My favorite theory at the moment is that life has always been here, before the planet.

Back when the universe was smaller, before mass was formed as stars and planets, the density of free floating mass was at some point high enough that there would have been temperatures around 20°c everywhere in the universe and water and other molecules were floating freely in space. At that point, for millions of years, life could have evolved in space.

When the universe expanded, mass got separated and formed into galaxies, planets and stars from gravity, while life was just dormant luggage hiding everywhere possible. On Earth the conditions once again got just right to step out of hibernation and evolve, but it could potentially happen on any other planet as well.

I like this theory because it doesn't involve as much random chance and it could mean that we can discover life elsewhere in our own solar system, instead of only hoping that future generations will eventually encounter it in other star systems..

fully fledged whole body animals that may have distributed microbes that had hitched a ride on or inside of them on their ride through open space, or on the surface of a meteor