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It took Earth ~ four billion years to go from molten rock to multicellular life. Let's take that as an absolute minimum requirement of time for organic life to evolve and produce sapient species (I know, that's a stretch, but just roll with it).

So how ancient can my Ancient Precursors realistically be if by lore they're originated from literally one of the first terrestrial planets with advanced organic life in the Universe, if not the first?

I've read that the necessary elements for the creation of earth-like planets began popping in existence not even one billion years after the Big Bang, but as I understood it the universe was still too volatile back then for any planet to exist long enough without some cosmic cataclysm sterilizing it's hypothetically possible beginnings of life. So when things calmed down enough for 4 billion years of relative peace to become feasible?

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  • $\begingroup$ I am sure there is much variation in how long it takes for a planet to become habitable for oxygen breathing lifeforms like humans. The only discussion of planetary habitability specifically restricted to habitable for beings like Humans is Habitable Planets for Man, Stephen H. Dole, 1964, 2007, and it might be obsolete in some matters. Dole considered the minimum age for a planet to become habitable for humans to be three billlion years, not the four billion it took the Earth. rand.org/content/dam/rand/pubs/commercial_books/2007/… $\endgroup$ Feb 6 at 21:51
  • $\begingroup$ for a planet to be "habitable to humans" basically all that's required is magnetosphere and atmospheric oxygen in sufficient quantities. I'm not sure how that's useful regarding the question, since it doesn't ask when the planet becomes suitable for colonization. $\endgroup$ Feb 7 at 7:23
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Earlier Than You Might Think

The important elements are less for planetary formation and more for the creation of Life As We Know It. (Your ancients could be Life As We Don't Know It, but in that case they might be energy beings who live in the depths of space and wouldn't need a terrestrial world.)

So what elements are vital to life? In order of atomic number, Hydrogen, Lithium, Carbon, Nitrogen, Oxygen, Sodium, Magnesium, Phosphorus, Sulphur, Chlorine, Potassium, Calcium... and most at issue for your ancients and their world, Iron.

Other than Hydrogen and Lithium, none of these elements were around until the first stars had formed and died. So although some Sol-type stars might have formed as early as a few hundred million years after the Big Bang, they wouldn't have the necessary materials in their protostellar disc to make the kind of planets that could support our kind of life.

So you'd need to wait for the deaths of a lot of stars to seed the universe with the materials necessary. That still (depending on the astrophysicist you ask) could put you as early as 700Ma after the Big Bang, or about 13.3Ba ago. The universe (taken as an enormous whole) looked about the same thirteen billion years ago as it does now.

Assuming that all the odds rolled the right way (which we are, for the purpose of "earliest possible", once stellar furnaces and supernovae had forged the elements necessary, your terrestrial planet would be possible - and cosmic cataclysms were really not that common at that point. So between 13-13.3 billion years ago, a planet could've formed on which life could arise. Four billion years after that (about five billion years before our own protostellar disc formed), you could have your civilization of ancients.

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