They're gray, big-eyed, and smoother than a buffed Maserati. They're aliens à la Hollywood. Lacking noses, ears, hair, and a sense of humor, these short-statured creatures are omnipresent in sci-fi films and TV.
Not surprisingly, many members of the public assume that if we ever interact with real extraterrestrials, they'll probably resemble these colorless critters. Indeed, if you look at the drawings of aliens made by people who believe that Earth is under saucer attack, you'll quickly note that most of these invaders fit the Tinseltown mold.
But you have to admit: the grays are highly anthropomorphic. They look so much like us, if a squad of these cosmic beings moved in next door, they'd eventually be hit up for dues by the homeowners association.
In a movie it's often important to have aliens whose gestures and facial expressions can be "read" by humans. And in the days before sophisticated computer animation, most extraterrestrial bit players were guys in rubber suits. Such practical considerations forced Hollywood's hand when it came to aliens -- they look like us for good reasons. Logistical reasons.
In fact, a few biologists think that Hollywood may, by accident, have it right. They argue that Homo sapiens actually conforms to some sort of optimal design for a sentient species. Convergent evolution, a well-known selection mechanism ensuring that both dolphins and barracudas are built like torpedoes, will constrain intelligent aliens to have a vaguely human form.
But come on! Are two eyes, four appendages and an upright posture really essential for any creature that can ace the galactic SAT's?
Maybe not. In fact, I'd venture that any aliens we ever detect or (less likely) encounter will look quite different than this self-referential stereotype.
First, note that any extraterrestrials able to get in touch will be far beyond our technical level. After all, alien Neanderthals won't rocket to our world, nor will they transmit signals we might pick up with our SETI experiments. Detectable extraterrestrials will be an older intelligence than ours.
Most people know this. Consequently, guesses about alien appearance usually proceed by attempting to forecast our own descendants. We assume that advanced aliens will resemble advanced humans. Hence the hairless, small-bodied and big-headed grays.
But this approach is obviously dicey. Imagine if the dinosaurs had tried picturing the rulers of their planet 100 million years hence. They'd undoubtedly envision these creatures as ... dinosaurs! Conceiving of aliens as polished versions of ourselves is appealing, but unconvincing.
A more illuminating approach would be to conceive of the aliens, not as our distant descendants, but as our future intellectual progeny. This suggests that they won't be biological at all. E.T. won't be protoplasm.
The argument derives its power not from any detailed analysis but simply on the basis of time scales drawn from our own experience. Consider how quickly we might transition from a technical society to one where the most advanced intelligence is designed, rather than born.
Here's the relevant history. In 1900, practical radio became reality, giving humans the ability to signal their presence to the cosmos. Forty-five years later, the first computers were being soldered together. Sometime this century, it seems likely that we will construct strong artificial intelligence -- machines that can think as well as we can.
So in two centuries, we go from biological beings launching their first detectable signals into space to synthetic intelligence. And the latter will evolve incredibly quickly because the machines will design their own successors.
Assuming that this technical pathway also applies to other worlds, it's clear that if we find Klingons, they are not likely to be carbon-based, organic creatures. The time window during which detectable alien intelligence is biological is very, very short. Machine intelligence -- which could be durable and long-lasting far beyond the limits of a biological species -- will dominate the universe.
NASA's Kepler telescope is busy tracking down habitable planets around other stars. It's likely that, within a year, it will discover other worlds that are very much like our Earth. Such planets would be obvious candidates for incubating life, and possibly intelligent life. But the incubator is not necessarily where intelligence will stay. It will, I think, leave the cradle rather quickly.
In other words, biological intelligence might be only a stepping stone to something far cleverer, something that is both longer-lived and more widespread than its protoplasmic precursors.
There's a lesson in this: In our search for intelligence beyond the bounds of Earth, we should be careful not to be dinosaurs looking for other sauropods.