Whatever or whichever. As far as Earth is concerned life took hold exactly once. A little odd isn’t it?
there is an approximate time life began on Earth that for all intense and purpose has never stooped - are you saying that process could not still be possible. it may have been due to extreme ideal conditions that are no longer present due in part to the life that did evolve.
Im not saying that the process isn't still possible. I'm saying that it isn't happening...possible or not. And that leads to some very interesting speculations.
Science lags in the popular mind no matter what they tell you. We are long past Darwin's "warm little pond". Didnt happen that way. We are past thinking that it was an amazing one in a gazillion chance that took place after billions of years of bumping and cooking and shuffling molecules. We now know life was established for 90% of the history of the Earth if not more. Blink of an eye almost...within a few hundred million years of Earth's cooling off.
And we know it happened once and only once on Earth. Never before and never since. (The ones to watch and read on this are Nick Lane and Peter Mitchell.) ATP Synaphase is the key to this idea. As important to biology as the speed of light is to physics.
And, for other reasons, we can be fairly sure there is no life anywhere else in the Universe. I'll leave that alone for now though.
Buy why only once? Thats the question with philosophical ramifications to me. Scientifically it is unexpected. As you said "whats changed"? And as far as I know there are no theories addressing it.