yann,
You make a statement, flatly, like ‘mankind wasn’t always around, man existed for less than a million year’ and others you have, based on what the scientific view is at present. Another heap of bones excavated somewhere and that will change to two millions years, or three. And so it will go on, people accepting the latest scientific view as established fact.
Now I am alright with that, because this is the function and scope of science, to discover bit by bit, gradually. For that reason science does not have the complete picture, the ultimate ‘worldview’. So that is quite in order, as far as science goes.
‘So the 31 realms division isn’t valid anymore and wasn’t valid 2 millions years ago. But you don’t have the answer obviously” You are right; based on the ASSUMPTIONS you make, there is no answer. But the Buddha made no assumptions, arrived at no conclusions (even the right ones!), formulated no theories; He SAW these 31 realms, and saw also that they have always existed, and not only with regard to this solar system, but to innumerable others reaching out to infinity and dating back without a beginning.
Now to get to your point: consider the case of an exoplanet which is labelled as uninhabitable – but such must the Earth have looked like 15-30 billion years ago (when in its phases of destruction and formation). And before that not even a solid planet, or a star, for that matter, – just a mass of dust clouds and gas that were formerly a planetary system, on the way to forming a (re-)generation of it. So science cannot say which planets are inherently uninhabitable. Those that (would) harbour life will also have a ‘local humanity’ there at some point, and repeatedly, along with the other 30 realms.
As for myself – this is just my opinion here – I feel that there is nothing in Nature that is without a purpose. Life is everywhere, unbounded, timeless, and where there is sentient life, Nature applies the scheme of these 31 realms. The Buddha’s word, not mine.
When you take the time to examine the Teaching seriously and profoundly, you will see the absolute reasonableness of It. The INEVITABILITY of It. All other theories will seem like passing video games..’what shall we play next?’ That is where faith in the Buddha happens. Not a blind faith born of convenience and intellectual laziness, but born OF intellectual endeavour and introspective searching. The search is also an inner one. And that will be only the start.