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Skywatcher

The Shape of Intelligence
Why E.T. Should Look Familiar

Seth Shostak

Once life gets started, there's a bit of a wait before it develops interesting mental capability and spawns thinking beings. On Earth, it took nearly four billion years for life to progress from the first organisms to Homo sapiens. But the time scale is less important than the probability. How inevitable was this progression from protist to professor? Could humans be the only reasoning creatures in the Galaxy, the only ones able to invent science and write poetry, simply because the evolution of intelligence is a chance event of very low probability?

This is an unsettled, and controversial, question. At first blush, it might appear that mankind, the crown of creation, marks the inevitable endpoint of all that's gone before. It is a common belief that, given a life-bearing planet and enough time, intelligent beings will always arrive on the scene. But as evolutionary experts are fond of pointing out, the particular species that populate the Earth today were not inevitable at all. They are the result of chance, contingency, and circumstance. In his book Wonderful Life, paleontologist Stephen Jay Gould emphasizes that if we could rewind life's tapes and start the drama of evolution anew, slight differences in climatic, tectonic, and even astronomical circumstance would result in a different assortment of flora and fauna today.

That point is indisputable. As an obvious example, if the dinosaurs and the majority of all other species had not been wiped out by the impact of a large comet or asteroid 65 million years ago, mammals would not have had their big chance to speciate, radiate, and produce the readers of this magazine. The Cretaceous extinction episode was merely one fork in the multi-branched road that threads from the dim mists of the archaebacteria to the bright lights of the late twentieth century. On the billions of other Earth-like planets that we can expect to exist in our Galaxy, the evolutionary roads will most assuredly have meandered along different routes.

Harvard zoologist Ernst Mayr, while optimistic about the frequent development of life on other worlds, believes that intelligent life is nearly miraculous, and therefore rare. His principal argument is that an enormously large number of life forms have developed on this planet, but only one is truly intelligent. By 500 million years ago, virtually all the blueprints of modern life had been drawn, but as Mayr notes, "Of the 40 or so original phyla of animals, only one, that of the chordates, eventually gave rise to intelligent life.... In conflict with the thinking of those who see a straight line from the origin of life to intelligent man,...at each level of this pathway there were scores, if not hundreds, of branching points and separately evolving phyletic lines, with only a single one in each case forming the ancestral lineage that ultimately gave rise to Man." We are an accident, a highly unlikely accident, according to Mayr. If he's right, then we may be the smartest creatures in the Galaxy, no doubt a comfort to some readers.

However, the question is not whether other worlds have cooked up exact replicas of humans. That occurs only in low-budget science fiction. The question is whether intelligence of the sort that can understand and make use of science has evolved elsewhere. How often will nature produce creatures with the characteristics that set us apart from "mere animals"-- consciousness, cognition, and creativity?

The answer, despite Mayr's somber assessment, may be "often." Intelligence has sprung up on our planet in response to competitive pressure. When the first humans clambered out of the African savannas, they confronted an environment in which the number of species had reached an all-time high, twice as many as when the dinosaurs held sway. The world was a brutally competitive place. Brain power, like keen eyesight or sensitive hearing, conferred an advantage in this heightened battle for survival. Intelligence promotes adaptability. It may be a common solution to the challenges of a tough, terrestrial environment populated by sophisticated competitors. If life on the other planets has similarly become diverse and competitive, it may also produce intelligent creatures.

This argument for smart E.T.s is based on the concept of convergent evolution. In nature, a common set of environmental circumstances will result in animals whose designs are analogous. For example, in the viscous undersea environment, we find a plethora of torpedo-shaped animals, for the obvious reason that streamlining increases speed. Both the ancient reptilian ichthyosaur and the modern mammalian dolphin are built to the torpedo plan, although their evolutionary origins are radically different. Oceanic creatures on distant planets would surely be streamlined, too. Austrian astronomers Mircea and Jorg Pfleiderer point out that the back legs of both cats and crickets are built with the same hinge and lever arrangements. This is another example of how nature concocts analogous designs to solve a common problem. It is somewhat akin to the automobile industry. After a century of evolution, most of the cars on the road today are strikingly similar, with four wheels and piston engines which are usually situated up front.

For those who are convinced that intelligence is a universally applicable solution to a frequently occurring environmental problem, such as intense interspecific competition, convergent evolution will see to it that we have some smart company in the cosmos. Paleontologist Dale Russell has played out this idea by considering what would have happened if the dinosaurs hadnÕt bought the farm. He has "evolved" a small Cretaceous dinosaur, Stenonychosaurus, to the present day. His rendition of this putative creature looks more like a human than like its Stenonychosaurus predecessor, and the suggestion is that smart dinosaur descendants might be running the planet now if astronomical catastrophe had not occurred 65 million years ago.

Ernst Mayr's counter-argument is that if convergent evolution were applicable to intelligence, then sentient beings would have appeared in more than one species, in the same way that eyes, hinged legs, and torpedo-shaped torsos have. Humans would not be the only intelligent creatures to have walked the Earth. Astronomer Frank Drake rebuts this with the observation that someone has to be first. Is it really surprising that humans look around and note that we're the only smart kids on the block? Is the fact that we are the single intelligent species among the hundreds of millions that have sprung up on this planet remarkable, or merely a trivial consequence of being the first?

