We all know that the first life on earth is the single celled plant, but how did it come into existence?
We don't know yet!
The origins of life on Earth aren't known yet! Also, the first life wasn't a single-celled plant. We don't really know what the first forms of life were on this planet because they were probably so small they didn't leave fossil evidence and if they did, the rocks they were in have most likely been recycled by now. However, we can say that the first forms of life we're pretty certain about were most likely prokaryotic chemoautotrophs meaning they used CO2 and chemicals found on early Earth to grow.
Eventually, the ability to photosynthesize developed much later on and this is when the atmosphere was largely changed! High concentrations of oxygen were introduced into the atmosphere and this oxygen was highly toxic to most life forms on Earth at the time. After this change, the Earth started to resemble what we see now.