Saturday Night Science: Life 3.0

The Earth formed from the protoplanetary disc surrounding the young Sun around 4.6 billion years ago. Around one hundred million years later, the nascent planet, beginning to solidify, was clobbered by a giant impactor which ejected the mass that made the Moon. This impact completely re-liquefied the Earth and Moon. Around 4.4 billion years ago, liquid water appeared on the Earth’s surface (evidence for this comes from Hadean zircons which date from this era). And, some time thereafter, just about as soon as the Earth became environmentally hospitable to life (lack of disruption due to bombardment by comets and asteroids, and a temperature range in which the chemical reactions of life can proceed), life appeared. In speaking of the origin of life, the evidence is subtle and it’s hard to be precise. There is completely unambiguous evidence of life on Earth 3.8 billion years ago, and more subtle clues that life may have existed as early as 4.28 billion years before the present. In any case, the Earth has been home to life for most of its existence as a planet.