The rise and fall of the third chimpanzee

meant to go in front, but the VW engine compartment is m the back, so I would have to change the gearbox and transmission and other things. I would also have to change the shock absorbers and brakes, designed to smooth the ride and stop a car at 65 mph but not at 180 mph. By the time I had finished modifying my VW to take the BMW engine, there would not be much remaining from my original Beetle, and the Modifications would have cost me a big pile of money. I suspect that my puny 40-horsepower engine is optimal, in the sense that I could no increase my cruising speed without sacrificing other performance features of my car—as well as sacrificing other money-requiring features of my lifestyle.

While the marketplace eventually eliminates engineering monstrosities like a VW with a BMW engine, all of us can think of monstrosities that took quite a while to eliminate. To those of you who share my fascination with naval warfare, British battle-cruisers are a good example. Before and during the First World War, the British navy launched thirteen warships called battle-cruisers, designed to be as large and with as many big guns as battleships but much faster. By maximizing speed and firepower, the battle-cruisers immediately caught the public imagination and became a propaganda sensation. However, if you take a 28,000-ton battleship, keep the weight of the big guns nearly constant, and greatly increase the weight of the engines while still maintaining total weight around 28,000 tons, you have to skimp on the weight of some other parts. The battle- cruisers skimped especially on weight of armour, but also on weight of small guns, internal compartments, and anti-aircraft defence. The results of this suboptimal overall design were inevitable. In 1916 H.M.S. Indefatigable, Queen Mary, and Invincible all blew up almost as soon as they were hit by shells at the Battle ofjutland. H.M.S. Hoodblew up in 1941, a mere eight minutes after entering battle with the German battleship Bismarck. H.M. S. Repulse was sunk by Japanese bombers a few days after the Japanese attack on Pearl Harbor, thereby acquiring the dubious distinction of being the first large warship to be destroyed from the air while in combat at sea. Faced with this stark evidence that some spectacularly maximal parts do not make an optimal whole, the British navy let its programme of building battle-cruisers become extinct. In short, engineers cannot tinker with single parts in isolation from the rest of a machine, because each part costs money, space, and weight that might have gone into something else. Engineers instead have to ask what combination of parts will optimize a machine's effectiveness. By the same reasoning, evolution cannot tinker with single traits in isolation from the rest of an animal, because every structure, enzyme, or piece of DNA consumes energy and space that might have gone into something else. Instead, natural selection favoured that combination of traits that maximizes the animal's reproductive output. Thus, both engineers and evolutionary biologists have to evaluate the trade-offs involved in increasing anything; that is, its costs, as well as the benefits that it would bring. An obvious difficulty in applying this reasoning to our life-cycles is that they have many features seeming to reduce, not to maximize, our ability to produce offspring. Growing old and dying is just one example; other examples are human female menopause, bearing one baby at a time, producing babies only once every year or so at most, and not even starting to produce babies until the age of twelve to sixteen. Would not natural selection favour the woman who reached puberty at age five, completed gestation in three weeks, regularly bore quintuplets, never underwent menopause, put lots of biological energy into repair of her body, lived to 200, and thereby left hundreds of offspring?

But posing the question in that form pretends that evolution can change our bodies one piece at a time, and ignores the hidden costs. For example, a woman certainly could not reduce the length of pregnancy to three weeks without changing anything else about herself or her baby. Remember that we only have a finite amount of energy available to us. Even people doing hard exercise and eating rich food—lumberjacks, or marathon runners in training—cannot metabolize much more than about 5,000 calories per day. How should we allocate those calories between repairing ourselves and rearing babies, if our goal is to raise as many babies as possible? At the one extreme, if we put all our energy into babies and devoted no energy to biological repair, our bodies would age and disintegrate before we could rear our first baby. At the other extreme, if we lavished all our available energy on keeping our bodies in shape, we might live a long time but would have no energy left for the exhausting process of making and rearing babies. What natural selection must do is to adjust an animal's relative expenditures of energy on repair and on reproduction, so as to maximize its reproductive output, averaged over its lifetime. The answer to that problem varies among animal species, depending on factors such as their risk of accidental death, their reproductive biology, and the costs of various types of repair. This perspective can be employed to make testable predictions about how animals should differ in their repair mechanisms and rates of aging. In 1957 the evolutionary biologist George Williams cited some striking facts about aging that become comprehensible only from an evolutionary perspective. Let's consider several of Williams's examples and re-express them in the physiological language of biological repair, by taking slow aging as an indication of good repair mechanisms.

