From Scientific American:
Most scientists have long assumed that life on Earth is a homegrown phenomenon. According to the conventional hypothesis, the earliest living cells emerged as a result of chemical evolution on our planet billions of years ago in a process called abiogenesis. The alternative possibility–that living cells or their precursors arrived from space–strikes many people as science fiction. Developments over the past decade, however, have given new credibility to the idea that Earth’s biosphere could have arisen from an extraterrestrial seed.
New research indicates that microorganisms could have survived a journey from Mars to Earth
More here.