- 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