Thanks to microscopy, early biologists were able to make a binary distinction: there were eukaryotes and bacteria. The former had large, complex cells with internal compartments, while the latter were largely featureless. Which raised an obvious question: how did that apparently giant leap in complexity come about? While DNA sequencing provided some hints as to the relations among different branches on the tree of life, the unique features of eukaryotes and the genes that enabled them appeared to have no real antecedents.

Until recently, that is. Last year, a hydrothermal vent in the Arctic named Loki’s Castle yielded organisms that picked up the name Lokiarchaea. Now, researchers have used Lokiarchaea’s genome to find a large group of related species that they are naming the Asgard superphylum. Genetically, these organisms are the closest relatives of complex cells. The relationship is so close that all organisms with complex cells may simply be one branch of this group.

Domain names

One of the big ideas in biology is what’s called the three domains of life. Genetic data revealed that we couldn’t simply divide all living things into complex eukaryotes and simple bacteria. Instead, two very distinct groups lurked behind the seemingly simple cell architecture we used to call bacteria. While one of these groups retained the name bacteria, the second was called archaea to reflect its distinct lineage from early in life’s history. Bacteria, archaea, and eukaryotes thus made up the three domains of life.