Two Tiny Lifeforms Built a Bridge Too Small to See—and May Have Made Humanity Possible

3 min readHere’s what you’ll learn when you read this story:Billions of years ago, two single-celled organisms, an archaeon and a bacterium, teamed up to form the first eukaryotic cell. Scientists don’t know exactly how this happened, but a new study found evidence of Asgard archaea, the closest living relatives of early eukaryotes, interacting with bacteria via thin nanotubes in microbial mats. Asgard archaea provide a compelling example of how archaea and bacteria likely interacted on early Earth.We all know that life began as single-celled organisms. Somehow, the prokaryotes known as bacteria and archaea arose from a pool of chemicals on early Earth (possibly in two separate events), and they were the predominant forms of life for billions of years. Then, around 2.7 to 1.8 billion years ago, something strange happened. Scientists believe an archaeon and an alphaproteobacterium teamed up via endosymbiosis, eventually producing mitochondria and the cellular structures that define eukaryotic, or complex, life.How this major moment in the evolution of life actually happened is a matter of debate, though. Asgard archaea, first discovered in 2015, are believed to be the closest living relatives of early eukaryotes. But because they favor oxygen-free environments, researchers have long puzzled over how they merged with bacteria in the first place. Luckily, the planet provides researchers with “living fossils,” also known as stromatolites, that offer a way to gaze into the distant past and unravel this enduring mystery.Stromatolites, formed by photosynthesizing cyanobacteria within microbial mats, are considered by some to be the oldest lifeforms still in existence, and they were largely responsible for introducing oxygen into early Earth’s atmosphere. Western Australia is home to a large collection of them, especially in an area known as Shark Bay.It’s here that scientists from the University of New South Wales, the University of Technology Sydney, and the University of Melbourne collected samples and isolated a member of the Asgard archaea family (Nerearchaeum marumarumayae). Amazingly, these archaea were interacting with bacteria (Stromatodesulfovibrio nilemahensis) found within the stromatolite using a thin tube that the researchers called simply a “nanotube.” It’s one of the first pieces of direct evidence of these two microbial lifeforms interacting in this way. The results of the study were published in the journal Current Biology.“Stromatolites could be more than ‘just’ a cradle of life where early microbial life flourished,” University of New South Wales’ Brendan Burns, senior author of the study, said in a press statement. “They could also tell us how complex life first emerged … This could be a little model for how these kinds of partnerships started and ultimately formed eukaryotes.”Using a high-resolution 3D-imaging technique known as electron cryotomography, researchers could see the detail of these structures at one millionth of a millimeter. They found that not only were the two microbes linked with nanotubes, but each o them also produced compounds that the other could use, including vitamins, nutrients and hydrogen.It’s hard to know if this interaction is precisely how eukaryogenesis looked billions of years ago, but evidence of an Asgard archaea and bacteria interacting within these microbial mats suggest that scientists’ best guess at how complex life came to be might be right.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.