This is a cut of a 3,800,000,000 year old fossil known as a stromatolite.
Stromatolites are rocks formed from mats of prokaryotic organisms such as bacteria and primitive algae that were grew, reproduced and died over millions of years.
With the absence of decomposers they formed mats on the sea floor dozens of metres thick.
Over time they got crushed by the weight of sediment and other prokaryotes above them. Turning them into rock.
Now, fully intact stromatolite pillars can only be found in Australia and some select regions.
They are older than plants, they are older than animals, they are older than life on land and they are older than multicellular organisms.