We’ve actually shown it is possible. There’s been tests with tardigrades, and it’s been found that in their dormant state, they can survive in space, which lends a ton of feasibility to an evolved panspermia option at least. And a few generations in space showed they could make it through the evolutionary bottleneck. So not only are animals living in open space not only possible, it’s a thing that has happened. After that, it’s just a matter of evolutionary selection over thousands of years until you get something that outright survives off of starlight, cosmic rays (some kind of super-chlorophyll) , and space dust.
Heck, we could probably genetically engineer tardigrades and lichen together, if money and projects were so inclined, to make plants that would grow on a targeted barren asteroid.