New research suggests that the single-celled microbial ancestor of all complex life was far more sophisticated than previously believed. Until now, scientists assumed this ancient organism was a relatively primitive cell, similar to a modern bacterium. However, recent studies published in Nature Microbiology and Nature reveal that this ancestor possessed a surprisingly extensive molecular toolkit, laying the groundwork for the evolution of multicellular life, including humans, animals, and fungi. Because the exact moment of this ancestor's existence cannot be observed directly, researchers adopted a comparative approach. Thijs Ettema, a Professor of Microbiology at Wageningen University & Research and co-author of the studies, explains that the team analyzed two distinct lineages that share a common origin: eukaryotes, which include all complex life forms, and Asgard archaea, a group of single-celled microorganisms discovered only a decade ago in deep-sea sediments. By studying these "distant evolutionary cousins," scientists can infer the characteristics of their shared ancestor. The research involved collecting genetic material from over 400 different Asgard archaea samples from the Bohai Sea and the Gulf of California. While DNA sequence comparisons showed limited similarity due to roughly two billion years of independent evolution, the scientists shifted their focus to protein structures. Proteins, which are the functional machines encoded by DNA, fold into specific three-dimensional shapes that tend to evolve much more slowly than genetic sequences. Utilizing advanced artificial intelligence tools like AlphaFold, the researchers predicted the 3D structures of more than 35,000 Asgard archaeal proteins. The results were striking. The analysis identified approximately 1,300 proteins in Asgard archaea that were previously thought to exist exclusively in eukaryotes. These proteins are essential for processes such as intracellular transport, storage, and the formation of cellular compartments. Their presence in both lineages strongly suggests they were inherited from the common ancestor, indicating that the microbial precursor already possessed the complex machinery required for building eukaryotic cells. Further evidence comes from microscopic observations. Although culturing Asgard archaea is challenging due to their slow growth and preference for oxygen-poor environments, recent laboratory successes have revealed unexpected features. Some species display tentacle-like protrusions for movement and internal vesicles and membranes reminiscent of those found in human cells. Additionally, a new study has identified Asgard archaea living in oxygen-rich environments, carrying genes for oxygen processing. While it remains uncertain whether the ability to respire oxygen was inherited from the ancestor or developed independently, this adaptation suggests these microbes could have survived the gradual oxygenation of Earth's atmosphere. The rapid technological advancements in DNA sequencing and AI-driven protein prediction over the last decade have been crucial to these discoveries. When Asgard archaea were first identified ten years ago, genetic data from a single species provided only limited and unconvincing evidence of their complexity. Today, however, the expanded dataset and sophisticated analytical methods have confirmed early intuitions. While questions remain regarding the precise appearance of the ancient ancestor and the specific functions of its proteins at that time, the evidence is clear: our distant microbial ancestor was already equipped with the potential to evolve into the complex cells that make up life today.