Panspermia is a theory that suggests that the seeds of life are prevalent throughout the universe and are distributed by meteoroids, asteroids and planetoids. According to that theory, life on Earth began by such seeds landing on Earth and propagating. The theory has origins in the ideas of Anaxagoras, a Greek philosopher and was developed later by British astronomers Sir Fred Hoyle and Chandra Wickramasinghe.
Panspermia proposes that life that can survive the effects of space, such as extremophile bacteria, become trapped in debris that is ejected into space after collisions between planets that harbor life and Small Solar System Bodies (SSSB). Bacteria may travel dormant for an extended amount of time before colliding randomly with other planets or intermingling with protoplanetary disks. If met with ideal conditions on a new planets' surfaces, the bacteria become active and the process of evolution begins.
There is some evidence to suggest that bacteria may be able to survive for very long periods of time even in deep space (and may therefore be the underlying mechanism behind Panspermia). Recent studies out of India have found bacteria at heights greater than 40 km in Earth's atmosphere where mixing from the lower atmosphere is unexpected, while Streptococcus mitus bacteria that had accidentally been taken to the moon on the Surveyor 3 spacecraft in 1967, could easily be revived after being taken back to earth three years later.
The link between Comets and Panspermia was investigated further with a NASA Launch performed by NASA beginning in 2004, entitled "The Stardust Mission". Ion Propulsion spacecraft was loaded with machinery to bring back lab samples from the tail of a comet. Glycine and other building blocks that have been found in comets. Comets travel through space with these frozen potentially reproductive materials, and the tail of the comets appear when gases melt in the presence of our sun.
However, a consequence of panspermia is that life throughout the universe would have a surprisingly similar biochemistry, being derived from the same ancestral stock. So the high-altitude bacteria might be expected, whether of earth or extra-terrestrial origin, to have a biochemistry similar to terrestrial forms. This is not resolvable until life on another planet can have its chemistry analysed. Of material definitely known to originate off-earth, analysis of the rock sample known as ALH84001, generally regarded as originating on Mars, suggests it contains artifacts that may have been caused by life forms. This is the only indication of extraterrestrial life to date and is still widely disputed.
This theory has been explored in a number of works of science fiction, notably Jack Finney's Invasion of the Bodysnatchers (twice made into a film) and the Dragonrider books of Anne McCaffrey. In John Wyndham's book, The Day of the Triffids, the first person narrator, writing in historical mode, takes care to reject the theory of panspermia in favour of the conclusion that the eponymous carnivorous plants are a product of Soviet biotechnology.
Some works of science fiction advance a derivative of the theory as a rationalisation for the improbable tendency of fictional extra-terrestrials to be strongly humanoid in form.
Theories about extraterrestrial life