These microbes fall into four major groups: Lokiarchaeota, Thorarchaeota, Odinarchaeota, and Heimdallarchaeota. “There’s around 50 more Asgardian gods, we’re good to go for the next few years,” says Ettema.
These four lineages all belong to an overarching supergroup that the team have called Asgard. And we eukaryotes either sit within the Asgardian family tree, or just outside it. We descend directly from either members of the group (perhaps within the Heimdall branch), or from incredibly close relatives.
The genomes of the Asgardian microbes hold many surprises. When Ettema looked at their DNA, he was surprised to find genes that are supposedly unique to eukaryotes. Some are for building and remodeling internal skeletons, which archaea don’t have. Others are involved in pinching off bits of the outer membrane to create little pockets that can move molecules around large cells—and yet, archaea are small. Yet others are typically involved in shuttling molecules between the compartments inside eukaryotic cells—compartments that typically don’t exist in archaea.
When Ettema first found these genes in Loki two years ago, other scientists were skeptical. Perhaps they came from eukaryotes that had contaminated Ettema’s samples. But the presence of such genes in other members of Asgard, collected by different teams from different corners of the world, suggests that they’re real.
This doesn’t mean that the Asgardians are eukaryotes themselves, or even anything close. As James McInernery and Mary O’Connell write, “The distribution of genes formerly thought to be eukaryote-specific is patchy,” so that no single Asgardian has “a full set.” Instead, it seems that “the building blocks for those eukaryote-defining traits were most likely already present in our archaeal ancestors,” says Ettema.
This feeds into a long-running and still unsettled debate about the origin of eukaryotes. Some scientists believe that the bacterium that would become the mitochondria was swallowed by a host cell that was already big, complex, and most of the way to becoming a eukaryote. Others hold that the host was still a true archaeon, and that it was the merger that allowed it to become more complex.
On the face of it, the presence of eukaryote-y genes in Asgard supports the former idea. But Ettema disagrees. Based on their genomes, “they’re not very sophisticated cells, but they have many of the right components in place,” he says. “They were primed to become eukaryotes.”
This debate is likely to rage on for a while, because no one has actually seen the Asgardians themselves. In the 1980s, scientists developed ways of identifying microbes by searching for their DNA amid environmental samples. This is now the main way of finding new species, but it means that many microbes—like Loki and its kin—are only known through their DNA. No one knows what they look like.
“It’s high on our priority list,” says Ettema. “It’s crucial that we have a look at the cells to see what they’re doing, but that’s extremely hard. Loki was discovered in deep-sea ocean sediments, where you don’t go every day. Thor and Heimdall are present in shallower environments, but they’re less than 0.1 percent of the total microbial community. It’s like looking for a needle in a haystack, but we’re working on it.”