Panspermia

Panspermia (Greek: pan "all" and spermia "seed") is the hypothesis that life, or the "seeds" of life, is all around the universe and was transported to Earth, either as life or seeding life. Often the hypothesis is that either rich organic material or extremely basic microbes, from dust clouds or other planets, made their way to Earth via meteors, survived the entry and thrived on Earth. Although the theory originally suggested that life was everywhere in space, waiting for a planet to fall upon like a seed into fertile ground, today it refers to any theory of an extraterrestrial origin of life on earth, for instance that life developed first on another planet and was brought here by meteorite or other space debris. The less common term exogenesis is also used for the suggestion that life originated somewhere other than on Earth and later spread here, by any method, natural or artificial, without necessarily being widespread in the universe.

As a hypothesis that life might have started outside of Earth, panspermia contradicts Earth-based abiogenesis (although the seeding of organic matter did need to come from somewhere, i.e. from extraterrestrial abiogenesis), but is not in conflict with the theory of evolution which says nothing about the origin of life. Notwithstanding this, some members of the intelligent design community have attempted to use the hypothesis to claim that some scientists, such as Francis Crick, support ID. Given that ID is (apparently) an attempt to provide an alternative to the theory of evolution, these claims may be disingenuous. Moreover, this is a possible solution to the creationists' first cause: natural rather than god-given origin. However, it merely pushes the problem back one step, for now we must ask where the seeds came from.

The idea that life could have existed somewhere else before coming to earth is respectable although lacking in evidence, but there are many crazier versions, such as the idea of directed panspermia, that aliens deliberately seeded life on Earth for some purpose.

Theory
The usual materialistic version of the theory holds that a simple form of life arrived on the earth early in the planet's existence, probably during the period around 4.1-3.8 billion years ago, when there was extensive bombardment by meteorites. Life could have been carried by asteroids, comets, or similar space debris, or drifted through space as microscopic particles.

For panspermia to be plausible, there are two main requirements. Firstly that life arose somewhere else in the galaxy in reasonable travelling distance from Earth, and secondly that this life was sufficiently robust to survive travelling through the extreme temperatures, high radiation, and vacuum of space, as well as not be destroyed by the heat of reentry or impact with the earth.

It's not clear what the odds are of life arising on a planet (see Drake Equation), but there's no specific reason it couldn't have originated on another planet: there is no definite reason why extraterrestrial life couldn't exist.

There is also some evidence that simple microorganisms could survive the extreme conditions in space, just as they can survive in the most extreme conditions that the Earth has to offer. This has become a practical issue for the space exploration program as it searches for life on other planets.

Other factors have been offered as evidence for panspermia, such as the dependency of life on relatively rare elements like molybdenum, posited by panspermia advocates Francis Crick and Leslie Orgel. This may point to an extraterrestrial origin somewhere that molybdenum is more common, or simply to evolution using a random, suboptimal method (which is common).

The other argument against panspermia would be a probabilistic one: if it's as likely for life to arise on the earth as anywhere, and it's unlikely for it to travel long distances, then why not take the simpler option and assume life originated here?

History
The idea has been traced back to the Pre-Socratic Greek philosopher Anaxagoras of Clazomenae in the 5th century BCE, who mentioned the universe as being made up of various kinds of matter including seeds (spermata in Greek). However, for centuries the dominant theory was creationism, that God created all life and did it here on Earth.

Charles Darwin's theory of evolution provided a mechanism for the development of life in its complexities, but said nothing about its source. As a result, the idea of panspermia became popular in the late 19th and early 20th century, due to a belief that the early earth would be too hot to sustain life and scepticism about how life could evolve from non-life; 19th century proponents included Hermann E. Richter, Sir William Thompson (Lord Kelvin), Jacob Berzelius, and Hermann Helmholtz. Evidence advanced for panspermia included the presence of carbon, hydrogen, and organic molecules in asteroids and the large amount of carbon observed in space. The 19th century French bacteriologist Emile Roux seemed to prove that microbes could survive ultraviolet light and vacuum, providing further support; but his experiments were later shown to be flawed.

Svante Arrhenius was perhaps most responsible for the development of panspermia as a theory, in an article "The Distribution of Life in Space" (1903), although he suggested that microbes could be transported through space by radiation pressure rather than carried on asteroids. However, in the mid 20th century, abiogenesis (origin of life on Earth from non-living molecules) became the primary theory of the origin of life, thanks to the work of Alexander Oparin, Stanley Miller, and others. But despite being a minority voice, some scientists including Fred Hoyle and continued to push the idea of panspermia in the latter part of the 20th century. They further contended that lifeforms continue to enter the Earth's atmosphere, and may be responsible for epidemics, new diseases, and the triggers for evolutionary spurts.

Imperial College London scientists have confirmed for the first time that important components of early genetic material, including uracil and xanthine, found in meteorite fragments are extraterrestrial in origin.

Pseudo-panspermia
Pseudo-panspermia, also called soft panspermia or molecular panspermia, is the idea that complex organic molecules that form the building blocks of life fell to earth from elsewhere and subsequently developed into life. Organic matter has been discovered on non-terrestrial bodies including meteorites and planetary bodies. But that isn't the same as proving that organic matter on earth came from space; it may just mean that organic matter is easy to create in the right conditions (a requirement for abiogenesis).

Directed panspermia
Directed panspermia is the hypothesis that life on Earth may have been deliberately started by some other alien life and that alien life may have interfered with (i.e., directed) its course throughout history. Francis Crick, one of the discovers of the structure of DNA, is often linked to this idea.

Accidentally transmitted panspermia
As a variant on the directed panspermia idea, it has been suggested that life on Earth was brought here accidentally by an extraterrestrial mission, or even from waste it left behind. While there is no evidence for such a claim, there have been occasional concerns that human probes could contaminate other worlds. A number of probes have been sent to Mars by the USSR, NASA, ESA etc and not all have been sterilised.

Concern about this was so great that the Galileo probe was crash landed into Jupiter rather than contaminating its moons.