Picture the scene not as science fiction, but as science possibility: a biosphere in its final hours. A planet, perhaps orbiting a star a few dozen light-years from our Sun, cradles an entire web of life. Forests breath and photosynthesize. Oceans teem with creatures of staggering diversity. Somewhere in those oceans, or in the mineral-rich mud of hydrothermal vents, something resembling our earliest microbial ancestors pulses with chemistry. And then, the end comes. Perhaps a supernova irradiates the system. Perhaps a planetary collision tears the world apart. Perhaps a runaway greenhouse effect turns its oceans to vapor. The biosphere has decades or centuries, at most, to find a way out.
What if it did? What if, millions or even billions of years ago, some fraction of life on that dying world survived the catastrophe? Not by evolving quickly enough, not by hiding in deep rock, but by leaving entirely, crossing interstellar space, and landing on a young, receptive world called Earth?
This is not the premise of a novel. It is a scientifically coherent hypothesis with a name, a literature, and, as of 2024 and 2025, a growing body of circumstantial evidence that makes it harder to dismiss than at any previous point in human history. The question “What if all life on Earth came from another planet?” sits precisely at the intersection of panspermia theory, planetary catastrophe science, astrochemistry, and the fastest-moving field in modern biology: the genomics of extremophile organisms. To ask it seriously is to discover that the boundary between wild speculation and legitimate science is thinner than we assumed.
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