The vastness of this universe leads me to believe that it's impossible to dismiss the idea that there is an intelligence/lifeform sufficiently advanced as to appear to be a God to us.
If you have a few minutes start here for a broader context (starts of wholly unrelated but with some great concepts):
https://youtu.be/P-2P3MSZrBM?t=10084
If not, he drills into it here:
* We are a computer simulation (hence there is no colonization, we are in an experiment)
* There are like 1,000,000,000,000,000,000 habitable planets in the universe. Just because something is "extremely unlikely" doesn't mean that it didn't happen to us. I.e. whomever this happened to, would ask himself that question. We may just be the ones it happened to.
Also: Why would they settle species from millions of years in their own past. If they wanted to colonize, they could start with their own life-forms.
Could be several things as well:
* We are a genesis project and they will simply come back later and settle in what we left behind (i.e. they let the ecosystem build for a few million years before trying out this settlement)
* They don't want to create competitors
* We are a population bomb, i.e. they just sent out billions of probes trying to populate any given habitable planet, and they couldn't just spawn themselves there and instead had to start from somewhere where a sustainable ecosystem would derive from
Depressing fiction idea: our progenitors return, look at what we've done with the world, shake their heads and leave us to suffer.
And it's unlikely that there is only a single recipe. Most likely there are many (just on Earth all the others died out)
If you look at how quickly life began after the oceans settled out, then look at precisely how complex even the most primitive forms of life are (in terms of likelihood of stochastic construction to the point of sophistication to support evolutionary mechanisms) it seems incredibly unlikely that we originated here.