I was always under the impression that life started in hydrothermal vents via a process known a chemosynthesis. Eventually some of the cells developed an evolutionary adaptation toward the surface of the water. Photosynthesis is now in play, and the rest as they say, is history. Welcome to the Great Oxygenation Event and all that followed thereafter.
The process seems pretty strait forward. Almost inevitable for any ocean baring planet. I guess will know soon enough should we detect oxygen rich small exo-planets like Earth.
Assuming the Universe is filled with copious "Earths", it's not a guerantee that life will be intelligent. And for the sake of argument, assuming the natives are intelligent, their motives most likely would be completely different than anything we could imagine. Something akin to comparing Humans and Dolphins. Both very intelligent, yet with completely separate ambitions in how they live their lives. Quite possibly, Humans are the only intelligent species looking up and beyond the confines of our own Planet. That may explain the Fermi paradox. We're the only living creatures that cares of such things!