How confident are scientists that life on Earth began in the conditions of present-day Earth?

My understanding is that life did not begin in conditions anything like present day Earth. In particular the oxygen you are currently consuming was produced by living things (https://en.wikipedia.org/wiki/Cyanobacteria). Prior to there being oxygen producing organisms there was very little oxygen available in the atmosphere.

So, perhaps they are looking for an earth-like atmosphere on the assumption that a similarly oxygen rich atmosphere would mean that there are oxygen producing organisms present.