The prokaryotic organism which are taken the earliest fossils are cyanobacteria.
What are cyanobacteria?
Cyanobacteria are aquatic and photosynthetic, as they resides in water and perform photosynthesis.
They are small and usually unicellular, though they frequently grow in visible colonies.
These are the known earliest fossils under prokaryotic organisms.
Thus, the answer is cyanobacteria.
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