Different as the cells from animals, plants, fungi and protozoa can be, they all share one prominent feature: a nucleus. They have other organelles, too, like the energy-producing mitochondria, but the presence of a nucleus — a well-defined porous pouch full of genetic material — is what inspired the biologist Édouard Chatton in 1925 to coin the term eukaryotes, which referred to living things with a “true kernel.” All the rest he labeled prokaryotes, for life “before kernel.” This dichotomy between nucleated and nonnucleated life became fundamental to biology.
No one knows exactly how the nucleus evolved and created that division. Growing evidence has persuaded some researchers, however, that the nucleus might have arisen through a symbiotic partnership much like the one believed to have produced mitochondria. A crucial difference, though, is that the partner responsible for the nucleus might not have been a cell at all, but a virus.
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