The origin of Life is in many ways a ‘blank spot’ in the pages of history, similar to the Big Bang or dark matter, scientists still only have a general idea of how life came to be and are somewhat fuzzy on the details. This is a subject of constant global research in an attempt to find a ‘clear narrative’ of how life came to be.
As the Earth was emerging from the chaos of its formation, it underwent 2 million years of torrential rainfall that cooled the surface and created the oceans that dominate the planet today. It is in these oceans, between 3.5-4 million years ago, that the first organisms emerge and the story of life on earth begins.
From Crash Course: Biology on YouTube by Hank Green.