PhysOrg Space Exploration - 2010 Feb 15
The iguanas of the Galapagos Islands have evolved many unique characteristics due to their isolation from mainland iguanas. Because they can't swim long distances, biologists believe that the first Galapagos iguanas arrived on natural rafts made from vegetation.
The same thing may have happened across the ocean of space. Some researchers speculate that life on Mars - if there is any - may be composed of "island species" that were carried away from Earth on interplanetary meteorites.
Or perhaps both planets were seeded by life from an even more distant "mainland."