Deep inside stromatolites, ancient rock-like mounds built by microbes, researchers have captured something never seen before: an Asgard archaeon physically tethered to a bacterium, exchanging materials through tiny tube-like connections. The discovery, published in Current Biology, offers a direct visual clue to one of biology's biggest unanswered questions: how simple cells first joined forces to create complex life.
A partnership frozen in time
Stromatolites and their close relatives, microbial mats, look like dark, unremarkable rocks. But they are actually dense, layered communities of microbes that have existed for billions of years. Long before animals or plants appeared, these formations helped pump some of the earliest oxygen into Earth's atmosphere. Today, they still grow in Shark Bay, a World Heritage site in Western Australia. Samples collected there led a team from UNSW Sydney, the University of Technology Sydney, and the University of Melbourne to isolate a member of the Asgard archaea, a group of microbes thought to be close relatives of the ancestors of eukaryotes, the cell type that makes up all plants and animals, including humans.
The research team, led by Associate Professor Brendan Burns, an evolutionary microbiologist at UNSW Sydney, found that this Asgard archaeon was living in close association with another organism. The two microbes were connected by extremely thin, tube-like structures called nanotubes. Through these connections, the microbes appeared to exchange nutrients and other compounds. This is the first direct visual evidence of an Asgard archaeon physically interacting with a bacterium in this way.
A long-standing theory gains new support
For decades, biologists have proposed that the first complex cell arose from an intimate partnership between an ancient archaeon and a bacterium. The theory holds that one organism eventually engulfed the other, and that relationship gave rise to mitochondria, the energy-producing structures inside complex cells. But direct evidence of what such an early partnership might have looked like has been missing.
The new images provide a possible model for that ancient event. "This could be a little model for how these kinds of partnerships started and ultimately formed eukaryotes," said Burns. The discovery suggests that stromatolites, already known as a cradle of early microbial life, may also hold clues to how complex life first emerged.
Years of patient lab work
Getting to this point was not easy. Genetic sequencing had shown that the organisms' DNA was present in the samples, but growing the microbes in the laboratory took years of effort. The team eventually succeeded, allowing them to observe the partnership directly. The finding does not prove that this exact relationship led to complex cells, but it offers a rare, tangible glimpse of a process that may have happened billions of years ago.
Stromatolites, it seems, are not just relics of Earth's distant past. They are still active laboratories where ancient biological processes may be playing out today. By studying these living fossils, scientists are getting closer to understanding how the leap from simple to complex life was made.
