The main concept of Panspermia is that life, or the fundamental building blocks of life, exists throughout the universe. This theory posits that life arrived on Earth by 'piggy-backing' on an asteroid, planetoid or meteroid. This ancestral form of life would have been delivered to Earth by the impact of said meteroid, planetoid or asteroid with the surface of the Earth. This idea was first proposed in the 5th century BC by the Greek philosopher Anaxagoras. Jons Jacob Berzeilius in 1834, William Thomas (also known as Lord Kelvin) in 1871, and a few other scientists reawoke this idea of the origins of life. Since the main concept of panspermia involves life existing on extrasolar bodies and traveling through the universe unprotected against solar radiation and living in a vacuum, this field of research involves a lot of investigation into extremophiles on Earth that can overcome these challenges. One such organism, as depicted to the right, is the tardigrade (Ph. Tardigrada). They are an extremely interesting organism who has fossils dating back to 530 million years ago. They are important to the hypothesis of Panspermia because they have been shown to survive being exposed to absolute zero temperatures (−273 °C, which is the temperature in space). They can resist 1,000 times more radiation than any other animal known, and have been shown to survive without water for almost a decade. They are also the first animal shown to survive the vacuum of space.
Resistance to this hypothesis stems from the fact that we view life as having specific conditions (element densities and temperatures, for example), which do not appear at this time to be common in the observable universe. Researchers have determined that bacteria found on Earth would not be able to survive the immense force and heat that would be experienced during an impact. And finally, if life were traveling around the universe and were able to survive space and impact on a planet, we would expect to see the evidence or at least remnants of life on Mars and Venus, which we currently do not.
Panspermia is not necessary incompatible with the RNA World or any other origin of life hypothesis. This is because panspermia posits that an ancestral life form was delivered to Earth, but does not theorize on the original origin of that life form.