We Are Alone

Drake expressed these assumptions in a letter to Science defending SETI against the eminent biologist Ernst Mayr. Mayr had noted that only one of the fifty million species on earth had developed civilizations, so the probability that life on a given planet would include an intelligent species might very well be small. Drake replied:

The first species to develop intelligent civilizations will discover that it is the only such species. Should it be surprised? Someone must be first, and being first says nothing about how many other species had or have the potential to evolve into intelligent civilizations, or may do so in the future Similarly, among many civilizations, one will be the first, and temporarily the only one, to develop electronic technology. How else could it be? The evidence does suggest that planetary systems need to exist in sufficiently benign circumstances for a few billion years for a technology-using species to evolve.

To see why this thinking runs so afoul of the modern theory of evolution, consider an analogy. The human brain is an exquisitely complex organ that evolved only once. The elephant's trunk, which can stack logs, uproot trees, pick up a dime, remove thorns, powder the elephant with dust, siphon water, serve as a snorkel, and scribble with a pencil, is another complex organ that evolved only once. The brain and the trunk are products of the same evolutionary force, natural selection. Imagine an astronomer on the Planet of the Elephants defending SETT, the Search for Extraterrestrial Trunks:

The first species to develop a trunk will discover that it is the only such species. Should it be surprised? Someone must be first, and being first says nothing about how many other species had or have the potential to evolve trunks, or may do so in the future. . . . Similarly, among many trunk-bearing species, one will be the first, and temporarily the only one, to powder itself with dust. The evidence does suggest that planetary systems need to exist in sufficiently benign circumstances for a few billion years for a trunk-using species to evolve. . . .

This reasoning strikes us as cockeyed because the elephant is assuming that evolution did not just produce the trunk in a species on this planet but was striving to produce it in some lucky species, each waiting and hoping. The elephant is merely "the first," and "temporarily" the only one; other species have "the potential," though a few billion years will have to pass for the potential to be realized. Of course, we are not chauvinistic about trunks, so we can see that trunks evolved, but not because a rising tide made it inevitable. Thanks to fortuitous preconditions in the elephants' ancestors (large size and certain kinds of nostrils and lips), certain selective forces (the problems posed by lifting and lowering a huge head), and luck, the trunk evolved as a workable solution for those organisms at that time. Other animals did not and will not evolve trunks because in their bodies and circumstances it is of no great help. Could it happen again, here or elsewhere? It could, but the proportion of planets on which the necessary hand has been dealt in a given period of time is presumably small. Certainly it is less than one hundred percent.

WHY US?

Why did some miocene ape first enter the cognitive niche? Why not a groundhog, or a catfish, or a tapeworm? It only happened once, so no one knows. But I would guess that our ancestors had four traits that made it especially easy and worth their while to evolve better powers of causal reasoning.

First, primates are visual animals. In monkeys such as the rhesus macaque, half the brain is dedicated to sight. Stereoscopic vision, the use of differences in the vantage points of the two eyes to give a sense of depth, developed early in the primate lineage, allowing early nocturnal primates to move among treacherous fine branches and to grab insects with their hands. Color vision accompanied the switch of the ancestors of monkeys and apes to the day shift and their new taste for fruits, which advertise their ripeness with gaudy hues.

Why would the vision thing make such a difference? Depth perception defines a three-dimensional space filled with movable solid objects. Color makes objects pop out from their backgrounds, and gives us a sensation that corresponds to the stuff an object is made of, distinct from our perception of the shape of the stuff. Together they have pushed the primate brain into splitting the flow of visual information into two streams: a "what" system, for objects and their shapes and compositions, and a "where" system, for their locations and motions. It can't be a coincidence that the human mind grasps the world—even the most abstract, ethereal concepts—as a space filled with movable things and stuff (see Chapters 4 and 5). We say that John went from being sick to being well, even if he didn't move an inch; he could have been in bed the whole time. Mary can give him many pieces of advice, even if they merely talked on the phone and nothing changed hands. Even scientists, when they try to grasp abstract mathematical relationships, plot them in graphs that show them as two- and three-dimensional shapes. Our capacity for abstract thought has co-opted the coordinate system and inventory of objects made available by a well-developed visual system.

