The Hadean Eon when the Earth Cooled Down and the Earth Crust were formed

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The Hadean Eon occurred roughly 4.5 to 3.8 billion years ago. It is the time when torrential rains beat down relentlessly on to the red-hot crust of our planet, accompanied by violent outgassing, planetesimal bombardment, and extended periods of rain, which cooled the crust and formed the oceans where the first life would eventually take shape.

Very slowly, over a few million years, the Earth cooled down and large seas formed.  It was in these seas that the first living animals were born. But, they were certainly not as large and complex as most of the animals we know today, having only one cell.

TRIVIA:
Do you want to know how many cells are there in a human's thumb? There are millions of cells in the thumb of your right finger, yes just in a single human finger, now wonder how many millions are there inside a human body so that is why humans and animals of today are very complex. Human is certainly a billion years in the making.