They are fossils cyanobacteria from the Archaean eon.
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The oldest cyanobacteria fossil found is about 3.5 billion years old, known as the Apex chert fossils from Australia. These fossils provide evidence of the early existence of cyanobacteria, which played a significant role in shaping the Earth's atmosphere and environment.
Stromatolites have been dated back to as far as 3.4 billion years ago, and are amongst the oldest known fossils found on Earth. Stromatolites are still present today, and can be found in very salty lakes, and marine lagoons.
Prokaryotes are estimated to have been around for about 3.8 billion years, while eukaryotes are thought to have evolved around 1.6 billion years ago. Therefore, prokaryotes are approximately 2.2 billion years older than eukaryotes.
The oldest known fossils are prokaryotic. These fossils, dating back over 3.5 billion years, are believed to be ancient bacteria and archaea. Eukaryotic cells are thought to have evolved later.
In undisturbed strata, the oldest fossils will be the deepest from the surface.