Four billion years ago the ocean was acidic, saturated with carbon dioxide. Where the alkaline fluid from the vents met the acidic water, there was a steep proton gradient across the thin iron-nickel-sulphur walls of the pores that formed at the vents. That gradient had a voltage very similar in magnitude to the one in a modern cell. Inside those mineral pores, chemicals would have been trapped in a space with abundant energy, which could have been used to build more complex molecules. These in turn – as they began to accidentally replicate themselves using the energy from the proton gradients
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