Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 123

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Life of the Oceans
The Earth was born 4.56 billion years ago amid the violent collision of
planetesimals. Sometime in the ensuing 100 million years or so it was
reborn, at white heat, in the aftermath of the Moon-forming impact.
Only after that could temperatures cool to levels that allowed the first
oceans to form—although these were likely intermittently vapour-
ized as impacts continued.
About 3.8 billion years ago the last great flurry of asteroids sub-
sided. From then until now the oceans have been a constant feature of
this planet. From about the same time, too, there is evidence for life,
preserved as the products of early microbial activity. So rapid was this
appearance of life, after the period of heavy asteroid bombardment,
that microbes might have evolved sometime in those perilous Hadean
times, 71 clinging on in rock fractures deep beneath the surface before
emerging into a calmer Archaean oceanic realm.
From this beginning, all life in the oceans and on Earth has evolved
in an unbroken chain of increasing diversity. How did the complex-
ity of modern ecosystems arise and how long did this process take?
For nearly the first half of Earth history the ocean was devoid of
oxygen, although rich in dissolved iron. It teemed with microbes
adapted to those conditions. The primitive life in those early seas
was already changing (and being changed by) the chemistry of the