The earliest Eon is known as the Hadean.It begins with the very formation of the Earth itself, around 4.6 billion years ago and ends billion years ago. And this is the only Eon that doesn’t have fossils. Because, back then, the world was just hell. Named after the Greek underworld Hades, the Hadean lived up to its name.
How the earliest eon started...
Named for the Greek word for ‘origin’, the Archean was once thought to be when the first signs of life appeared. But at the very least, it’s fair to say it was the first time that life flourished, forming mats of microbes in the primordial seas. The fossils that these microbes left behind are called stromatolites, or sometimes, stromatoliths, and the very oldest of them -- like those found in western Australia -- date from the Archaean. During this time, the atmosphere was mostly carbon dioxide, but the appearance of cyanobacteria was about to change all that.
Then 2.5 billion years ago, the Archean gave way to the Proterozoic Eon, meaning ‘earlier life’. And around this time, photosynthetic bacteria, along with some multicellular forms of life, spewed tons of oxygen into the atmosphere. This probably wiped out much of the anaerobic life on Earth. BUT! It cleared the path for crucial, new organisms, including the ancestral Eukaryotes, whose cells each have a nucleus and organelles wrapped up in membranes. Eukaryotes developed into the first really big, complex, and sometimes kinda weird forms of life, like the frond-like Charnia and the plate-shaped Dickinsonia These new, larger organisms quickly diversified.
Its name means ‘visible life,’ and the Phanerozoic was when life really became … obvious. This is the eon that’s home to trees, dinosaurs, newts, aardvarks, and humans.
According to www.britannica.com The Paleozoic Era began 541 million years ago with the Cambrian explosion, an unparalleled diversification of marine organisms, and concluded about 252 million years ago with the end-Permian catastrophe, the biggest extinction event in Earth history.
According to www.britannica.com, The Mesozoic Era is the second of Phanerozoic time's three major geologic epochs. The Mesozoic Era began 252.2 million years ago, following the end of the Paleozoic Era, and ended 66 million years ago, at the start of the Cenozoic Era.