How many billions of years ahead of us could they be or millions or whatever?
-
First-generation planetary systems couldn't have supported life because the interstellar medium wasn't rich enough yet in what astronomers refer to as "metals." By the time that second-generation stars formed, first-generation stars had already forged metals in their cores and spread them into the interstellar medium. So the earliest life might have formed on planets around the first second-generation stars. That might have been about 11 or 12 billion years ago. That would have been about seven or eight billion years before life appeared on earth.
-
Tough to say. Earth is probably one of the earliest planets to develop life (or at least, life as we know it) based on the time it would take for the earliest stars in the universe to produce enough heavy elements to make life possible. But once life has started somewhere, there are innumerable variables that might alter how long it would take a 'thinking' species to develop and evolve.
Let's take Earth as an example, since it's the only one we really have. Single-celled organisms dominated the biosphere for billions of years, mass extinctions and various population explosions came an went, mostly due to environmental changes or solar/celestial events... the timing of these events would not be universal across the universe, it would vary from planet to planet. Perhaps a planet that developed life before us is still in the single-celled era, or its equivalent of dinosaurs never went extinct, not allowing its equivalent of mammals to dominate the land. Perhaps other worlds have greater ice-ages, or are completely covered in oceans, which would also affect adaptation traits that evolution would allow to prosper or perish.
So, while it is relatively safe to say that 5 billion years ago is a (very approximate) ballpark figure as to when the first life forms could probably exist, everything that has happened since then is essentially impossible to make an educated guess about.
Let's take Earth as an example, since it's the only one we really have. Single-celled organisms dominated the biosphere for billions of years, mass extinctions and various population explosions came an went, mostly due to environmental changes or solar/celestial events... the timing of these events would not be universal across the universe, it would vary from planet to planet. Perhaps a planet that developed life before us is still in the single-celled era, or its equivalent of dinosaurs never went extinct, not allowing its equivalent of mammals to dominate the land. Perhaps other worlds have greater ice-ages, or are completely covered in oceans, which would also affect adaptation traits that evolution would allow to prosper or perish.
So, while it is relatively safe to say that 5 billion years ago is a (very approximate) ballpark figure as to when the first life forms could probably exist, everything that has happened since then is essentially impossible to make an educated guess about.