Another idea is that life – or at least the chemical building blocks – came from space. Many comets were crashing into Earth early in its history. Comets are rich in carbon-based chemicals from which life could have started.
It is also possible that life could transfer from one planet to another. It seems that life is tough enough to survive a long journey through space. A large comet or meteorite crashing into the Earth could throw large numbers of rocks into space. Any bacteria that are riding as passengers on these rocks could eventually land on another planet.
In the same way, a meteorite from Mars could carry Martian life (if it existed) to Earth. In 1996, scientists told the world that they had found just such a Martian rock in Antarctica. However, other scientists question their research.
Last update: 6 December 2004