The Third Chimpanzee: The Evolution and Future of the Human Animal

by Jared Diamond

  None of these motives, however, goes to the heart of the paradox: Why do we actively seek what we know to be harmful? I suggest another motive, one that is related to a wide range of seemingly selfdestructive traits in animals. It may explain a risky or self-destructive human behavior.

  The Clue in the Long Tail

  I arrived at this idea while studying an entirely different paradox, one involving bird evolution. While watching a male bird of paradise in New Guinea, I wondered why it had evolved a three- foot-long tail, which surely made it more difficult to fly and walk in the jungle.

  Males of other bird of paradise species had evolved other bizarre features, such as long plumes growing out of their eyebrows, brilliant colors, and loud calls. All these features must threaten the birds’ survival. Bright colors and loud calls, for example, are likely to attract predatory hawks. Yet those same features serve as advertisements that help the male birds win mates. Like many other biologists, I found myself wondering why male birds of paradise use such handicaps as their advertisements, and why female birds find the handicaps attractive.

  At this point I recalled a paper published in 1975 by Amotz Zahavi, an Israeli biologist. Zahavi suggested a new theory about the role of costly or self-destructive signals in animal behavior. His idea was that male traits that make survival more difficult might attract females precisely because they are handicaps. Suddenly I realized that Zahavi’s idea might apply to the birds of paradise I studied. It could also explain another paradox I had noticed: our use of toxic chemicals, and the way we encourage it in ads that make smoking and drinking look glamorous, even though we know smoking and drinking are destructive.

  A Theory about Animal Communication

  Zahavi’s theory concerned the broad problem of animal communication. Animals need quick, easily understood signals to convey messages to other animals, including potential mates and predators. Say that a gazelle, for example, notices a lion stalking it. It would be in the gazelle’s interest to give a signal that the lion would understand as “I am a superior, fast gazelle! You’ll never catch me, so don’t waste your time and energy trying.” Even if the gazelle really can outrun a lion, a signal that discourages the lion saves the gazelle’s time and energy, too.

  But what signal could the gazelle give? It can’t run a demonstration hundred-yard dash in front of every lion that shows up. What about a quick, easy signal, such as pawing the ground with the left hind foot? The problem is that an easy signal opens the door to cheating, because any gazelle, even a slow one, could give the signal. Lions would learn to ignore it. The signal must convince the lion of the gazelle’s honesty.

  The signal that gazelles use is called stotting. Instead of running away as fast as possible, the gazelle runs slowly, repeatedly jumping high into the air with stiff-legged leaps. At first glance, this behavior appears self-destructive. It wastes time and energy, and it gives the lion a chance to catch up.

  Why would this young gazelle draw a lion’s attention by leaping high into the air? Surprisingly, behavior that looks dangerous or even self-destructive may save an animal's life. All too often, though, instinct drives humans to dangerous behaviors that have no good effects, only destructive ones.

  Zahavi’s theory goes to the heart of this paradox. Signals that put an animal at risk— whether the signals are structures such as long tails or behaviors such as stotting—are good indicators of honesty because they are handicaps. A signal that does not cost the signaling animal anything lends itself to cheating, because even a slow or inferior animal can afford to give that signal. Only costly or risky signals guarantee honesty. A slow gazelle that stotted at an approaching lion would seal its fate, but a fast gazelle could outrun the lion even after stotting. By stotting, the gazelle boasts, “I’m so fast that I can escape you even after giving you this head start.”

  I’ve described the problem of signals as if the gazelle chooses stotting from many possible behaviors, and as if the lion thinks it over and decides that the stotting is a good sign the gazelle is both speedy and honest. In reality, those “choices” are the result of evolution. They are directed by genes. Gazelles and lions that spare themselves unnecessary, wasteful chases save energy and tend to leave the most offspring. It’s a basic principle of evolutionary biology that genetically coded features or behaviors that help animals leave more offspring—in this case, stotting—get passed on.