It is harder to see how a standard mammal could have moved in that direction. Most mammals hug the ground sniffing the rich chemical tracks and trails left behind by other living things. Anyone who has walked a frisky cocker spaniel as it explores the invisible phantasmagoria on a sidewalk knows that it lives in an olfactory world beyond our understanding. Here is an exaggerated way of stating the difference. Rather than living in a three-dimensional coordinate space hung with movable objects, standard mammals live in a two-dimensional flatland which they explore through a zero-dimensional peephole. Edwin Abbott's Flatland, a mathematical novel about the denizens of a plane, showed that a twodimensional world differs from our own in ways other than just lacking one third of the usual dimensions. Many geometric arrangements are simply impossible. A full-faced human figure has no way of getting food into his mouth, and a profiled one would be divided into two pieces by his digestive tract. Simple devices like tubes, knots, and wheels with axles are unbuildable. If most mammals think in a cognitive flatland, they would lack the mental models of movable solid objects in 3-D spatial and mechanical relationships that became so essential to our mental life.

A second possible prerequisite, this one found in the common ancestor of humans, chimpanzees, and gorillas, is group living. Most apes and monkeys are gregarious, though most mammals are not. Living together has advantages. A cluster of animals is not much more detectable to a predator than a single animal, and if it is detected, the likelihood that any individual will be picked off is diluted. (Drivers feel less vulnerable speeding when they are in a group of speeders, because chances are the traffic cop will stop someone else.) There are more eyes, ears, and noses to detect a predator, and the attacker can sometimes be mobbed. A second advantage is in foraging efficiency. The advantage is most obvious in cooperative hunting of large animals, such as in wolves and lions, but it also helps in sharing and defending other ephemeral food resources too big to be consumed by the individual who found it, such as a tree laden with ripe fruit. Primates that depend on fruit, and primates that spend time on the ground (where they are more vulnerable to predators), tend to hang out in groups.

Group living could have set the stage for the evolution of humanlike intelligence in two ways. With a group already in place, the value of having better information is multiplied, because information is the one commodity that can be given away and kept at the same time. Therefore a smarter animal living in a group enjoys a double advantage: the benefit of the knowledge and the benefit of whatever it can get in trade for the knowledge.

The other way in which a group can be a crucible of intelligence is that group living itself poses new cognitive challenges. There are also disadvantages to the madding crowd. Neighbors compete over food, water, mates, and nest sites. And there is the risk of exploitation. Hell is other people, said Jean-Paul Sartre, and if baboons were philosophers no doubt they would say that hell is other baboons. Social animals risk theft, cannibalism, cuckoldry, infanticide, extortion, and other treachery.

Every social creature is poised between milking the benefits and suffering the costs of group living. That creates a pressure to stay on the right side of the ledger by becoming smarter. In many kinds of animals, the largest-brained and smartest-behaving species are social: bees, parrots, dolphins, elephants, wolves, sea lions, and, of course, monkeys, gorillas, and chimpanzees. (The orangutan, smart but almost solitary, is a puzzling exception.) Social animals send and receive signals to coordinate predation, defense, foraging, and collective sexual access. They exchange favors, repay and enforce debts, punish cheaters, and join coalitions.

The collective expression for hominoids, "a shrewdness of apes," tells a story. Primates are sneaky baldfaced liars. They hide from rivals' eyes to flirt, cry wolf to attract or divert attention, even manipulate their lips into a poker face. Chimpanzees monitor one another's goals, at least crudely, and sometimes appear to use them in pedagogy and deception. One chimp, shown a set of boxes with food and one with a snake, led his companions to the snake, and after they fled screaming, feasted in peace. Vervet monkeys are yentas who keep close track of everyone's comings and goings and friends and enemies. But they are so dense about the nonsocial world that they ignore the tracks of a python and the ominous sight of a carcass in a tree, the unique handiwork of a leopard.

Several theorists have proposed that the human brain is the outcome of a cognitive arms race set in motion by the Machiavellian intelligence of our primate forebears. There's only so much brain power you need to subdue a plant or a rock, the argument goes, but the other guy is about as smart as you are and may use that intelligence against your interests. You had better think about what he is thinking about what you are thinking he is thinking. As far as brain power goes, there's no end to keeping up with the Joneses.

My own guess is that a cognitive arms race by itself was not enough to launch human intelligence. Any social species can begin a never-ending escalation of brain power, but none except ours has, probably because without some other change in lifestyle, the costs of intelligence (brain size, extended childhood, and so on) would damp the positive feedback loop. Humans are exceptional in mechanical and biological, not just social, intelligence. In a species that runs on information, each faculty multiplies the value of the others. (Incidentally, the expansion of the human brain is no evolutionary freak crying out for a runaway positive feedback loop. The brain tripled in size in five million years, but that is leisurely by evolutionary timekeeping. There was enough time in hominid evolution for the brain to shoot up to human size, shrink back down, and shoot up again several times over.)

