> So we are probably just one of the first.
Those two sentences don't really mix. We have evidence that points to life forming basically immediately once the conditions are right. That means that for us to be the first, nowhere else in the galaxy and universe could have had the conditions. That just seems incredibly unlikely. Even in our own Solar System, other bodies like Mars had the conditions for life before Earth did.
In fact, given that most of the easily accessible coal and oil deposits will have been used up it might even be much more difficult for a subsequent species to get as far as humanity has. (Although that might be balanced a bit by lots of the metal ore being pre-mined and near the surface)
There'll be plenty of time for nature to compact and bury much of the biomass from the previous sentience attempts.
What's the evidence for this? Not asking rhetorically, I'm curious and want to read more.
Basically, as my understanding goes, life is effectively as old as the Earth is. As soon as it cooled down after forming and had liquid oceans, life started, which happened just a few hundred million years after Earth formed.
There is the book Every Life is on Fire as well, describing how life began to dissipate heat in the environment of molecules in water being heated. That seems to imply life will quickly start in such conditions. (Note: I don't strongly recommend the book, as it has a lot of religious stuff in it and honestly isn't that well written, and I have mainly learned about the author's theory elsewhere.)