Why were cyanobacteria considered as among the earliest life forms existed on Earth?

Question

Why were cyanobacteria considered as among the earliest life forms existed on Earth?

Answer 1

Because the young earth was depleted in oxygen. Any life at this time needed to survive without oxygen.

Answer 2

well just think about it if you were around during the time of the dinosaurs and still alive today you would be classed as the earliest human still living but i'm not teasing or anything it's just i've learnt all about it in science coz i'm still in yr 7