Scientific American considers the evidence:

This new understanding of life's origins has transformed the scientific debate over panspermia [i.e., the theory that life originated from tiny seeds across the cosmos]. It is no longer an either-or question of whether the first microbes arose on Earth or arrived from space. In the chaotic early history of the solar system, our planet was subject to intense bombardment by meteorites containing simple organic compounds. The young Earth could have also received more complex molecules with enzymatic functions, molecules that were prebiotic but part of a system that was already well on its way to biology. After landing in a suitable habitat on our planet, these molecules could have continued their evolution to living cells. In other words, an intermediate scenario is possible: life could have roots both on Earth and in space.

That's the debate, anyway. One crucial question is whether outer-space organisms coming to Earth on the meteor express could have survived the hot trip through the atmosphere. Hard to do. Not only that, but they'd also have to have survived being rocketed out of their planet of origin. Not fun, either. Both moves seem "plausible theoretically," but eventually scientists will have to scrounge up some bacteria and put them through interstellar hell to see whether the little critters actually survive, say, a trip from Mars to Earth after meteors dislodge chunks of rock from the former. Very cool. In a related vein, I was once told that the Yucatan meteor that wiped out the dinosaurs probably deafened every living organism in the world immediately on impact. Also cool, but it's surprisingly hard to find confirmation for this on the web.