August 19, 2006
Q: What would intelligent life on other worlds look like? - Layperson
A: In the movies, intelligent extraterrestrials generally look like us: theyâ€™re anthropomorphic. Mind you, the specifics of their appearance hinge on their motives. Good aliens, such as E.T. and the friendly little guys from Close Encounters of the Third Kind, are child-like and cuddly, while bad aliens (by far the majority) are often reminiscent of reptiles and insects â€“ creatures that we like to either turn into shoes or swat. There are occasional exceptions to these rules, such as R2 D2, from Star Wars, who looks like a canister vacuum cleaner that's lost its hose. But most extraterrestrials hardly deviate from the basic blueprint of Homo sapiens. Consequently if you ask a guy on the street to draw an alien, chances are he'll sketch a rather humanoid, grey, small, hairless, unwilling to smile, and shamelessly nude.
What about little, green aliens? Our blood is red because the iron in our hemoglobin is what lets it bind oxygen, and oxidized iron (rust) is red. Geen-blooded aliens who could breath the same atmosphere as us would probably have a copper-based hemoglobin-equivalent, as copper oxidizes green (see the statue of liberty). Copper's affinity for oxygen, is stronger than iron and would make a better oxygen-carrying blood. Copper-based blood could give a being with this type of blood, a green or blue-green skin color.
In fact, as a tour of any zoo will convince you, there's a breathtakingly wide variety of creature designs that work on our planet – and presumably on theirs, too. So why would intelligent extraterrestrial life look anything like us? Probably it wouldn't, although there's a mechanism known to biologists as convergent evolution- the evolution of species from different taxonomic groups toward a similar form; the development of similar characteristics by taxonomically different organisms- that argues for at least a bit of a resemblance. Convergent evolution is the idea that as nature works its way down a Darwinian path, selection favors good designs that enhance survivability.
These designs can be arrived at via different evolutionary routes. For instance, predators in the ocean will have a survival advantage if they can swim fast. Consequently many of them have a torpedo shape, simply because this streamlined form gives them a better chance at snagging a dinner. Dolphins and barracudas look similar in silhouette, even though they evolved from very different forebears. Their shapes are the result of convergent evolution. Some biologists have suggested that the same may be true of humans and extraterrestrial life -- that the human body plan is a good design for an intelligent creature. We have free appendages (these are known by the technical term: “arms”) terminated by graspy little hands useful for writing, wielding tools, or ferrying snacks to our mouths. We also have two eyes with overlapping vision, providing 3-D views of the world that facilitate tool use. The eyes are located high up, permitting us to peer over grass and brush to find a mate or a meal. And the list goes on. Humans, in many ways, are a reasonably functional design for a technically sophisticated creature.
But it is a bit extreme to maintain that we are the best design, and therefore convergent evolution will ensure that an intelligent alien looks like your brother-in-law. After all, an extra set of arms might be useful, as would an eye in the back of our heads. A double spine might allow faster and easier walking, and a few extra digits on each hand could make for better tool use or piano playing. The bottom line is that any biological creature we find that's at least as clever as we are might have, some features in common with us (two eyes, instead of one, for instance). But there's little reason to think our own design is so wonderfully optimal that all thinking beings will have converged on it. Intelligent extraterrestrials may look vaguely humanoid, but no more than vaguely.
Most scientists hold that if extraterrestrial life exists, its evolution would have occurred independently in different places in the universe. An alternative hypothesis, held by a minority, is panspermia, which suggests that life in the universe could have stemmed from a smaller number of points of origin, and then spread across the universe, from habitable planet to habitable planet. These two hypotheses are not mutually exclusive.
Given their relative abundance and usefulness in sustaining life it has long been assumed that life forms elsewhere in the universe will also utilize these basic components. However, other elements and solvents might be capable of providing a basis for life. Silicon is usually considered the most likely alternative to carbon, though this remains improbable. Silicon life forms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as planets closer to the sun. Life forms based in ammonia rather than water are also considered, though this solution appears less optimal than water .
Bracewell has proposed that life could make use of the chemistry of the
silicon atom rather than the carbon atom. Silicon based organisms would, for
example, breathe out silicon dioxide (sand) instead of carbon dioxide. The rock
eating creature has often been suggested as a product of this biological system.
[An example of this can be seen in the STAR TREK
Indeed, technically life is little more than any self-replicating reaction, which could arise in a great many conditions and with various ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive. Suggestions have even been made that self-replicating reactions of some sort could occur within the plasma of a star, though it would be highly unconventional, since plasma is essentially the fourth state of matter, where electrons are not bound in their orbits around atomic nucliae.
Theoretical evolution and morphology
Along with the biochemical basis of extraterrestrial life, there remains a broader consideration of evolution and morphology. What might an alien look like? Science fiction has long shown a bias towards humanoid or (often in the case of villains) reptilian forms. The classical alien is light green or grey skinned, with a large head, and the typical four limb and two to five digit structure—i.e., it is fundamentally humanoid with a large brain to indicate great intelligence. Other subjects from animal mythos such as felines and insects have also featured strongly in fictional representations of aliens.
In considering the subject more seriously, a useful division has been suggested between universal and parochial (narrowly restricted) characteristics. Universals are features which have evolved independently more than once on Earth (and thus presumably are not difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. These include flight, sight, photosynthesis and limbs, all of which have evolved several times here on Earth with differing materialization. There are a huge variety of eyes, for example, many of which have radically different working schematics as well as different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, by contrast, are essentially arbitrary evolutionary forms which often serve little utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Parochials include the five digits of mammals, the genitalia and sexual mechanics of animals, as well as the curious and often fatal conjunction of the feeding and breathing passages found within many animals.
A consideration of which features are ultimately parochial challenges many taken for granted notions about morphological necessity. Skeletons, in some form, are likely to be replicated elsewhere, yet the vertebrate spine—while a profound development on Earth—is just as likely to be unique. Similarly, it is reasonable to expect some type of egg laying amongst off-Earth creatures but the mammary glands which set apart mammals may be a singular case.
The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists do stress that the enormously heterogeneous nature of Earth life foregrounds even greater variety in space, others point out that convergent evolution dictates substantial similarities between Earth and off-Earth life. These two schools of thought are called "divergionism" and "convergionism", respectively .
Several physics laws would govern an alien or even an earthly body-heat transference, tensile strength, surface area, volume area, fluid pressure, and so on. A human over 10-12 feet tall would start to look different from smaller humans because of these laws. That is why the dinosaurs were built the way they were.
In short, if alien life formed under similar conditions as on earth, alien life might look similar or identical to us. If the conditions were different, they alien life could very well look different. Even now, humans are growing taller and heavier. A human say in 10,000 years may look very different from the current human population.
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