  Zahavi’s theory can be applied to the long tail on that male bird of paradise. Any male bird that has managed to survive in spite of the handicap of a long tail must have terrific genes in other respects. He has proved that he must be especially good at finding food, escaping predators, and resisting disease. The bigger the handicap, the harder the test he has passed.

  When the female bird of paradise picks a mate, she is like a fairy-tale damsel courted by knights. The damsel tests her suitors by asking them to slay dragons. When she sees a onearmed knight who can still slay a dragon, she knows she has found a mate with great genes. By displaying his handicap, the knight or bird of paradise is actually displaying his superiority.

  Expensive and Dangerous Human Behavior

  It seems to me that Zahavi’s theory applies to many costly things humans do to gain status. People who woo possible mates with expensive gifts and other displays of wealth are saying, in effect, “I have plenty of money to support a family. You can believe my boast, because you can see how much money I’m spending now without a care.” People who show off jewels or sports cars gain status, because everyone knows that those objects are expensive.

  Zahavi’s theory can also be applied to more dangerous human behaviors, including the abuse of chemicals. Especially in adolescence and early adulthood, the period when abuse is likely to begin, we devote a lot of energy to establishing our status. I suggest that we share the same unconscious instinct that leads birds and gazelles to indulge in dangerous displays. Ten thousand years ago we “displayed” by challenging a lion or a tribal enemy. Today we do it in other dangerous ways, such as driving fast or consuming dangerous drugs.

  THE DANGEROUS DIVES OF MALEKULA’S MEN

  AMERICAN INDIANS OF THE PACIFIC NORTHWEST used to seek status by giving away as much wealth as possible in ceremonies known as potlatch rituals. Before modern medicine, tattooing was not only painful but also dangerous because of the risk of infection. Tattooed people were advertising two forms of their strength, tolerance of pain and resistance to disease. These are just two of the many ways people seek status through behavior that is costly, risky, or harmful.

  A more dramatic example comes from the Pacific island of Malekula, where men have traditionally shown off by the insanely dangerous practice (now imitated around the world by recreational bungee jumpers) of building a high tower and then jumping off it headfirst. The jumper gets a couple of sturdy vines and ties one end of each vine to the tower. The other end of each vine goes around one of the jumper’s ankles. The length of the vines is calculated to stop the plunge while the jumper’s head is still a few feet above the ground. Survival demonstrates that the jumper is courageous, able to do a careful calculation, and a good builder.

  On the Pacific island of Malekula, part of the island nation Vanuatu, young men traditionally built towers and then leaped from them to show their skill and bravery. If this jumper has calculated correctly, the vines tied to his ankles will stop him before he crashes into the ground.

  False Messages

  The message of dangerous displays is “I’m strong and superior. I must be strong enough to get past the burning, choking sensation of my first puff on a cigarette, or the misery of my first alcohol-caused hangover. To do it often and remain alive and healthy, I must be superior”— or so I imagine. In reality, although the message of the male bird’s long tail is true, for us that message is false. Like so many of our animal instincts, the dangerous display works against us in modern society.

  Someone who has smoked several packs of cigarettes a day for years and still hasn’t
developed lung cancer may have a gene for resistance to lung cancer, but that proves nothing about his or her intelligence, skills, or ability to create a happy life for a spouse and children. In fact, given what we now know about the harmful effects of smoking, that person’s behavior may be a sign of negative qualities, such as poor judgment, that would make the smoker a bad choice as a mate.

  Animals with brief lives and courtships must develop quick signals. Potential mates don’t have enough time to measure each other’s real qualities. But humans, with our long lives and courtships and business associations, have plenty of time to study one another’s worth. Drug abuse is a classic example of a once-useful instinct that has turned foul in us. In this case, it is the instinct to rely on handicap signals as a sign of strength. Whiskey and cigarette ads are cleverly directed at that old instinct, with their messages that smoking and drinking will bring status and make us attractive. Our buried instinct does not see that these messages are false, but we can use our human abilities to learn, reason, and choose different goals to override the false messages.

