The proterozoic age was a period of time 542-2,500 million years ago. During the proterozoic age of the earth, basic organisms of algae and invertebrates were common. Mammals were not present on earth yet.
The oldest fossils of complex cells known as eukaryotes are from the proterozoic age. The proterozoic age occurred before any complex life existed on earth.
This is an image of a stromatolite, a record of some of the most ancient life on earth preserved in rock.