Meet an atheist ... who believes in God

[Humanist_Activist]

67. I think the reason we make assumptions about the basic building blocks of life is because...

we know they are basic building blocks on at least one planet, Earth, and the laws of chemistry and physics work the same everywhere in the universe, so we would assume that there are plenty of places where, when the conditions are right, life with similar, though not necessarily exact, biochemistry, has evolved. It also helps that we find things like amino acids and other organic molecules literally floating in space in molecular clouds that condense to form stars and planets.

Indeed, its likely that the first chemicals on Earth that were the precursors to life, amino acids, RNA, etc. were frozen in asteroids that condensed out of our planetary disc/molecular cloud, and arrived here soon after the crust cooled and liquid water was becoming common on the surface in the form of asteroid strikes. Our oceans came from space after the planet formed, so why not our biochemicals? Given that they were much less likely to survive the conditions that formed the planet in the first place, it was, quite literally, hot enough to melt rock at that point.

We do have a big type of confirmation bias, only one planet as an example of life, and that can be artificially limiting as you say, but it is, right now, the best starting off point we have.