neither too big nor too small. The whole animal is more successful than it would be if a given trait were bigger or smaller.
Once again, we can see how this principle works if we look at a complex piece of machinery, such as a car. Engineers can’t tinker with single parts in isolation from the rest of the machine, because each part costs money, space, and weight that could have gone into something else. Engineers have to ask what combination of parts will make the machine most effective.
In a way, evolution is like an engineer. It can’t tinker with single traits in isolation from the rest of an animal, because every organ, enzyme, or piece of DNA consumes energy and space that might have gone into something else. Instead, natural selection favors the combination of traits that gives the animal the greatest reproductive success. Both engineers and evolutionary biologists must consider the trade-offs involved in increasing anything. They must weigh the costs as well as the benefits the change would bring.
THE LESSON OF THE BATTLE CRUISERS
FOR AN EXAMPLE OF A SPECIES IN WHICH one trait became huge, leading to the species’ extinction, consider the British battle cruiser. Before and during World War I (1914-1918), the British navy launched thirteen of these warships. They were designed to be as large as battleships, with as many big guns as a battleship, but much faster. By maximizing speed and firepower, the battle cruisers immediately caught the public’s imagination and became a propaganda sensation.
But . . . if you take a 28,ooo-ton battleship, keep the weight of the big guns almost the same, and greatly increase the size of the engines for greater speed, all while keeping the overall weight of the vessel at 28,000 tons, you have to skimp on some of the other parts. The battle cruisers skimped especially on the weight of their armor, but also on the weight of their smaller guns, internal compartments, and antiaircraft defense.
The HMS Queen Mary after il was hit by German shells in WWI in 1916.
The result of this unbalanced design was inevitable. In 1916 the battle cruisers HMS Indefatigable, Queen Mary, and Invincible all blew up almost as soon as they were hit by German shells in a single battle. HMS Hood blew up in 1941, eight minutes after entering battle with the German battleship Bismarck. A few days after the Japanese attack on Pearl Harbor in 1941, HMS Repulse was sunk by Japanese bombers, becoming the first large warship to be destroyed from the air while in combat at sea. Faced with this clear evidence that spectacularly beefing up some parts doesn’t make a well-balanced whole, the British navy let its program of building battle cruisers go extinct.
Evolution and Aging
Our life cycles have many features that seem to limit, not maximize, our ability to produce offspring. Growing old and dying is one example. Others include our late puberty, our nine-month-long pregnancies, our single births, and the female menopause (the point in a woman’s life when she stops being able to bear children). Why wouldn’t natural selection favor a woman who entered puberty at five, completed a full pregnancy in three weeks, always bore five or more children, never entered menopause, and lived to two hundred, leaving behind hundreds of offspring?
Asking that question pretends that evolution can change our bodies one piece at a time, and it ignores the hidden costs. Humans could not reduce the length of pregnancy to three weeks, for example, without changing other things about ourselves and our babies. Remember that we have only a limited amount of energy available to us. Even people doing hard work, such as lumberjacks or marathon runners, can turn only about six thousand calories a day into energy. If our goal is to produce as many babies as possible, how would we divide those calories between rearing babies and repairing ourselves to live longer?
At one extreme, if we put all our energy into babies and none into biological repair, our bodies would age and disintegrate before we could rear our first baby. At the other extreme, if we spent all our energy on keeping our bodies functioning well, we might live a long time but we would have no energy for the exhausting business of making and rearing babies.
What natural selection must do is adjust the amounts of energy a species spends on repair and reproduction to arrive at the maximum number of offspring averaged over a lifetime. The result is a balance between life span and the reproductive traits of the life cycle. That balance varies from one animal species to another.
A two-month-old mouse, for example, can make baby mice, while it takes at least a dozen years, often longer, for a human to become physically capable of reproduction. Even a well- fed and cared-for mouse, though, is lucky to reach its second birthday. A well-fed and cared-for human is unlucky not to reach his or her seventy-second.
