From Ross Pomeroy at RealClearScience:
The oldest-known zircons, discovered in the Jack Hills of Western Australia, originally crystalized 4.4 billion years ago! It was within one of these zircons that geochemist Elizabeth Bell and her team discovered the carbon they think was produced by life. Life that old, whatever it was, would not have bones, or even a clearly-defined shape, so a true fossil find will probably never be unearthed. Instead, whatever carbon-based life existed back in the Hadean would simply leave traces of itself in the form of carbon itself. Bell’s co-author, Mark Harrison, referred to the stuff as “the gooey remains of biotic life.” More.
See also: Earth’s crust cooled only 160 million years after solar system formed, says new Australian zircon study
What we know and don’t know about the origin of life
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