While the battle rages about whether life frequently spawns intelligence, we have to face the uncomfortable fact that argument alone is unlikely to settle the matter. Clearly, if intelligence is rare, then we are highly exceptional. But both modesty and the Principle of Mediocrity caution us about making such an assertion. And besides, it's an unsettling and unsatisfying thought that the crew of the Starship Enterprise might boldly go where no man has gone before, only to encounter dumb beasts.

Let us continue to suppose that we truly share the universe with other intelligences. Would they be, as in Star Trek, human-looking folk clad in Greek tunics, spouting insipid philosophy in perfect English? Or would they have insect-like faces that only a plastic surgeon could love?

The only extraterrestrials that Earthlings have yet seen are those appearing at the local cinema. But Hollywood aliens are like winter vegetables: there isn't much variety. Traditionally, most have been recognizably human, with heads, eyes, and multi-fingered hands. This preference for anthropomorphic aliens can be partially attributed to the practical necessity of designing a rubber suit for a human actor. However, modern screen aliens are frequently animated by computers rather than protoplasm. Hollywood's special effects teams can now endow extraterrestrial folk with convincing tentacles, steely teeth, and four quarts of mucus. Yet they often still look a little like the guy next door.

Does this reflect some sort of universal truth? Is there only one way to build an intelligent critter? Should we expect E.T. to look like us?

Let's consider what good engineering practice can tell us about an intelligent extraterrestrial's appearance. To begin with, we have asserted that any worthy E.T. must be able to hold up his side of the conversation. Consequently, we can assume a high degree of complexity, in a biological sense. Capability implies complexity. To prove this point, note that in the late 1970s, microprocessor chips combined thirty thousand transistors on a single, small slab of silicon. These chips could handle simple word processing and modestly sized spreadsheets. Two decades later, Pentium chips incorporated one hundred times as many transistors and were able to dazzle their owners with snazzy graphics and multiple application windows (usually in the service of word processing or spreadsheets). The universal link between complexity and ability ensures that E.T. will be multicellular.

This fairly obvious fact of intelligent life rules out the possibility that our alien pals will assume the shape of a malevolent mound of Jell-O, such as Steve McQueen confronted in The Blob. A single, large cell (as the carnivorous blob seemed to be) would surely be stupid. But of greater concern, the bulk of its protoplasmic innards would be situated far from its outer membrane. This would fatally hinder the exchange of gases and nutrients needed for survival, and the central parts of that blob would become gangrenous. Another liability for a single-celled sentient is the fact that any disease that succeeds in killing one cell will demolish the entire organism. Such critters are awfully vulnerable.

Clearly, enormous blob-like cells are non-starters. And at the other end of the scale, there's a limit on how small a cell can be, too. Mycoplasma bacteria, organisms that measure about 10 millionths of an inch across, are typical of nature's best miniaturization efforts here on Earth. A substantially smaller cell would have a volume too puny to support the necessary chemistry of existence, a fact that will also apply to aliens. Consequently, since we anticipate that the complexity required of a thinking being will involve at least billions of cells, there's no chance that E.T. will fit on the head of a pin.

Another reason to expect that intelligent extraterrestrials will be large enough to trip over derives from simple geometric scaling. If you take any animal and somehow shrink it to half its normal dimensions (that is, reduce its size by a factor of two), you'll find that its surface area--the amount of skin needed to cover it--has decreased by the square of the size change, or a factor of four. In other words, the shrunken animal ends up with a higher epidermis-to-endoplasm, or skin-to-innards ratio. Like an automobile radiator, it will lose body heat quickly. For this reason, hummingbirds eat constantly and copiously, whereas elephants don't. This sets a limit on how small a warm-blooded animal can be. If they're less than a certain size, they won't be able to eat fast enough to avoid a fatal cooling off.

This is relevant because E.T. is probably warm-blooded. Thermostatically controlled creatures are the high-performance members of the animal kingdom. They are both more active than their cooler brethren and better able to adjust to climate variations.

So if you believe that E.T. prefers the faster lifestyle and greater adaptability that come with being warm-blooded, then it's unlikely he'll be much smaller than a hummingbird. Indeed, you should expect him to be considerably larger. If he's bird-sized, he'll probably be bird-brained, and not terribly bright. He'll also be spending all his time the way that hummingbirds and mice spend theirs: foraging for their next meal.

The scaling laws set a limit to E.T.'s maximum size too, since they govern his agility. Imagine enlarging a creature to twice its normal dimensions. Strength, which depends on muscle cross-section, will grow by the square of the size increase, or a factor of four. This sounds pretty good until you consider that the scaled-up creature's weight, which goes up as the cube of the size, has increased by a factor of eight. The animal now has a power-to-weight ratio that's half of what it was, thus reducing physical ability. This simple scaling explains why an ant can carry many times its own weight, while an elephant can't. If you drop an ant from ten times its height, it will hardly notice (mind you, ants barely have a brain, and don't seem to notice much of anything). Drop an elephant from ten times its height, and you will be faced with an unsettling clean-up.