Animals like us, who start having offspring after a number of years, must devote a lot of energy to self-repair so that we live long enough to reach reproductive age. As a result, we age far more slowly than mice, probably because we repair our bodies much more effectively. (Much of our maintenance and self-repair, remember, goes into the invisible, scheduled replacement o
f our cells.) A human who invested no more energy into self-repair than a mouse would die long before reaching puberty.
IS THERE A CAUSE OF AGING?
RESEARCHERS IN GERONTOLOGY, THE STUDY of aging, focus on the physiological aspects of age and death. They search for a Cause of Aging, or at most a few causes. Evolutionary biology, though, suggests that they will not succeed. There should not be a single cause of aging, or even a few. Instead, natural selection should act to match the rates of aging in all our systems, so that getting older and dying involves many changes happening at the same time.
There’s no point in doing expensive maintenance on one part of the body if other parts are deteriorating faster because so much energy goes into that maintenance. There’s also no point in allowing a few parts or systems to deteriorate long before the rest if spending energy to repair just those few systems would bring a big increase in life expectancy. Natural selection doesn’t make pointless mistakes. The best strategy is to repair all parts at whatever rates allow everything finally to collapse all at once.
I believe that the evolutionary ideal of total collapse describes the fates of our bodies better than the physiologists’ long-sought single Cause of Aging. Most people as they age experience tooth wear or loss, decreases in muscle strength, and significant losses in hearing, vision, smell, and taste. Weakening of the heart, hardening of the arteries, brittleness of bones, decrease in kidney function, lowered resistance of the immune system, and loss of memory are also common symptoms of aging. Evolution does seem to have arranged things so that all our systems deteriorate.
From a practical viewpoint, this is disappointing. If there were one single or dominant cause of aging, curing that cause would give us a fountain of youth. Natural selection, though, would not permit us to deteriorate through a single mechanism with a simple cure. Perhaps that’s just as well. What would the world be like if we all lived for centuries? What use would we make of our extra time?
Life after Reproduction
For a key example of how evolution can explain some facts about aging, let’s examine a unique feature of the human lifestyle, which is that we survive past reproductive age. Passing one’s genes to the next generation is what drives evolution. Animals of other species rarely live on after they stop reproducing. Nature programs death to happen when fertility ends, because there’s no evolutionary benefit in keeping a body in good repair when it is longer making babies.
So why are human women programmed to live for decades after menopause, and why are human men programmed to live to an age when most of them are no longer busy fathering babies?
The answer lies in human parental care. In the human species, the intense phase of parental care is unusually long: nearly two decades. Even older people whose own children have reached adulthood are important to those children. By helping to care for their grandchildren and other youngsters, they contribute to survival—not just of their own children and grandchildren but of their whole tribe. Especially in the days before writing, older people were carriers of essential knowledge. For this reason, nature has programmed us to keep our bodies in reasonable repair at relatively advanced ages, even after women reach menopause and can no longer bear children.
But why did natural selection program female menopause into us in the first place? Most mammals, including human males and gorillas and chimpanzees of both sexes, merely experience a gradual decline and eventual end of fertility as they grow older. Only human females experience the abrupt shutdown of fertility that is menopause. Wouldn’t natural selection favor the woman who remained fertile until the bitter end?
Human female menopause probably resulted from two other uniquely human characteristics. One is the exceptional danger that childbirth poses to the mother. Compared with other species, human babies are enormous relative to their mothers’ size. Childbirth can be a difficult, even dangerous, matter. Before modern medical care, women often died while giving birth, and this still happens today, although it has become much rarer than it used to be. Among other primates, it has always been rare for mothers to die in childbirth.
The other characteristic is the danger that a mother’s death poses to her children, who are extremely dependent on her for care. Because children need parental care for a long time, even after they are no longer nursing, the death of a hunter-gatherer mother would probably have meant that her children likely died, too. This would have remained true up to a later age in childhood than for any other species of primate.
A hunter-gatherer mother with several children, then, risked the lives of those children every time she gave birth to a new baby. As each of her children grew older, her investment in that child’s care grew larger. At the same time, her own risk of dying in childbirth also increased as she got older. This meant that the danger to her existing children got worse and worse with each new pregnancy. When you already have three living children still dependent on you, having a fourth runs the risk of leaving those three motherless.
