See, in its early days, life wasn't all that interesting. There were only two types of organisms floating around, bacteria and archaea — small, single-celled creatures with only the most primitive internal infrastructure. These cells didn't have the organelles required for complexity, like mitochondria (which produce energy) and nuclei (where DNA is stored and transcribed).
To gain complexity, and ultimately, multicellularity, cells needed an energy boost. So, some 2 billion years ago, scientists think that one archaeon engulfed a bacterium.
It was the start of a beautiful friendship. The smaller creature gave its host energy, while the archaeon made sure the bacterium was safe and fed. All eukaryotic cells — the kind that make up all multicellular life — owe their existence to the two creatures that underwent this fateful merger.
But who were they?
Phylogenomic reconstructions suggest that the bacterium came from a clan called Alphaproteobacteria, which share some important commonalities with mitochondria, their modern day successors.
Then, in 2015, a team led by biologist Thijs Ettema of Uppsala University in Sweden discovered a new type of microbe a mile and a half beneath the surface of the Atlantic Ocean. The organism, named Lokiarchaeota (Loki for short) for the wily trickster god of Norse myth, was an archaeon, but it looked like it could be related to more complex life-forms.
Next a separate team of researchers uncovered genetic material from a related type of archaea in a North Carolina estuary. Keeping with the Norse theme, they called it Thorarchaeota (for the thunder god Thor). Though neither the Thor or Loki microbe was the unknown ancestral archaeon, both hinted at its identity.
Now, using DNA from the two microbes, Ettema and his colleagues have reconstructed the entire phylogenetic group from which they came. They also populated their family tree with two more member groups: the Odinarchaeota (for Odin, god of healing, magic and knowledge) and Heimdallarchaeota (for Heimdall, guardian of Asgard).
The scientists think that the archaeon ancestor was either a member of the Asgard group or a very close relative. We multicellular organisms may also be members of the group, or originate from cousins that sit just outside it.
Though newly discovered Asgard group are not eukaryotes, they share some important similarities — including genes that were thought to exist only in complex cells. Members have the genetic material needed to build an internal skeleton, even though these primitive cells don't have a skeleton. They have genes for encoding proteins capable of crossing membranes, even though archaea — which lack organelles — don't really have membranes to cross. These findings suggest that many of the genetic tools needed for complex life may have evolved in the Asgard archaea — even though most of those organisms never achieved complexity.
“The things we thought made a eukaryote a eukaryote, we’re now finding in these new archaea,” co-author Brett Baker, an assistant professor at the University of Texas Marine Science Institute, said in a news release. “We’re essentially rewriting the textbook on basic biology.”
Ettema said his next goal is to isolate these cells and attempt to grow them under a microscope.
“I am convinced that this will reveal more important clues about how complex cells evolved,” he said. “Ultimately our microbial ancestry will be uncovered.”