Life On The Drift
Hard panspermia theory suggests that life originated in just one stellar system in the Galaxy about ten billion years ago, and spread out to planets around other stars, including Earth. It assumes that life is unlikely to arise more than once, that big whacks like the dinosaur-killing Chixulub impact could knock debris off Earth's surface and into interplanetary space, and that microbes could survive inside rocks for the million-odd years it would take to drift to another star system. Hara et al's paper suggests that more than one fragment of debris could reach stellar systems within twenty light years of the sun, and microbes might survive if those fragments were embedded in icy material like comets, which would shelter it from cosmic radiation.
It's a neat idea. Doughtly little microbes minding their own business when they're suddenly knocked off Earth by a fiery cataclysm, snoozing away a million years inside a centimetre-long spaceship of solid rock, plunging into an alien ocean and getting busy with the business of evolution. But like all ideas associated with panspermia, it is based on the very big assumption that abiogenesis, the spontaneous generation of life, is a highly unlikely event; an assumption that tends to degrade into an argument about First Cause that is as yet is impossible to answer because we have only one example of a life-bearing planet. (And it's possible that life on Earth arose more than once, but other forms were wiped out by catastrophic impacts, or were out-competed by our very early ancestors, leaving only fossils or refuge populations surviving in niche habitats. In that context, discovery of microbes with characteristics radically different from all other known species would be as important as discovery of life on another world. That's why NASA made a big noise about the possible (and now largely discredited) discovery of a microbe that appeared to substitute arsenic for phosphorous in its metabolism.)
If we found life on Mars, and that life closely resembled life on Earth, then we'd certainly have to take the idea of panspermia (or at least its weaker cousin, exogenesis) within the solar system seriously. But given that one estimate puts the number of Earth-like planets in the Galaxy at around ten billion, abiogenesis would have to be an extremely rare event for it to have occurred only once. Martians may be from Earth (or we may be Martians), but despite the calculations of Hara et al about the probability of the chain of steps required to transfer life from one stellar system to another, it still seems most likely that if ever do meet any aliens, they'll be genuinely alien, products of a creation utterly separate from our own.
(Thanks to James Bradley for pointing me towards the article.)