  Costs and Benefits to Animals and Humans

  All animals have had to evolve signals for quickly communicating messages to other animals. To be believable, a signal had to carry some cost, risk, or burden that only superior individuals could afford. Many animal signals that seem to have a negative effect on the signaler— such as stotting in front of a lion, or having a long, burdensome tail in the jungle—can be understood in this light.

  This viewpoint may explain the evolution of both human art and human chemical abuse. Art and abuse are widespread hallmarks of human society, and it is not immediately obvious how they help us survive or attract mates. I argue in chapter 7 that art often serves as a reliable signal of an individual’s superiority or status, because it requires skills to create, and it also requires status or wealth to acquire. Now I’m taking that argument a step further to say that humans seek status through many other costly displays besides art. Some of those displays, such as diving off towers or consuming toxic drugs, are surprisingly dangerous.

  I don’t claim that this perspective gives us total understanding of either art or chemical abuse. Complex behaviors take on a life of their own and go far beyond their original purpose. At first those behaviors may have served more than one function.

  Even from an evolutionary perspective, there remains a basic difference between animal behavior and human chemical abuse. Stotting, long tails, and other animal signals involve costs, but the benefits are greater than the costs. A long-tailed male bird does pay a cost. He has the burden of those feathers when he searches for food or escapes a predator. That cost, however, is more than made up for by the mating advantages the bird gains because his tail attracts females. The end result is more offspring, not fewer, to carry on his genes. The tail only appears to be a self-destructive feature. In reality, it favors the survival of the bird’s genes.

  Human chemical abuse is different. The costs are greater than the benefits. Drug addicts and alcoholics not only lead shorter lives, but they also become less attractive, not more, to possible mates. They also lose the ability to care properly for children. Unlike the gazelle’s stotting or the bird’s long tail, the trait of human chemical abuse does not continue because there are hidden benefits that outweigh the costs. It continues because the toxic substances are chemically addicting. Overall, drinking, smoking, and using drugs are self-destructive behaviors.

  Gazelles may occasionally miscalculate in stotting. That’s how lions wind up dining on gazelle from time to time. But gazelles don’t commit suicide through addiction to the excitement of stotting. Our self-destructive abuse of chemicals has gone beyond its origins in the instinctive behavior of animals.

  In 1938, a radio broadcast based on H. G. Wells's novel War of the Worlds convinced some people that the United States was being invaded by Martians. Panic and confusion reigned for a few hours.

  CHAPTER 1O

  ALONE IN A CROWDED UNIVERSE

  THE NEXT TIME YOU’RE OUTDOORS ON A CLEAR night away from city lights, look up at the sky. Get a sense of the number of stars. Next, find a pair of binoculars and turn them on the Milky Way, that stream of brightness across the sky. See how many more stars are visible now. That multitude of stars is just the beginning.

  Our universe contains billions of galaxies, each with billions or even trillions of stars. Many of those stars, we now know, have planets revolving around them. Once those numbers have sunk in, you’ll be ready to ask: How could humans possibly be unique in the universe? How many civilizations of intelligent beings like us must be out there, looking back at us? How long before we are in communication with them, or visit them, or are visited by them?

  On Earth, we are unique. No other species possesses language, art, or agriculture remotely close to ours in complexity. Most human hallmarks would not show up at a distance of many light-years. (Distances between stars are measured in light-years. A light-year is the distance light travels in a year, almost six trillion miles). But there are two signs of intelligent beings elsewhere that we might be able to detect on Earth, if those beings existed: space probes and radio signals. We are sending out both; other intelligent creatures should be sending them out, too. Where are they?

