Get a life III
Alive at last
It is not known at this time the actual mechanism through which life started. What we do know is it started almost as soon as conditions allowed, and that it probably did so more than once, given catastrophic collisions may have sterilized the Earth eventually in several occasions.
The basic ingredients were in place: an excellent solvent and carrier like water, abundant carbon dioxide and probably other organic compounds like methane and ammonia (known to be present in interstellar matter). It could have started deep in the oceanic rifts, or in sunlit, tidaly washed shores. It may have used orderly clay layers as scaffolding for carbon polymers to arrange, maybe lighting synthesized aminoacids, or they formed through alternate unknown processes. Long chain organic molecules may even have arrived from outer space carried by meteorites, though it is doubtful they could survive a fiery reentry.
Whichever the path, it was swift and effective, for there is fossil evidence of cyanobacteria mats or stromatolites dating almost to the same moment liquid water was freely available. Stromatolites can be found even today, these hardy life forms almost unchanged in four billion years.
Oxygen certainly was not present in that atmosphere, for it is a very reactive and therefore poisonous substance for those early living cells - unless it can be harnessed in useful chemical cycles as more advanced forms did later. By the way, the Sun was at that time shining much weaker than today (it will eventually keep increasing and engulfing the Earth in some billion years in the future), which is fortunate for lacking oxygen to be dissociated by UV in Ozone, there was not much of a protective layer to shield those tender first steps.
Surprisingly, things kept almost unchanged for a couple of billion years thereafter. As much as it was fairly easy for life to start, it took far longer to reach the stage of association among individual cells in functionally differentiated colonies and to establish a new hierarchical level of organization.
During this period, oxygen was released as a living function byproduct, and started to accumulate, slowly growing to the current atmospheric equilibrium 20 percent concentration.
Adrift, on molten rock
We mentioned above in connection with the origins of Earth's atmosphere, the widespread volcanism of earlier epochs, lasting in subdued form to these days. Differentiation of the original metal and siliceous compounds in the core, mantle and crust of
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