Near an underwater hydrothermal vent field called Loki's Castle between Greenland and Norway, researchers found a new group of archaea whose genomes contained proteins previously thought exclusive to complex life. Named the Asgard Archaea, including Lokiarchaeota, Thorarchaeota, and Heimdallarchaeota, they suggest that all complex life, every animal, plant, fungus, and you, may have evolved from within these ancient microbes, meaning the tree of life may have only two domains, not three.
Hiroyuki Imachi, Masaru K. Nobu,Nozomi Nakahara,Yuki Morono, Miyuki Ogawara, Yoshihiro Takaki, Yoshinori Takano, Katsuyuki Uematsu, Tetsuro Ikuta, Motoo Ito, Yohei Matsui, Masayuki Miyazaki, Kazuyoshi Murata, Yumi Saito, Sanae Sakai, Chihong Song, Eiji Tasumi, Yuko Yamanaka, Takashi Yamaguchi, Yoichi Kamagata, Hideyuki Tamaki, and Ken Takai, CC BY-SA 4.0, via Wikimedia Commons
A Castle of Black Smokers
At 73.5° N on the Arctic Mid-Ocean Ridge, 2,352 meters beneath the sea, sits a field of active hydrothermal vents. Discovered in 2008 by a University of Bergen expedition, it was named Loki's Castle because its shape resembled a fantasy fortress. This field consists of at least five "black smoker" chimneys, some over 12 meters tall, which vent mineral-rich water at temperatures up to 320°C. The vents sit on a massive mound of sulfide mineral deposits roughly 250 meters in diameter.
It was in the sediments near this extreme environment that scientists made a deep discovery about the history of life. In 2010, researchers took a sediment core and subjected it to metagenomic analysis, sequencing the DNA of all the organisms within. This analysis, published in 2015, revealed a completely new phylum of Archaea. Named Lokiarchaeota after the vent field, these microbes were a branch on the tree of life—they were the closest known relatives to all eukaryotes.
Rewriting the Tree of Life
The genetic material from the Lokiarchaeota contained genes for proteins previously believed to exist only in eukaryotes, the domain of life that includes animals, plants, and fungi. These "eukaryote-specific proteins" are involved in creating a cytoskeleton and forming internal membrane structures—hallmarks of cellular complexity. Subsequent discoveries in other environments identified related lineages, which were given names from Norse mythology: Thorarchaeota, Odinarchaeota, and Heimdallarchaeota. Together, they form the Asgard superphylum.
This evidence strongly supports a "two-domain" model of life, challenging the long-held three-domain (Bacteria, Archaea, Eukaryota) system. In the two-domain model, eukaryotes are not a separate, equal domain but instead evolved from within the Asgard archaea. This suggests an ancient Asgard archaeon was the "host" cell that, in a key moment for life on Earth, engulfed a bacterium that would eventually become the mitochondrion.
For years, Asgard archaea were only known from their DNA. No one had ever seen one. That changed after a grueling 12-year effort by a Japanese team, which successfully cultured a strain of Lokiarchaeota. Named Candidatus Prometheoarchaeum syntrophicum, this organism is a slow-growing anaerobe that lives in a tight, energy-exchanging partnership (syntrophy) with other microbes. It has long, branching cellular protrusions, providing a visual model for how its ancient relative might have entangled and absorbed its bacterial partner, led to for all complex life.
💡Fun Facts
The Loki's Castle vent field was discovered in July 2008.
The name refers to the Norse trickster god Loki and was chosen because the field was so difficult to locate.
It took a Japanese laboratory 12 years of continuous effort to finally grow a single Asgard archaeon in a culture.
The cultured archaeon, *Prometheoarchaeum*, has an extremely slow doubling time, estimated at 14 to 25 days.
