The evolution of the first organisms
The first living organisms we have proof of thanks to fossils, are three and a half billion years old. They are the so called “stromatolytes”; structures made of several layers piled one on top of the other like a stack of pancakes. Today one can find organisms similar to the fossilized stromatolytes in Australia’s hot seas. These present day stromatolytes are made up by the growth of bacteria communities and blue algae on which grains of sand deposit. Both bacteria and blue algae are prokaryote organisms, meaning that they do not have a differentiated nucleus, and therefore they are more primitive. But blue algae are able to perform photosynthesis and so we can believe that the first forms of life on Earth date even further back than 3 and a half billion years ago.
We don’t have eukaryote cell fossils over a billion years old, so we can assume that life’s evolution in the first two or three billion years was very slow and affected only unicellular organisms. Instead researchers believe that the step from unicellular to multicellular organisms happened very quickly, because the first fossils of complex organisms were already plentiful six hundred million years ago.
Precisely six hundred million years ago the Precambrian Era finished and the Palaeozoic era began, of which we have sure fossil proof, when life existed only in the sea . Subsequently algae made their first attempts to colonize the land. After the appearance of the first plants on dry lands came the first herbivores, of which some subsequently evolved into carnivores.
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