Comment by rekenaut

Potentially a much greater filter is going from unicellular to multicellular life, no? If it likely took billions of years to get from unicellular to multicellular life on Earth, and only (hundreds of) millions of years to get to life that can conduct spaceflight, then perhaps microbes wouldn’t be the best way to attack this problem (I’m assuming you’re talking primarily about unicellular microbes, of course).