In December 2025, Brett Baker led a research cruise to collect microbial genomes off the coast of Uruguay. In this photo, Tyler Smith pilots a remotely operated vehicle collecting shallow coastal sediments. Credit: Brett Baker Scientists widely agree that complex life emerged after two very different microbes formed a close partnership. That merger eventually gave rise to plants, animals, and fungi, collectively known as eukaryotes. Yet one key question has lingered for years. How did these two