Answer: Cynobacteria
Step-by-step explanation:
The Precambrian time is the earliest period of earth history. It is preceded by the Cambrian time hence got the name. The earliest living organism in the Precambrian time was the Cynobacteria. It has been found in the fossil record of 3.4 billion years ago. They were the primitive organisms which were capable of conducting photosynthesis and producing oxygen. The abundance of Cynobacteria has resulted in the increase in abundance of oxygen into the atmosphere.