I closed the first chapter with some big questions about the evolution of life on earth. Why did life arise so early? Why did it stagnate in morphological complexity for several billion years? Why did complex, eukaryotic, cells arise just once in 4 billion years? Why do all eukaryotes share a number of perplexing traits that are never found in bacteria or archaea, from sex and two sexes to ageing? Here I am adding two more questions of an equally unsettling magnitude: why does all life conserve energy in the form of proton gradients across membranes? And how (and when) did this peculiar but
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