Panspermia is a theory (more directly described as a hypothesis, as there is no compelling evidence yet available to support or contradict it) that suggests that the seeds of life are prevalent throughout the universe and life on Earth began by such seeds landing on Earth and propagating. The theory has origins in the ideas of Anaxagoras, a Greek philosopher.
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. 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.
Another objection to Panspermia is that bacteria would not survive the immense heat and forces of an impact on earth; no conclusive conclusions (whether positive or negative) has been reached yet on this point.
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.
Some have taken the theory as an answer to those arguing the improbability of the origin of life, in that wherever life first began, it spread throughout the universe by panspermia.
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.
See also: Theories of the origin of humans