3.5 billion years ago: The earliest fossils
The earliest fossils found are simple cyanobacteria, placed in the domain Bacteria: unicellular organisms characterized by their small size, lack of a nucleus and organelles, with a very simple genetic material. Cyanobacteria obtain their energy through photosynthesis, producing glucose and releasing oxygen.
These fossils, which are dated 3.5 billion years ago, were found in Western Australia, in an area called Pilbara. They are fossilized organic material trapped in sediments, which prevented its decomposition and allowed the replacement of the original pore spaces with minerals over time. Most probably these bacteria changed the composition of the planet’s atmosphere. Thanks to them, oxygen-breathing species were able to survive. Cyanobacteria evolved very slowly, so the ones that currently exist are identical or nearly identical to these earliest fossils found.