Eukaryotes first emerged during the Paleoproterozoic, likely as flagellated cells.- Meet the Eukaryote, the First Cell to Get Organized (quantamagazine.com)
Three billion years ago, life on Earth was simple. Single-celled organisms ruled, and there wasn’t much to them. They were what we now call prokaryotic cells, which include modern-day bacteria and archaea, essentially sacks of loose molecular parts. They swirled together in shallow, primordial brews or near deep-sea ocean vents, where they extracted energy from the environment and reproduced by dividing one cell into two daughter cells. Then, one day, that wilderness of simple cells cooked up something more complex: the ancestor of all plants, animals and fungi alive today, a cell type known to us as the eukaryote
- Eukaryote (Wikipedia)
The eukaryotes (/juːˈkærioʊts, -əts/) constitute the domain of Eukarya, organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes: the Bacteria and the Archaea. Eukaryotes represent a small minority of the number of organisms, but due to their generally much larger size, their collective global biomass is much larger than that of prokaryotes.