Why does this matter for evolution?

Biologists have long proposed that the first eukaryotic cell — the kind found in all plants, animals, and humans — emerged from an intimate partnership between an ancient archaeon and a bacterium. In that scenario, one organism eventually engulfed the other, giving rise to mitochondria, the energy-producing structures inside complex cells.

Direct evidence for such a partnership has been elusive. The new images offer the first visual proof that an Asgard archaeon can physically interact with a bacterial partner in the present day.

Associate Professor Debnath Ghosal of the University of Melbourne, a coauthor, said: "This discovery brings us a few steps closer towards understanding how complex cells evolved from relatively simpler microbial life forms."

Why was the work so difficult?

Asgard archaea are notoriously hard to cultivate outside their natural habitats. The team spent four to five years trying to grow the microbes in the lab.

"It took four or five years in the lab," Burns said. "A lot of time, optimizing and chasing different shadows."

The researchers could never isolate the archaeon in pure culture. Burns said that failure may itself reveal something about the organism's biology.

"The fact that we could never get these organisms into pure culture is probably because they always depend on other organisms to survive," he said.

What role did machine learning play?

The team also used deep learning, a form of machine learning, to predict the structures of proteins in the microbes. Coauthor Associate Professor Katharine Michie of UNSW said this approach lets researchers "see ancient versions of the cellular machinery that later became central to complex life."

How was the new species named?

Naming the microbe involved consultation with Kymberly Oakley, identified as the world's foremost Malgana language expert, and with Malgana elders. The Malgana language, spoken by traditional owners of central Shark Bay, was incorporated into the species name with the elders' permission. Indigenous people have inhabited the Shark Bay region for around 30,000 years.

What's next?

Burns said he hopes to identify additional microbial partnerships and expand what he called a "little primordial Asgard soup," giving scientists more pieces of the puzzle surrounding the earliest stages in the evolution of complex life.

Coauthor Associate Professor Iain Duggin of the University of Technology Sydney pointed to the deep timescale of such partnerships: "It's if we have slowly arisen from the bottom of the sea."

The team cautioned that the new images, while striking, come from a single study in one location. Further work will be needed to determine how representative this partnership is of the kinds of interactions that may have driven the origin of eukaryotes billions of years ago.