Figure 1. Phylogenomic relationships between Asgard archaea and eukaryotes

With the support of the National Natural Science Foundation of China (Grant No. 91851105 and 92051102), research team of Professor Meng Li (Institute for Advanced Study, Shenzhen University), in cooperation with research team of Professor Eugene V. Koonin (National Institutes of Health, USA), discovered six new phyla of Asgard archaea from samples of coastal wetlands and offshore sediments in China, to the Western Pacific abyss. The authors established the first Asgard archaea Clusters of Orthologous Gene database (AsCOG). The study analyzed the metabolic potential of Asgard archaea through their genome contents, reconstructed the Tree of Life composed of prokaryotes (bacteria and archaea) and eukaryotes, and elucidated the relationship between Asgard archaea and eukaryotes in detail in terms of phylogenetic relationships, eukaryotic signature proteins and metabolic interactions, providing new insights into the origin of eukaryotes. This study, entitled "Expanded diversity of Asgard archaea and their relationships with eukaryotes", was published by Nature on April 28, 2021 (link: https://doi.org/10.1038/s41586-021-03494-3).

Archaea are an essential part of prokaryotes that have a unique position in the Tree of Life on Earth and are crucial research materials for exploring the origin of life. Asgard archaea are the fourth recently established archaeal superphylum. Based on genomic analysis, some scholars believe that eukaryotes originated from Asgard archaea. The first five Asgard archaea phyla were named after gods in Norse mythology. Professor Meng Li et al. discovered six new Asgard Archaea phyla, five of which were also named after gods in Norse mythology, and the sixth more ancient novel branch of Asgard archaea, named Wukongarchaeota after Sun Wukong in the classical Chinese novel "Journey to the West". Wukongarchaeota have a chemoautotrophic metabolic potential of hydrogen oxidation, which is significantly different from other Asgard archaea with mixotrophic or heterotrophic metabolic capacities.

This study systematically analyzed the evolutionary relationship between Asgard archaea and eukaryotes. It determined that eukaryotes originated from within Asgard archaea, the common ancestral branch of Heimdall-Wukongarchaeota, or from other deeply branching unknown archaeal lineages (Figure 1). Furthermore, the study identified a large number of novel eukaryotic signature proteins (ESPs) encoded in Asgard archaeal genomes. Finally, it reconstructed the evolution of key metabolic pathways in Asgard archaea superphylum, suggesting that eukaryotes may have originated from a metabolic symbiosis between autotrophic Asgard archaea and fermentative bacteria, providing new insights to answer a basic scientific question on the origin of eukaryotes.