In the fall of 2019, however, a culture of a Loki organism started by Rodrigues-Oliveira began to inch along. It divided in about half the time as the Japanese strain, and it reached densities 50 to 100 times higher. Even so, working with it could still be like leafing through a Where’s Waldo? book: In 36 hours of scanning samples through an electron microscope, Schleper said, the team spotted just 17 individual specimens.

Last December, they debuted their results in Nature. This Loki, too, had tentacle-like filaments that Schleper’s group speculate might entangle other organisms and interact with them. Scooping the Japanese team, they showed that the tentacles were made of a protein, Lokiactin, that closely resembles the actin with which eukaryotic cells build supportive cytoskeletons. So not only is the Lokiactin gene like a eukaryotic gene, but it performs a eukaryote-like function.

The Lokiactin gene also pops up in every one of the 172 or so Asgard genomes that scientists have encountered. That implies that the ancestor of the entire group — and maybe the ancestor of all eukaryotes — might have had a similar proto-skeleton.

So what is Schleper’s lab trying to do with the organism now? “Everything!” she said, laughing.

Reaching Out to Form Complex Cells

Within the now-dominant two-domain picture to which the Asgard archaea are contributing, the big story of life on this planet goes something like this. Some 4 billion years ago, life forked into two single-celled branches, the archaea and the bacteria.

Genetic evidence implies that the two branches crossed again 2 billion years later when an archaeon — likely from the Asgard group — somehow ingested a bacterium. The process domesticated what was once a distinct, free-living cell and turned it into the organelles called mitochondria that persist inside eukaryotic cells. The descendants of that fateful union branched into other single-celled organisms like dinoflagellates, and then later into multicellular creatures that grew to macroscopic sizes, left fossils behind, and colonized both sea and land.

But even theorists who stand behind this narrative belong to divided camps. Some argue that gaining mitochondria was the defining event in eukaryogenesis. Others insist that mitochondria arrived late in an ongoing transition.

“You might have had Asgard archaea that were already quite complex and quite eukaryote-like,” said Tom Williams, a computational microbiologist at the University of Bristol, describing the latter position. “Then they acquired mitochondria, in an extreme form of this view, as a sort of icing on the cake.” Williams, however, thinks that mitochondria were acquired earlier than that: The complexity of the Asgards has just tipped the discussion toward an intermediate view, he said.

But the data from research on Asgards has also constrained the eukaryogenesis debate in other ways. For one thing, both of the Asgards cultivated so far have proved hard to separate from an entourage of other microbes. Like the Japanese Loki, the Austrian organisms seems to prefer — even depend on — having an extra species of archaeon and another sulfate-reducing bacterium in culture with them. Scholars working on eukaryogenesis, such as Purificación López-García at the French National Center for Scientific Research, have long promoted the idea that mitochondria were first captured from within just this kind of “syntropic” partnership, where multiple species live interdependently.

The finding that Lokis have actin tentacles adds plausibility to a eukaryogenesis scenario called the inside-out model, Spang and Schleper said. In 2014, the cell biologist Buzz Baum at University College London and his cousin, the evolutionary biologist David Baum of the University of Wisconsin, Madison, proposed an idea they had kicked around at family events: that the first eukaryotes were born after a simple ancestral cell extended protrusions past its cell walls. First these arms reached toward a symbiotic bacterium. Eventually they closed around that partner, turning it into a proto-mitochondrion. Both the original archaeal cell and the captured symbiote were enveloped within a skeleton provided by the arms.

Back when Asgard archaea were still known only from scraps of environmental DNA, Baum had asked attendees at a conference to draw what they thought the organisms would look like. His own drawing based on the inside-out ideas, which predicted that they would sport protruding arms, surprised the other assembled scientists. At the time, Schleper said, it seemed “so odd that he makes this funny suggestion.”

A Competitive Atmosphere

The events of eukaryogenesis have been so obscured by intervening time and gene-swapping that we may never know them with certainty.

The two Loki species currently in culture, for example, are modern-day organisms that differ from ancient archaea in the same way that a living, singing cardinal differs from the ancestral dinosaur from which it evolved. The Loki group isn’t even the subset of Asgard archaea that genetic analyses suggest is most closely related to eukaryotes. (Based on known Asgard genomes, a preprint posted by Ettema and his colleagues in March argued that the ancestor of eukaryotes was a Heimdall archaeon.)

Still, labs around the world are gambling that bringing more diverse representatives of the Asgard group into cultivation will yield a bonanza of new clues about their — and our — common ancestor. Schleper is trying. So is Ettema. So is Baum, who said his lab is soon welcoming a new colleague who will bring vials of archaea from groups like Heimdall and Odin. So is Imachi, who declined to speak to Quanta for this story.

“If I were to be interviewed by you now, I would most likely talk about new data that has not yet been published,” he explained in an email, adding that his group applauded the Schleper team’s efforts. “It is very competitive now (although I do not like this kind of competition),” he added.

Other sources also bemoaned the overly pressurized atmosphere. “It would be nice if the field would be more open to sharing,” Spang said. The pressure weighs heaviest on the young scientists who tend to take on the high-risk, high-reward cultivation projects. Success can add a glowing Nature paper to their resume. But wasting years on a failed effort can stunt their chances of ever getting a job in science. “It’s really an unfair situation,” Schleper said.

For now, though, the race continues. When the Baum cousins published their ideas about eukaryogenesis in 2014, Buzz Baum said, they assumed we’d probably never know the truth. Then suddenly the Asgards showed up, offering new glimpses of the liminal, transitional stages that boosted life from single-celled simplicity into overdrive.

“Before we destroy this beautiful planet, we should do a bit of looking, because there’s cool things on planet Earth we know nothing about. Maybe there are things that are sort of living fossils — states in between,” he said. “Maybe it’s on my shower curtain.”

Correction added April 11, 2023:
In an earlier version of this story, when Tom Williams described a “mitochondria late” view of eukaryogenesis, it was not clear that he was describing a view with which he disagrees.