New Delhi: All plants, animals and fungi on Earth are collectively known as eukaryotes, and them emerged when an Asgard archaeon microbe developed a symbiotic relationship with an alphaproteobacterium. The two became one organism, with the alphaproteobacterium evolving into the power house of eukaryotic cells, called the mitochondria. The Asgard archaea live primarily in the deep sea, and other anoxic environments with little oxygen. Scientists were long puzzled over how the two types of microbes came together in the first place. New research, published in Nature, suggests that some Asgards use or at least tolerate oxygen. The research supports the idea that complex life emerged in an oxygen-rich environment.

While most Asgards today live in anoxic environments, the ones most closely related to eukaryotes live in places with oxygen, such as shallow coastal sediments and floating freely in the water column. They have a lot of metabolic pathways that use oxygen, suggesting that our eukaryotic ancestor had these pathways as well. The researchers uncovered new lineages, and explored their metabolic pathways. The research is aligned with the reconstruction of the history of the Earth by geologists and palaeontologists. The research indicates that Asgards adapted rapidly to the oxygen-rich environment after The Great Oxidation Event, where cyanobacteria produced tremendous amounts of oxygen and forever altered the atmosphere of the planet.

How complex life emerged on Earth

Till about 1.7 billion years ago, the atmosphere of the Earth had very little oxygen. The emergence of photosynthesising cyanobacteria resulted in a dramatic spike in oxygen levels. The first microfossils of eukaryotes appeared within a few hundred thousand years of The Great Oxygenation Event, suggesting that the presence of oxygen was essential to the emergence of complex life. The Asgards found an energy advantage in using oxygen, and evolved into eukaryotes.

The researchers used the AI model AlphaFold2, for which the Nobel Prize in Chemistry was awarded in 2024 to predict how proteins in microbes would fold into three-dimensional shapes. These shapes or structures of the proteins determines their functions. The research showed that several proteins of Heimdallarchaeia, a subset of Asgard archaea closely resembled those used by eukaryotes for oxygen-based, energy-efficient metabolism.