Very large animals are impressive, but not often for their agility. They depend on massive appendages to stand up and move around. (For this reason, scaled-up ants, spiders, and grasshoppers, such as occasionally appear in Hollywood creature features, are impossible. The relatively spindly legs of these giant insects wouldn't allow them to get off their bellies.)

Elephants, whose legs are chunky enough to be turned into umbrella stands, are currently the largest creatures walking the Earth. The only living entities with greater heft hang out underwater (whales) or underground (fungi). Either environment provides help in supporting Brobdingnagian bulk. But for obvious reasons, such subsurface giants aren't likely to develop astronomy or make sophisticated tools. These arguments suggest that if E.T. inhabits an Earth-like planet, it's most probable that, size-wise, he'll be somewhere between ten pounds and ten tons. This might be scaled up or down a bit depending on his planet's gravitational tug, of course.

In the discussion so far, we've made the implicit assumption that E.T. is an animal. Animals have mobility, and we expect that ambulatory life forms will be the first to develop intellect. A dog, which has to find and catch its next meal, requires more brain power than a dogwood. So our sentient extraterrestrial will have some method to get up and go. On Earth, most locomotion for larger animals is afforded by appendages, either legs, wings, or fins.

A detectable, intelligent E.T. with fins is unlikely. Underwater creatures might never put together the technology required to visit or signal distant worlds. (One impediment is the fact that radio waves don't penetrate sea water.) Marine environments have also been characterized as "too easy," and not prone to produce smart creatures in a hurry. In the ocean, movement is relatively simple, the temperature changes only slowly, and the weather's always the same. Most probably it will be land animals that first develop substantial brain power--intelligence will be the evolutionary response to the climatic and topographic rigors of the high and dry. In support of this idea, note that dolphins, considered to be among the brightest of briny folk, didn't evolve underwater. They were land-dwelling mammals that returned to the sea. The stark facts are that most marine life is dumb, and even the celebrated cetaceans haven't produced either science or great literature. So it seems safe to say that space-faring extraterrestrials who live underwater, such as featured in the movie The Abyss, are all wet.

Might E.T. have wings? Flying aliens are certainly a possibility, although the heaviest airborne animals on Earth weigh in at 35 pounds or so, and animals of this heft seem too small to have a great deal of intelligence. On the other hand, planets with thick atmospheres, lower gravity, or especially oxygen-rich air to turbocharge metabolism, might conceivably spawn some soaring intelligence.

Having considered the air and the sea, we return to everyone's favorite extraterrestrial habitat: dry land. For locomotion on terra firma, legs and their derivatives, arms, are the equipment of choice. No animals have wheels, despite their efficiency. Wheels function well only on prepared geography, either rails or roads. (Snakes manage to get around without either appendages or wheels, but snakes don't make tools.) The number of appendages, normally four for the bigger animals on Earth, should be large enough to permit both movement and manipulation (thus, more than two), and small enough not to burden E.T.'s brain with an enormous processing load (less than, say, a dozen, assuming they have individually movable digits). Insects sport six appendages, and dominate the species count on our planet. So six appendages is a number nature surely finds agreeable. The fact that humans, hippos, and hyenas all have four extremities is the accidental consequence of the evolution of backboned animals from a four-finned ancestral fish, Euthenopteron. Had Homo sapiens been blessed with six appendages, we would be more adept at piano duets and handball.

We can also expect an intelligent alien to have eyes, useful for gathering information about the local environment. Indeed, eyes are so handy that nature independently developed them in many species on Earth. They seem to be an inevitable product of evolution in a world flooded with light. It's most likely that advanced life will be found on planets orbiting sun-like stars. In such cases, the spectral quality of the light illuminating these worlds will be similar to what we experience on Earth. If the atmosphere lets at least some of this sunshine in, we can anticipate that E.T. will have an eyeball or two. Actually, two is much better than one, as a pair will offer depth perception, always an advantage when it comes to catching dinner. The benefit of having more than two needs to be weighed against the cost of the additional cerebral processing. None of the higher animals on Earth have opted for more than two eyes.

Hearing is another universally useful talent because it permits communication in environments, such as jungles, where sight-lines are short. It also allows you to warn someone whose back is turned. Smell, although not overly developed in humans, is widely used by other animals, and can convey highly specific information. The disadvantage of relying on scent alone is that odors are wafted to you via diffusion in the air, and are therefore hard to pinpoint. It's easy to know that there's a dead skunk somewhere along the highway. It's less easy to know exactly when your car passes it on a dark night.

The gist of this discussion is that all the human senses also make sense for E.T. And it's only good engineering practice to put the sensory organs--eyes, ears, nose, or their extraterrestrial equivalents--up high where they can get a better "view," and to situate them close to the brain to minimize reaction time. In other words, E.T. will have a head, and his brain will be in it.

Spring 1998

Vol. 51:2