The origins of complex life on Earth have long puzzled scientists, with the prevailing theory suggesting that eukaryotes—plants, animals, and fungi—evolved from a symbiotic relationship between two distinct types of microbes. The mystery lay in understanding how these microbes, one requiring oxygen and the other thriving in oxygen-free environments, came into close proximity. Now, researchers from The University of Texas at Austin have published findings in the journal Nature that may solve this enigma.
The study reveals that one of our microbial ancestors, belonging to the Asgard archaea group, is capable of using or tolerating oxygen. This discovery supports the theory that complex life evolved in an oxygen-rich environment, as predicted. Brett Baker, an associate professor of marine science and integrative biology at UT, explained,
“Most Asgards alive today have been found in environments without oxygen. But it turns out that the ones most closely related to eukaryotes live in places with oxygen, such as shallow coastal sediments and floating in the water column, and they have a lot of metabolic pathways that use oxygen. That suggests that our eukaryotic ancestor likely had these processes, too.”
The Asgard Archaea Connection
Baker and his team have been delving into the genomes of Asgard archaea, uncovering new lineages and exploring their metabolic pathways. Their latest findings align with geological and paleontological reconstructions of Earth’s history. The Great Oxidation Event, which occurred around 1.7 billion years ago, saw a dramatic increase in atmospheric oxygen levels, coinciding with the appearance of the first known eukaryotic microfossils. This suggests that oxygen played a crucial role in the emergence 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 noted. “Oxygen appeared in the environment, and Asgards adapted to that. They found an energetic advantage to using oxygen, and then they evolved into eukaryotes.”
Genomic Expansion and New Discoveries
Scientists believe eukaryotes emerged when an Asgard archaeon formed a symbiotic relationship with an alphaproteobacterium, eventually evolving into mitochondria within eukaryotes. The new study significantly expands the number of Asgard archaea genomes, identifying specific types like Heimdallarchaeia, which are closely related to eukaryotes but less common today.
Kathryn Appler, a co-author and postdoctoral researcher at the Institut Pasteur in Paris, France, highlighted the importance of comprehensive sequencing efforts:
“These Asgard archaea are often missed by low-coverage sequencing. 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 for this research came from 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. The research began with Appler’s Ph.D. work at The University of Texas Marine Science Institute in 2019, involving the assembly of over 13,000 new microbial genomes from marine sediments.
Implications and Future Research
This groundbreaking work has nearly doubled the known genomic diversity of Asgard archaea. By comparing proteins produced by Heimdallarchaeia to those involved in eukaryotic energy and oxygen metabolism, researchers used an AI model, AlphaFold2, to predict protein structures. The results indicated that several proteins from Heimdallarchaeia closely resemble those used by eukaryotes for oxygen-based, energy-efficient metabolism.
Other contributors to the study include former UT researchers Xianzhe Gong, Pedro Leão, Marguerite Langwig, and Valerie De Anda, as well as James Lingford and Chris Greening from Monash University in Australia, and Kassiani Panagiotou and Thijs Ettema from Wageningen University in the Netherlands.
The discovery not only sheds light on the origins of complex life but also opens new avenues for research into the evolutionary processes that shaped life on Earth. As scientists continue to explore the genetic and metabolic intricacies of Asgard archaea, further insights into the evolution of eukaryotes are expected, potentially rewriting our understanding of life’s history.