Those worsening odds probably led to menopause through natural selection. Shutting down female fertility protects a mother’s investment in the children she has already borne. But because childbirth carries no risk of death for fathers, men did not evolve menopause. Like aging, menopause is a feature of our life cycle that is hard to understand without the context of evolution. It’s even possible that menopause evolved only within the past sixty thousand years, when Cro-Magnons and other anatomically modern humans began regularly living to the age of sixty and beyond.
The longer life span of modern humans rests not only on cultural adaptations, such as tools for getting food or fighting predators, but also on the biological adaptations of menopause and increased investment in self-repair. Whether those biological adaptations developed at the time of the Great Leap Forward or earlier, they rank among the life history changes that made the third chimpanzee human.
PART THREE
UNIQUELY HUMAN
Hawaiian school-children around 1914. At that time young people like these, from many ethnic backgrounds, were turning the pidgin language of their plantation-worker parents into a fully functioning creole language.
BIOLOGY IS THE BASIS FOR SOME OF OUR uniquely human traits, as we saw in parts 1 and 2. Our large brains and the fact that we walk upright are determined by our genes. So are some features of our bodies and life cycles.
If those were our only unique traits, we wouldn’t stand out among animals. Ostriches walk on two legs. Some other animals have big brains relative to the size of their bodies. Seabirds live in large colonies the way we do, and tortoises, like humans, have long life spans. Our uniqueness lies in the cultural traits that rest on our genetic foundations. These cultural traits— spoken language, art, tool-based technology, and agriculture—give us our power.
If we stopped there, we’d have a one-sided, positive view of our uniqueness. Archaeology shows that our invention of agriculture was a mixed blessing that has seriously harmed many people while benefiting others. And we have other, darker traits. One is chemical abuse, our tendency to consume things, such as toxic drugs, that are harmful to us. At least that doesn’t threaten our survival as a species. Two of our other cultural practices—to be discussed in parts 4 and 5—do threaten our survival. One is genocide, the killing of whole groups of people. The other is the mass extermination of other species, which often goes along with the destruction of the environment, our own habitat. These traits make us uncomfortable. Are they occasional, unnatural outbreaks, or are they features as basic to humanity as the traits we’re proudest of?
None of these human traits, good or bad, could have arisen from nothing. For each of them, we need to ask: What behavior in the animal world might have given rise to this human trait? Can we trace the appearance and evolution of this trait in our family tree? The next four chapters consider these questions for traits that are noble, two- edged, or only mildly destructive: language, art, agriculture, and chemical abuse. I’ll end this part by examining the search for
intelligent life on other planets, and by showing what we can learn about life in the universe from studies of woodpecker evolution right here on earth.
CHAPTER 6
THE MYSTERY OF LANGUAGE
THE ORIGIN OF LANGUAGE IS THE MOST important mystery in how we became uniquely human. Language lets us communicate with each other far more precisely than any animals can do. It lets us make group plans, teach one another, and learn from what people have experienced in other places or in the past. This is why I think that the Great Leap Forward—the stage in human history when our ability to invent new ways of doing things finally appeared—was made possible by the development of spoken language as we know it.
The Lack of a Time Machine
Animals communicate, but between human language and the sounds made by any animal lies a gulf that doesn’t seem crossable. How was it crossed? We evolved from animals that lacked human speech, so our language must have evolved over time, along with other human features such as the shape of our skulls or our ability to make tools and art.
Unfortunately, the origins oflanguage are harder to trace than the origins of our skulls, tools, or art. The spoken word vanishes in an instant. I often dream of a time machine that would let me place tape recorders in the camps of our ancient ancestors. Perhaps I’d discover that australopithecines, the African man-apes of millions ofyears ago, uttered grunts not too different from those of chimpanzees. Maybe Homo erectus used recognizable single words and then, after a million years or so, progressed to two-word sentences. Before the Great Leap Forward, Homo sapiens may have gotten as far as longer strings of words, but without much grammar, and the full range of modern speech arrived only with the Leap.
The Third Chimpanzee: The Evolution and Future of the Human Animal Page 7