The first example concerns the age at which an animal first breeds and produces offspring. That age varies enormously among species: few humans are so precocious as to produce babies before the age of twelve years, while any self-respecting mouse a mere two months old can already make baby mice. Animals belonging to a species whose age of first breeding is late, like us, need to devote much energy to repair, in order to ensure that they survive to that reproductive age. Hence we expect investment in repair to increase with age at first reproduction.

For instance, correlated with our having a much later age of first reproduction than do mice, we humans age far more slowly than mice and are thus presumed to repair our bodies much more effectively. Even with plenty of food and the best medical care, a mouse is lucky to reach its second birthday, while we would be unlucky not to reach our seventy-second birthday. The evolutionary reason: a human who invested no more of his/her energy in repair than does a mouse would be dead long before reaching puberty. Hence it is more worthwhile to repair a human than a mouse.

What might that postulated extra energy expenditure of ours actually consist of? At first, our human repair capabilities seem unimpressive. We cannot regrow an amputated arm, and we do not regularly replace our skeleton, in the way that some short-lived invertebrates do. However, such spectacular but infrequent replacements of a whole structure probably are not the biggest items in an animal's repair budget. Instead, the biggest expense is all that invisible replacement of so many of your cells and molecules, day after day. Even if you spend all day every day just lying in bed, you need to eat about 1,640 calories per day if you are a man (1,430 for a woman) just to maintain your body. Much of that maintenance metabolism goes to our invisible scheduled replacement. And so I would guess that we cost more than a mouse in the respect of putting a bigger fraction of our energy into self-repair, and a smaller fraction into other purposes like keeping warm or caring for babies.

The second example I shall discuss involves the risk of irreparable injury. Some biological damage is potentially reparable, but there is also damage that is guaranteed to be fatal (for example, being eaten by a lion). If you are likely to be eaten by a lion tomorrow, there is no point paying a dentist to start expensive orthodontic work on your teeth today. You would do better to let your teeth rot and start having babies immediately. But if an animal's risk of death from irreparable accidents is low, then there is a potential payoff, in the form of increased lifespan, from putting energy into expensive repair mechanisms that retard aging. This is the reasoning by which Mercedes-owners decide to pay for lubrication of their cars in Germany and the US but not in New Guinea.

Biological analogies are that the risk of death from predators is lower for birds than for mammals (because birds can escape by flying), and lower for turtles than for most other reptiles (because turtles are protected by a shell). Thus, birds and turtles stand to gain a lot from expensive repair mechanisms, compared to flightless mammals and shell-less reptiles that will soon be eaten by predators anyway. Indeed, if one compares longevities of well-fed pets protected from predators, birds do live longer (that is, do age more slowly) than similarly sized mammals, and turtles live longer than similarly sized shell-less reptiles. The bird species best protected from predators are seabirds like petrels and albatrosses that nest on remote oceanic islands free of predators. Their leisurely life-cycles rival our own. Some albatrosses do not even breed until they are ten years old, and we still do not know how long they live: the birds themselves last longer than the metal rings that biologists began putting on their legs a few decades ago in order to age them. In the ten years that it takes an albatross to start breeding, a mouse population could have gone through sixty generations, most of which would already have succumbed to predators or old age. As our third example, let's compare males and females of the same species. We expect more potential payoff from repair mechanisms, and lower rates of aging, in that sex with the lower accidental mortality rate. For many or most species, males suffer greater accidental mortality than females, partly because males put themselves at greater risk by fighting and bold displays. This is certainly true of human males today and has probably been so throughout our history as a species—men are the sex most likely to die in wars against men