A Break in a Longstanding Mystery about Origin of Complex Life

Baker and his team research Asgard archaea genomes, uncovering new lineages, expanding enzymatic diversity and exploring their metabolic pathways. The team's latest finding agrees with the picture geologists and paleontologists have reconstructed of Earth's history. Until about 1.7 billion years ago, Earth's atmosphere had very little oxygen. Then, oxygen levels spiked dramatically, like levels seen today. Within a few hundred thousand years after this Great Oxidation Event, the first known microfossils of eukaryotes appeared, suggesting that the presence of oxygen might have been important for the origin of complex life.

"The fact that some of the Asgards, which are our ancestors, were able to use oxygen fits in with this very well," Baker said. "Oxygen appeared in the environment, and Asgards adapted to that. They found an energetic advantage to using oxygen, and then they evolved into eukaryotes."

Scientists believe eukaryotes arose when an Asgard archaeon developed a symbiotic relationship with an alphaproteobacterium. Eventually, they become one organism with the latter evolving to become an energy-producing organelle within eukaryotes called the mitochondria. In the new paper, the scientists vastly expand the number of Asgard archaea genomes and point to specific types of Asgard archaea, such as Heimdallarchaeia, which are closely related to eukaryotes but less common today.

"These Asgard archaea are often missed by low-coverage sequencing," said co-author Kathryn Appler, a postdoctoral researcher at the Institut Pasteur in Paris, France. "The massive sequencing effort and layering of sequence and structural methods enabled us to see patterns that were not visible prior to this genomic expansion."

Funding was provided for this work in part by the Gordon and Betty Moore and Simons Foundations, the National Natural Science Foundation of China and the National Health and Medical Research Council of Australia.