Extraterrestrial life

:This article is about the scientific study of extraterrestrial life; for treatment in popular culture, see Extraterrestrial life in popular culture.

Possible basis of extraterrestrial life

Biochemistry

All life on Earth is based on the building block element carbon with water as the solution in which bio-chemical reactions take place. 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 (Main article: Alternative biochemistry). Silicon is usually considered the most likely alternative to carbon, though this remains improbable. Life forms based in ammonia rather than water are also considered, though less frequently. Nor can we reject the possibility that a completely new substance may be found that may react in a similar way to carbon or that wholly unique, non-chemical life-forms may possibly flourish through exotic physics.

Related Topics:
Carbon - Water - Life - Alternative biochemistry - Silicon - Ammonia

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Along with a building block element and a solvent, life also requires an energy source. Energy from a parent star is the most obvious source for extraterrestrial life but this is not the only possibility, as the example of terrestrial extremophiles shows. Geothermal energy from a planet's interior, for instance, may drive sub-surface or oceanic life, while tidal flexing (e.g., for bodies orbiting a gas giant) provides another possible motor to sustain living things.

Related Topics:
Star - Extremophiles - Geothermal - Tidal flexing

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The scientific study of the possible biochemical basis for extraterrestrial life is often called xenobiology.

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See also: Back-contamination

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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 (in the case of villains) reptilian forms. The classical alien is light green or grey skinned, with an enormous head, small body, 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 our animal mythos (felines, insects) have also featured strongly in fictional representations of aliens. While such bias is predictable, it is also curiously unimaginative and almost certain to be proven wrong should human beings encounter extraterrestrials.

Related Topics:
Evolution - Morphology - Reptilian - Classical alien - Brain - Mythos - Felines - Insects

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In considering the subject more seriously, a useful division has been suggested between universal and parochial 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 eye above nose above mouth structure of the animal face (science fiction almost always employs this appearance though it is largely arbitrary) as well as the curious and often fatal conjunction of the feeding and breathing passages found within many animals.

Related Topics:
Been suggested - Flight - Sight - Photosynthesis - Limbs - Visual spectrum - Infrared - Polarity - Echolocation - Mammals

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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.

Related Topics:
Skeletons - Vertebrate - Spine - Mammary glands

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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 http://www.daviddarling.info/encyclopedia/E/etlifevar.html.

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