Eukaryotic cells are generally much bigger than either bacteria or archaea. They also have larger genomes. They have internal compartments that act like our organs, each with its own special job. They have an internal skeleton that acts as a transport network for molecules. There’s this huge gulf of complexity that separates them from the other two domains. It’s a gulf that has only ever been crossed once in life’s history. Bacteria and archaea are capable of amazing feats of evolution, but in over 3.7 billion years of existence, none of them have ever evolved into anything approaching a eukaryote-like cell—except that one time. Why?

One possible answer, which I’ve written about before, says that eukaryotes were created through an incredibly unlikely merger between members of the other two domains. Somehow, a bacterium found its way inside an archaeon and, rather than being digested or destroyed, became a permanent part of its host. In doing so, it provided the archaeon with an extra source of energy, which allowed it to get bigger, accumulate more genes, and evolve down new paths that were previously inaccessible to it. That fusion cell gave rise to the eukaryotes, and the bacterium eventually turned into the mitochondria—little bean-shaped structures that still power eukaryotic cells to this day.

Once the eukaryotes evolved, they repeatedly engulf microbes and fused with them—a process called endosymbiosis. But that’s much easier to do when the host cell is already big, and can engulf smaller neighbors. If the host is an archaeon, the feat becomes much harder and far more improbable. That’s maybe why the merger between an archaeon and a bacterium—the one that gave rise to mitochondria and may have spawned the eukaryotes—has only happened once.

What were those two ancient partners like? We know that the bacterium belonged to a group called the alphaproteobacteria (which also includes Wolbachia, a microbe that I’ve repeatedly written about here.) But until recently, no one knew anything about the archaeon host.

Ettema changed that in 2015. His team at Uppsala University collected samples of sediment from Loki’s Castle—a field of hydrothermal vents that lie between Greenland and Norway, at depths of 2,300 meters. By analyzing the DNA within their sample, the team identified a new archaeon that they called Lokiarchaeota (low-key-ar-kay-oh-tuh). Their DNA revealed that they are the closest living relatives of all eukaryotes. They’re the best approximations that we have for that ancestral archaeon that gave rise to us all.

Loki isn’t alone. Last year, another group led by Brett Baker at the University of Texas in Austin discovered a related group of archaea in the mud of North Carolina’s White Oak River; continuing the theme, they called it Thorarchaeota. Now, by teaming up, Ettema, Baker, and others have found even more relatives in sites around the world: Yellowstone National Park, a hot spring in New Zealand, deep-sea vents lying near a Japanese island, and many more. “This work is fantastic: going out to the sediments of the planet and digging up the weird and wonderful bugs that live there,” says James McInerney from the University of Manchester.