A new theory proposes the first complex life forms evolved through the interactions of an archaeon (blue) a sulfate-reducing bacterium (red) and an oxygen-breathing bacterium, which formed symbiotic relationships exchanging gases and nutrients. Masaru K. Nobu, Hiroyuki Imachi and JAMSTEC, NPG Press/YouTube

After 12 years of patient efforts, scientists have succeeded in culturing an organism that may reveal how complex life evolved, possibly being the closest living relative to the species that began it all.

After the origins of life itself, the transformation from single-celled organisms to multi-cellular animals is probably the greatest step in life’s history. Around 2 billion years ago some life forms gained nuclei and mitochondria, which respectively stored their genome and provided their power.

The domain Eukaryota is descended from this event, being made up of all life that has these features, including not only plants, animals, and fungi, but some single-celled microorganisms as well. The most popular explanation for eukaryotes’ evolution is that they evolved from the combination of a bacterium, and a member of the third great domain of life, the Archaea. The idea runs that an archaeon consumed a bacterium, with the two forming a symbiotic relationship where the one provided energy to the other, eventually becoming mitochondria.

We can't travel back in time to witness this seminal moment, but microbiologists think the key to understanding how this occurred lies in studying the closest living relatives to that archaeon. Our capacity to do so has been limited, however, because growing the leading prospects in the lab has been beyond scientists.

Dr Hiroyuki Imachi of the Japan Agency for Marine-Earth Science and Technology has announced in Nature this barrier has finally been overcome. Imachi was working on organisms collected from sediment in the depths of the Pacific Ocean that have been named for Norse gods, and are collectively known as the Asgard archaea.

The Asgard archaea’s genomes have been studied intensively since it was realized they have many genes that are not found in other archaea, or bacteria for that matter, but are common in eukaryotes. However, our inability to culture them has prevented observations of their shape or behavior.

Surprisingly, Imachi found the best way to grow his targets is not to replicate the cold dark conditions on the seafloor where they normally live. Instead, he warmed them to room temperature and fed them amino acids, peptides, and even baby-milk powder. Crucially, he found Asgard archaea do not grow on their own, but only flourish as part of a symbiotic system. Even then they grow around a thousandth of the rate of E. coli.

In a mixing of ancient mythologies, Imachi named the species he cultured Prometheoarchaeum syntrophicum. Prometheoarchaeum has long tendrils stretching out from the 550-nanometer-wide main body of the cell. Imachi proposes these tendrils ensnared the bacteria that eventually became mitochondria.

It is proposed an archaeon responded to the rise of oxygen by forming a symbiosis with a sulfate-reducing bacterium, which consumed the hydrogen it produced, and then later an aerobic bacterium, which later became incorporated into it to become the mitochondria of the first eukaryotes. Imachi et al./Nature

Prometheoarchaeum generates hydrogen on which its symbiotic partners feed, and observations of this relationship led Imachi and co-authors to a new model for how mitochondria were captured. The paper proposes the archaea formed symbiotic relationships with two different types of bacteria to survive the rise of oxygen occurring at the time. One bacterium consumed waste hydrogen, while the other breathed the newly available oxygen, which would otherwise have poisoned the archaeon. After initially tangling in the tendrils, the aerobic bacterium became absorbed inside the cell.

If Imachi is right, we are all descended from a microbial threesome, in which the archaeon and oxygen-breathing bacterium eventually merged to become a single-life form. The sulfate reducer maintained its distinct identity, but was essential to providing the chemistry under which the archaeon and aerobic bacterium could come together.