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.
“What we [eukaryotes] are is a classic case of what they call emergent complexity,” explained Philip Bell, the head of research for the yeast biotechnology company MicroBioGen. Bell proposed a viral origin for the eukaryotic nucleus back in 2001 and refreshed the theory in September. “It’s three organisms that came together to make a new community, which eventually integrated to such an extent that it became, effectively, a new life-form.”
He and other researchers take their confidence from findings such as the demonstration that giant viruses build “viral factories” inside prokaryotic cells — compartments that, much like the nucleus, uncouple the processes of transcription (reading genes) and translation (constructing proteins). “I think it’s now the strongest model,” he said.
Most researchers who study the origins of eukaryotes might not agree with him; some still describe it as an idea on the fringe. But proponents of a viral origin point out that several recent discoveries line up conveniently with a viral model — and they believe that conclusive evidence in their favor is finally within reach. [Continue reading…]