Microfossils

The earliest known fossils were in rocks from the Cambrian era, a little over 600 million years old. Now microfossils, identified in ancient rocks, indicated early bacterial life on Earth from 3,500 million years ago.
 Elso Sterrenberg Barghoorn United States 1965 Microfossils

The earliest known fossils were in rocks from the Cambrian era, a little over 600 million years old. Now microfossils, identified in ancient rocks, indicated early bacterial life on Earth from 3,500 million years ago.