No one knows how long it took for hetorotrophs to develop chlorophyll which was responsible to put free oxygen into the atmosphere. From fossil evidence, it appears that by the time of the oldest known cynobacteria (blue-green algae), which appeared almost 3.5 billion years ago, chlorophyll was present, sustained in part by the presence of some chemicals that are usually found in aerobic organisms. From this, it can be deduced that cynobacteria was one of the first plants. Of course, this can not be proven, as there is no way of telling if there were chlorophyll-producing organism living the early atmosphere. Judging form the chemical composition of cynobacteria, though, it would seem as if it was one the first, if not the first organism to live in a place other than the atmosphere. This primitive plant is the ancestor of many current day plants.