Microbes named after Norse gods may hold the origin of complex cells
Mud near an Arctic hydrothermal vent called Loki's Castle yielded genomes of microbes carrying proteins once thought unique to cells with nuclei. Named for Loki, then joined by relatives named for Thor, Odin and Heimdall, these Asgard archaea suggest that animals, plants and fungi all sprang from within an archaeal branch.
The story began with sediment drilled in 2010 from a rift on the Knipovich ridge in the Arctic Ocean. Genetic analysis of layers rich in unfamiliar archaea led a team at Uppsala University to propose Lokiarchaeota in 2015, a nod both to the vent and to a trickster god famous for provoking scholarly argument. A Texas-led group found Thor's namesake in North Carolina's White Oak River in 2016, and samples from Yellowstone to Japan added Odin and Heimdall. The whole collection became Asgard, now formally the kingdom Promethearchaeati.
What makes them remarkable is their toolkit. They encode so-called eukaryotic signature proteins, including membrane-reshaping machinery and a ubiquitin tagging system. Their actin skeleton is regulated by proteins that can interact with actin from our own cells, and a tubulin from the Odin group may be an evolutionary step between bacterial division proteins and the microtubules in eukaryotes. In 2017 the Heimdall group turned out to have histone tails, previously thought to exist only in eukaryotes.
Most live without oxygen, and they are found in seabed sediments, soils, freshwater, plant roots and animal bodies. In 2020 researchers watched a cultured species, Promethearchaeum syntrophicum, swapping nutrients with two bacterial partners, a relationship they likened to the merger thought to have produced complex cells around two billion years ago.
A 2023 study placed eukaryotes deep inside the Asgard tree, as sister to a Heimdall subgroup called Hodarchaeales. In the favoured scenario, an Asgard host lived in mutual dependence with other microbes and absorbed an alpha-proteobacterium that became the mitochondrion. That supports dividing life into two domains rather than three.
Source: Asgard archaea
