The Proterozoic era (the era of early life) began 2.6 billion years ago.
The surface of the Earth was a bare desert. The oxygen content in the atmosphere was low. There was no ozone screen. Living organisms could live only in the aquatic environment.
Eukaryotic unicellular and multicellular organisms developed in the seas. Photosynthetic organisms were improved. All divisions of algae (green, brown, red, etc.) arose.
Animals appeared. The first animals were probably sponges and coelenterates.
Ancient coelenterates gave rise to flatworms that were three-layered and had bilateral symmetry.
Better roundworms with a through digestive tube evolved from the ancient flatworms. Then there were secondary cavity animals – annelids.
The ancient marine polychaetes probably gave rise to the formation of types of arthropods, molluscs, and chordates.
By the end of the Proterozoic, all types of invertebrates and primitive chordates (skullless) existed.
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