Asgard Archaea as Living Fossils: Illuminating the Origins of Eukaryotic Complexity

Asgard archaea represent one of the most compelling microbial lineages for under-standing the evolutionary transition from prokaryotes to eukaryotes. First identified through metagenomic surveys and later enriched in laboratory culture, these organisms are distinguished by their repertoire of eukaryotic signature proteins (ESPs), which suggest deep evolutionary links to the emergence of complex cellular structures. Recent advances, including single-cell genomics, metatranscriptomics, proteomics, and cryo-electron microscopy, have begun to reveal aspects of their biology that were pre-viously inaccessible. These studies highlight unique cellular architectures, novel meta-bolic strategies, and ecological interactions that allow Asgard archaea to thrive in extreme environments such as hydrothermal vents and deep marine sediments. Framed within the concept of “living fossils,” Asgard archaea offer an unparalleled glimpse into ancestral states of life that have persisted for billions of years. Their ability to survive under anoxic and energy-limited conditions also has profound implications for astrobiology, particu-larly in refining biosignature detection strategies for Mars, Europa, and Enceladus. By synthesizing current knowledge of Asgard biology and the cutting-edge methods applied to their study, this review emphasizes their central role in reconstructing the origins of complex life and in guiding the search for extraterrestrial life in analogous environments.