  This seems to me one of the greatest puzzles in science. Given the billions of stars, and given the abilities that developed in our own species, we ought to be detecting the spacecraft, or at least the radio waves, of other species that also developed those abilities. But we have not done so. Could we really be unique, not just on Earth but also in the universe?

  COUNTING UP THE ALIEN CIVILIZATIONS

  THE FIRST SEARCH FOR EXTRATERRESTRIAL RADIO signals, in 1960, took place at the National Radio Astronomy Observatory in Green Bank, West Virginia. It was carried out by an astronomer named Frank Drake. The next year Drake arranged a meeting of scientists at Green Bank to discuss the possibility of detecting extraterrestrial intelligence. In preparing for the meeting, Drake created a formula to calculate how many alien civilizations might exist. Known as the Green Bank formula, or the Drake equation, it goes like this:

  N = R* • fp • ne • fl • fi • fc • L

  R* = The rate of formation of stars suitable for the development of intelligent life.

  fp = The fraction of those stars with planetary systems.

  ne = The number of planets per solar system with an environment suitable for life.

  fl = The fraction of suitable planets on which life actually appears.

  fi = The fraction of life-bearing planets on which intelligent life emerges.

  fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence (such as radio waves) into space.

  L = The length of time such civilizations release detectable signals into space.

  Astronomers plug their best estimates into each of these variables, using the most recent discoveries or ideas about such things as star formation, the number of stars that have planets, and so on. Multiplied together, the variables produce N, which is the number of civilizations in our Milky Way galaxy that produce signals we could detect.

  The formula suggests a large number of extraterrestrial civilizations. Physicist Enrico Fermi pointed out that based on the formula, we should have been visited by intelligent aliens by now, or at least have detected radio signals from their civilizations—but we have no solid evidence of alien visits or interstellar radio signals. This is called Fermi’s paradox.

  Some researchers believe that an unknown effect, called the Great Filter, keeps the number of civilizations far lower than expected. Two of the possible Great Filters that have been suggested are that it is very rare for intelligent life to arise, and that technological civilizations don’t last very long.

  Is There Anybody Out There?

  We have already tried to communicate with extraterrestrial life, or life outside Earth. The first attempt came in 1960, when scientists listened (unsuccessf
ully) for radio transmissions from two nearby stars. Since then, we have sent radio transmissions as well as spacecraft and probes out into space, and we have listened for radio signals that might be a sign of intelligent beings outside our solar system.

  Astronomers have tried to calculate the number of advanced civilizations in the universe by using the Green Bank formula, which multiplies a string of estimated numbers, such as the number of stars in the galaxy, the number of those stars that have planets, and the number of those planets that support life. Multiplying all the estimates together, they conclude that the universe must contain billions of billions of planets with life.

  A fraction of those life-bearing planets—even if only 1 percent—must have advanced civilizations with the technical skills to send radio signals between the stars. If the Green Bank formula is correct, our galaxy alone must have about a million planets supporting advanced civilizations. So where are they? The silence is deafening.

  Something must be wrong with the astronomers’ calculations. Astronomers know what they’re talking about when they estimate the number of stars and planetary systems, and also the percentage of those systems where life is likely to occur. Instead, the problem may lie in the idea that advanced technical civilizations will evolve on a significant percentage of life-bearing worlds. That idea is based on what biologists call convergent evolution. To understand what convergent evolution means, and its limits, let’s look at woodpeckers as a test case.

  An acorn woodpecker stores food. in spite of the advantages of the woodpecking lifestyle, only one group of creatures on earth has evolved to follow it. Does this tell us anything about the probability of intelligent life in outer space?

  The Case of the Woodpeckers

  Convergent evolution is how biologists describe the fact that many groups of creatures can evolve independently to have the same traits or to occupy the same ecological niche. The evolution of these species converges, or comes to the same place. For example, birds, bats, pterodactyls, and insects all evolved independently to fly. Another example of convergent evolution is eyes, which evolved independently in many different animal groups.