While evolution is an accepted theory of how species evolve over time, and how new species spring from existing ones, the fundamental question of what actually started life on this planet is still the subject of a lot of conjecture.
Some scientists postulate that rocks from space carrying life from other planets or perhaps asteroids landed on earth and found a favorable environment, and the evolutionary process was on!

New research indicates that microorganisms that crashed to Earth embedded in the fragments of distant planets might have been the sprouts of life on this one, according to research from Princeton University, the University of Arizona and the Centro de Astrobiología (CAB) in Spain.

The researchers report in the journal Astrobiology that under certain conditions there is a high probability that life came to Earth — or spread from Earth to other planets — during the solar system’s infancy when Earth and its planetary neighbors orbiting other stars would have been close enough to each other to exchange lots of solid material. The work will be presented at the 2012 European Planetary Science Congress on Sept. 25.