Asgard Archaea

The Asgard archaea are a group of archaea that are the closest known relatives of the eukaryotes. Their genomes contain many genes once thought to be unique to complex cells, and they are central to the question of how the first eukaryotic cell arose.

In 2015 researchers sequenced DNA from sediment near the hydrothermal vent field called Loki's Castle in the Arctic Ocean and found a new lineage, named Lokiarchaeota. Related groups were named after Norse gods, such as Thorarchaeota, Odinarchaeota and Heimdallarchaeota, giving the collective name Asgard. In 2019 a Japanese team led by Hiroyuki Imachi grew one member, Prometheoarchaeum syntrophicum, after more than a decade of effort, from deep-sea mud.

Asgard genomes encode versions of proteins that eukaryotes use for the cytoskeleton, such as actin-like proteins, for membrane trafficking, and for tagging proteins for destruction. The cultured Prometheoarchaeum is small and grows slowly, and it forms long, branching protrusions. It probably lives by feeding on amino acids in partnership with other microbes.

The data support the view that eukaryotes arose from an Asgard-like host that took in a bacterium, the ancestor of the mitochondria, a hypothesis explained under endosymbiotic theory and eukaryogenesis. Under this view the tree of life has only two primary domains, bacteria and archaea, with eukaryotes nested within the archaea. Heimdallarchaeota appear to be the closest relatives of eukaryotes.

• Archaea — the domain they belong to
• Eukaryote — the cells they are related to
• Mitochondria — the organelle from the bacterial partner