I grew up reading northern European myths and legends, including tales of Odin, Thor, and the other gods of Asgard. I was largely unaware of the Marvel comics appropriation of Thor and other Asgardians as superheros until the Marvel cinematic universe started to hit the big screens in the last 12 years.

I have only recently come across the growing evidence that an Asgardian was one of our ancestors, way back near the beginning of the evolution of life, when the first eukaryotic cell appeared. Eukaryotes are organisms whose cells contain membrane-wrapped internal structures, such as a nucleus, unlike prokaryotes (Bacteria and Archaea), which have no membrane-bound internal structures.

In an earlier post on mitochondrial DNA, I mentioned that the mitochondria are thought to have arisen when a primitive cell incorporated a bacteria inside it. Increasing evidence is being found to suggest that an Asgardian was responsible for this. Mitochondria have a separate circular ring of DNA, like bacteria, which is separate to the nuclear DNA comprising the chromosomes.

Fifty years ago, scientists split life on Earth into two categories: eukaryotes; and prokaryotes, single-celled organisms that generally lack internal membranes. Bacteria were the only prokaryotes that biologists knew about. Then, in 1977, evolutionary biologist Carl Woese and his colleagues described archaea as a third, distinct form of life — one that reached back billions of years. Archaea are also prokaryotes that are somewhat like bacteria, in that they lack nuclei and membrane-bound organelles, and get around using flagella. But there are a few key differences. They divide differently. Their cell walls are made of slightly different stuff. And their RNA is different enough to separate them on the phylogenetic tree. Archaea are able to survive in very harsh environments, such as those found in undersea volcanic vents, and to derive energy from hydrocarbons such as methane or butane.

In 2015, A Spang and colleagues [1] recovered several unusual Archaea from the sea floor near a “black smoker” hydrothermal vent between Greenland and Norway. This vent field had been discovered seven years earlier and had been named Loki’s Castle as its shape reminded its discoverers of a fantasy castle. Loki is the trickster god of Norse mythology, son of the giant Farbauti [2]. He is a friend of Thor and the blood brother of Odin, not the adopted son of Odin as in the Marvel Cinematic Universe.

The unusual archaea discovered near Loki’s Castle were named Lokiarchaeota and share several genetic features with us, the Eukaryotes. DNA analysis of these organisms suggests that modern eukaryotes belong to the same archaeal group. According to Spang and colleagues, the ancestor of the Asgard archaea probably fed on carbon-based molecules, such as methane and butane. This diet would have generated byproducts that could nourish partner bacteria. Such food-sharing agreements — common among microbes — could have evolved into a more intimate relationship. An archaeon might have snuggled up next to its bacterial partner to ease nutrient exchanges, leading eventually to the ultimate embrace.

This has been an area of controversy and sometimes bitter argument, but I will review recent evidence below that indeed suggests we are descendants of ancient Asgardian archaea.  In the five years since the discovery of the Lokiarchaeota, several other archaea were discovered that had eukaryotic characteristics [3]. These archaea were named after other Norse gods: Thorarchaeota, Odinarchaeota and Heimdallarchaeota. The entire grouping of new phyla was named Asgard after the realm of the Norse gods. When these Asgardian gods are included in the tree of life, eukaryotes do not cluster anymore as a separate domain. Instead, they fall within the domain of archaea.

Eukaryotes are thought to have arisen when two types of single cell merged, with one engulfing the other. A cell from the Asgardian domain of archaea is proposed to have engulfed a bacterial cell of a type know as alphaproteobacterium, and the engulfed bacterium evolved into the energy generating mitochondria of eukaryotes, which later evolved into multicelled organisms, among which are humans. This event is estimated to have occurred 1.8 to 2 billion years ago [4]

Ancient Asgardians are thought to be the archaea who took this step, because many of the Asgardian genes involved in DNA transcription and replication share a common ancestor with (are genetically similar to) the corresponding eukaryotic genes.  And the Lokiarchaeota are the closest living relatives.

Last week’s issue of Nature reports the growth of an Asgardian in the laboratory for the first time ever. Imachi and colleagues [5,6] report that they have cultured an Asgardian from the Lokiarchaeota phylum that they have named Prometheoarchaeum syntrophicum. Although Prometheoarchaeum was obtained from deep ocean sediments, Imachi and colleagues found that it grew best at 20° C with amino acids, peptides and even baby-milk powder for food.

They found that Prometheoarchaeum would only grow in the presence of one or two other microbes, the archaeon Methanogenium and the bacterium Halodesulfovibrio. Prometheoarchaeum breaks down amino acids into hydrogen, which the other microbes eat. But the hydrogen-producing process requires the breakdown of sulphates into sulphides which only the other two microbes can do. So these organisms have a syntrophic relationship in which they are all needed for mutual survival.

Prometheoarchaeum develops lengthy appendages with multiple branches, and suggest the microbe may have used the tentacles to grab hold of oxygen-producing organisms. Imachi and coauthors speculate that the origin of mitochondria was a result of the need for the host cell to adapt to oxygen use as a consequence of rising oxygen levels on ancient earth, and that a bacterial partner became entangled with and then engulfed by an ancestral Asgardian.

The diagram below gives a more detailed summary of the tree of life showing the family relationships between bacteria, archaea and eukaryotes. It is from a 2016 paper, and does not include all the Asgardian phyla now discovered (see previous diagram for those).

I cannot resist concluding with a picture of Heimdall from the first of Marvel’s Asgardian movies.

References

1. Spang, A., Saw, J. H., Jørgensen, S. L., Zaremba-Niedzwiedzka, K., Martijn, J., Lind, A. E., … Ettema, T. J. G. (2015). Complex archaea that bridge the gap between prokaryotes and eukaryotes. Nature, 521(7551), 173–179. doi:10.1038/nature14447
2. Neil Gaiman. Norse Mythology. Bloomsbury: London, 2017.
3. Zaremba-Niedzwiedzka K,  Caceres EF, Saw JH, et al. Asgard archaea illuminate the origin of eukaryotic cellular complexity. Nature 2017, 541:XXX. doi10.1038/nature21031
4. Betts HC, Puttick MN, Clark JW, Williams TA, Donoghue PCJ, Pisani D. Integrated genomic and fossil evidence illuminates life’s early evolution and eukaryote origin. Nat Ecol Evol. 2018;2(10):1556–1562. doi:10.1038/s41559-018-0644-x
5. Imachi H, Nobu MK et al. Isolation of an archaeon at the prokaryote-eukaryote interface. Nature 577, 519–525 (2020)
6. Schleper C, Sousa FL. Meet the relatives of our cellular ancestors. Nature 2020, 577: 478-479.
7. Hug LA, Baker BJ, Anantharaman K, Brown CT, Probst AJ, Castelle CJ et al. (2016). A new view of the tree of life. Nat Microbiol 1: 16048.