The cell nucleus is what distinguishes animals, plants, fungi, etc., from bacteria:
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.”Christie Wilcox, “Did Viruses Create the Nucleus? The Answer May Be Near.” at Quanta
He points to the “viral factories” that giant viruses build inside prokaryotic cells. A similar theory holds that bacteria became our mitochondria. These sorts of theories are called “endosymbiosis.” There are many conflicting subtheories, as Wilcox details. One is that cells “learned” how to construct a nucleus from the giant viruses. If so, they are smarter than we think—or something is, anyhow.
We’re waiting for the book to come out, “The Mind of the Cell,” which details this discovery on the cells’ part. 😉
See also: Why do many scientists see cells as intelligent? Bacteria appear to show intelligent behavior. But what about individual cells in our bodies?