A third pilot of intelligence, alongside good vision and big groups, is the hand. Primates evolved in trees and have hands to grasp the branches. Monkeys use all four limbs to run along the tops of branches, but apes hang from them, mainly by their arms. They have put their well developed hands to use in manipulating objects. Gorillas meticulously dissect tough or thorny plants to pick out the edible matter, and chimpanzees use simple tools such as stems to fish out termites, rocks to bash open nuts, and mashed leaves to sponge up water. As Samuel Johnson said about dogs walking on their hind legs, while it is not done well, you are surprised to find it done at all. Hands are levers of influence on the world that make intelligence worth having. Precision hands and precision intelligence co-evolved in the human lineage, and the fossil record shows that hands led the way.

Finely tooled hands are useless if you have to walk on them all the time, and they could not have evolved by themselves. Every bone in our bodies has been reshaped to give us our upright posture, which frees the hands for carrying and manipulating. Once again we have our ape ancestors to thank. Hanging from trees calls for a body plan that is different from the horizontal four-wheel-drive design of most mammals. Apes' bodies are already tilted upward with arms that differ from their legs, and chimpanzees (and even monkeys) walk upright for short distances to carry food and objects.

Fully upright posture may have evolved under several selection pressures. Bipedal walking is a biomechanically efficient way to retool a treehanging body to cover distance on the flat ground of the newly entered savanna. Upright posture also allows one to peer over grass like a marmot. Hominids go out in the midday sun; this zoologically unusual work shift brought in several human adaptations for keeping cool, such as hairlessness and profuse sweating. Upright posture might be another; it is the opposite of lying down to get a tan. But carrying and manipulation must have been crucial inducements. With the hands free, tools could be assembled out of materials from different locations and brought to where they were most useful, and food and children could be carried to safe or productive areas.

A final usher of intelligence was hunting. Hunting, tool use, and bipedalism were for Darwin the special trinity that powered human evolution. "Man the Hunter" was the major archetype in both serious and pop accounts through the 1960s. But the macho image that resonated with the decade of John Glenn and James Bond lost its appeal in the feminist-influenced small planet of the 1970s. A major problem for Man the Hunter was that it credited the growth of intelligence to the teamwork and foresight needed by men in groups to fell large game. But natural selection sums over the lives of both sexes. Women did not wait in the kitchen to cook the mastodon that Dad brought home, nor did they forgo the expansion of intelligence enjoyed by evolving men. The ecology of modern foraging peoples suggests that Woman the Gatherer provided a substantial portion of the calories in the form of highly processed plant foods, and that requires mechanical and biological acumen. And, of course, in a group-living species, social intelligence is as important a weapon as spears and clubs.

But Tooby and DeVore have argued that hunting was nonetheless a major force in human evolution. The key is to ask not what the mind can do for hunting, but what hunting can do for the mind. Hunting provides sporadic packages of concentrated nutrients. We did not always have tofu, and the best natural material for building animal flesh is animal flesh. Though plant foods supply calories and other nutrients, meat is a complete protein containing all twenty amino acids, and provides energy-rich fat and indispensable fatty acids. Across the mammals, carnivores have larger brains for their body size than herbivores, partly because of the greater skill it takes to subdue a rabbit than to subdue grass, and partly because meat can better feed ravenous brain tissue. Even in the most conservative estimates, meat makes up a far greater proportion of foraging humans' diet than of any other primate's. That may have been one of the reasons we could afford our expensive brains.

Of course no one really knows whether these four habits formed the base camp for the ascent of human intelligence. And no one knows whether there are other, untried gradients to intelligence in biological design space. But if these traits do explain why our ancestors were the only species out of fifty million to follow that route, it would have sobering implications for the search for extraterrestrial intelligence. A planet with life may not be enough of a launching pad. Its history might have to include a nocturnal predator (to get stereo vision), with descendants that switched to a diurnal lifestyle (for color) in which they depended on fruit and were vulnerable to predators (for group living), which then changed their means of locomotion to swinging beneath branches (for hands and for precursors to upright posture), before a climate shift sent them from the forest into grasslands (for upright posture and hunting). What is the probability that a given planet, even a planet with life, has such a history?