Some evidence of photosynthesis was found on rocks in Greenland dating from 3.7 billion years ago. Danish researchers reached that conclusion analyzing the relative abundance of uranium and thorium in some rocks, and the high abundance of uranium in those suggests that there was oxygen present. Their conclusion was that microorganisms like present day cyanobacteria were converting sunlight to chemical energy through oxygen-producing photosynthesis.
What is the oldest evidence of life?