>>...minerals that don’t form when oxygen is present led me to conclude that the 2.5-billion-year-old microfossils in the photos above gained their energy by oxidizing sulfur. ...
I'm far from a chemist (didn't even take chemistry in high school) but for anyone wondering how an organism could 'oxidize' sulfur in an environment where oxygen was close to nonexistent, here it just means electron exchange, using the change in sulfur's state for energy.
The big mystery for me is the emergence of eucaryotes (from what we now know were an archaea and a bacteria) and without which multicellularity was impossible - just given the way both archaea and bacteria generate energy using their cell membranes, limiting their possible size because of the way volume vs surface area changes as an object gets larger.
Is there a way to tell whether a microfossil you're examining is a eucaryote or procaryote? I would *guess* they might have been around before the great oxidation event (big die-off of everything that couldn't tolerate the oxygen levels greated by cyanobacteria) - or is it more likely that the oxygen levels were a selection pressure that eucaryotes were a response to?
Fascinating stuff!