Life, for all its complexities, has a simple commonality: It spreads. Plants, animals and bacteria have colonized almost every nook and cranny of our world.
But why stop there? Some scientists speculate that biological matter may have proliferated across the cosmos itself, transported from planet to planet on wayward lumps of rock and ice. This idea is known as panspermia, and it carries a profound implication: Life on Earth may not have originated on our planet.
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