Earth, early history
The Earth was formed about 4.55 billion years ago through the accretion of small particles within the protoplanetary
nebula that surrounded the infant Sun. Its primordial atmosphere probably
consisted of a mildly reducing atmosphere of carbon monoxide, carbon dioxide, nitrogen, water vapor, and a small amount
of hydrogen, derived from volatile matter that permeated the outer layers
of the planet. Heat released by the breakdown of short-lived radioactive
elements led to further massive outgassing
of these substances. What little free hydrogen there was soon escaped into
space, allowing the formation of an ozone layer which subsequently protected the surface against the Sun's ultraviolet radiation. However, conditions for the evolution of life were hardly clement
during the first few hundred million years after the Earth's formation.
In that early period, between about 4.55 and 3.8 billion years ago, the
Earth is thought to have been struck repeatedly by objects measuring 100
kilometers or more across and traveling at around 30 kilometers per second.
Each such impact would have seared the surface at temperatures of up to
3000°C, stripping away any atmosphere, boiling away any oceans, and
rendering life impossible down to a depth of about 1 kilometer below the
Recent discoveries of extremophiles in deep
subsurface rocks and in high temperature environments, however, suggest
that once life had appeared it may have survived the traumas of bombardment
by retreating deep underground.
AND EVOLUTION OF LIFE