Spanish soldier and military governor Juan Ponce de Leon explored Florida in 1513. Later, some historians claimed that he was searching for the Fountain of Youth, a mythical spring whose waters cured disease and granted eternal life. Some scientists still hope to find the secret of endless—or at least longer—life.
IS THERE A BELOVED GRANDPARENT IN YOUR life? Or maybe a respected teacher? You can probably think of at least one older person who has enriched your life.
We enter life surrounded by people who are older than we are: our parents, grandparents, aunts and uncles, and maybe older brothers and sisters. They become our protectors, guides, families, and friends. We cherish our relationships with these loved ones. it is hard to accept the fact that we will lose them someday, but life naturally draws to an end as old age is followed by death. it is a fate that all of us will eventually share.
Like members of every other species on earth, individuals of our species, Homo sapiens, have a life expectancy that is a feature of our life cycle. Life expectancy is the term scientists use to describe the average period that a member of any species can expect to live. Many factors influence life expectancy. For humans, a key factor is where you live. People born in different countries may have different life expectancies, based on things such as the quality of the food, water, and medical care available to them. The life expectancy for adults in the United States is now seventy-six years for men and almost eighty- one years for women. Few of us, though, will survive to one hundred.
Why is it so easy to live almost 80 years, so hard to live 100, and almost impossible to survive to 120? Why do humans with access to the best medical care, and animals kept in a cage with plenty of food and no predators, inevitably grow weak or sick and die? Death is one of the most obvious features of our life cycle, but there’s nothing obvious about what causes it.
Slow Aging
We age more slowly than our closest relatives. Not a single ape of any species has been recorded as achieving the current life expectancy of American humans. Only exceptional apes reach their fifties. Some of our slow aging may have developed fairly recently in our evolutionary history, around the time of the Great Leap Forward, sixty thousand years ago. Few Neanderthals survived past the age of forty. Among the Cro-Magnons who replaced them, quite a few lived into their sixties.
Slow aging is vital to the human life cycle, which depends on shared information. As language evolved, we became able to pass on far more information than before. Today we can pass it in written or recorded form, but writing is a fairly recent development in our history. For tens of thousands of years before writing, old people were our libraries. They served as keepers of a group’s shared information and experience, just as they continue to do in tribal societies today. Under hunter-gatherer conditions, the knowledge possessed by even one seventy- year-old could mean the difference between starvation and survival for a whole clan.
Our ability to survive to a ripe old age had something to do with our advances in culture and technology. it’s easier to defend yourself against a lion with a spear than with a hand-held stone, easier still with a high-powered rifle. But advances in culture and technology would not have been enough to give us longer lives unless our bodies had also been redesigned to last longer. As we’ll see in this chapter, our biology became remolded to match the increased life expectancy made possible by our cultural and technological advances.
Aging is studied by two groups of scientists who take very different approaches. Physiologists explore the body and its structures, searching for the mechanisms within our cells that bring about aging. Evolutionary biologists try to understand how natural selection could ever permit aging to occur. I think that aging can’t be understood unless we seek both explanations. I expect that the evolutionary explanation (why we age) will help us find the physiological explanation (what specific features and processes in our bodies cause us to age).
Repair and Replace
Physiologists tend to think that something about our bodies and their systems makes aging unavoidable. One theory is that aging occurs because our immune systems find it harder and harder to tell the difference between our own cells and foreign cells from outside our bodies. This is a fatal defect in our immune systems. Could natural selection have created an immune system without that flaw? To answer that question, we need to look at how our bodies maintain themselves.
Aging can be viewed simply as damage or deterioration that doesn’t get repaired. We are unconsciously but constantly repairing ourselves at every level, from molecules to tissues to whole organs. In the same way, we spend money to repair our cars. Our bodies’ self-repair mechanisms, like car repairs, fall into two categories: damage control and regular replacement.
For a car, damage control means things such as repairing a flat tire or replacing a smashed fender. For us, the most visible example of damage control is wound healing, which repairs damage to our skin. Some animals can achieve more spectacular damage control. Lizards regrow tails they have lost, starfish regrow severed limbs, and sea cucumbers can even regrow their intestines. At the invisible, molecular level, we have enzymes that recognize and fix damaged sites in our genetic material, DNA.
The other type of repair is regular replacement, also familiar to any car owner. We periodically change the oil, air filter, and other parts without waiting for the car to break down first. In the animal world, teeth are replaced on a scheduled basis. Humans go through two sets in their lifetime, elephants six sets, and sharks an indefinite number of sets. Lobsters and insects are among the creatures that regularly replace their external skeletons, or hard shells, by shedding them and growing new ones. Hair growth is another example of regular replacement. Hair keeps steadily growing, no matter how short we cut it.
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.
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 of 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?
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.