Life may have arisen near hydrothermal vents rich in iron and sulfur. The earliest cells incorporated these elements into small peptides, which became the first and simplest ferredoxins — proteins that shuttle electrons within the cell — to support metabolism. As cells evolved, ferredoxins mutated into more complex forms. The ferredoxins in modern bacteria, plant and animal cells are all derived from that simple ancestor. (Credit: Illustration by Ian Campbell/Rice University)

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