This action funds an NSF Postdoctoral Research Fellowship in Biology for FY 2025. The fellowship supports research and training of the fellow that will contribute to biology in innovative ways. This research addresses a fundamental evolutionary question in biology: how did environmental forces shape the emergence of animals from unicellular ancestors? To explore this, the fellow will study choanoflagellates, the closest living relatives of animals. Despite their evolutionary significance, research on choanoflagellates has been largely limited to a few model species. This project focuses on the evolutionary adaptations of Barroeca monosierra, a recently discovered multicellular choanoflagellate isolated from Mono Lake, California, and its novel internal microbiome, to extreme environmental conditions. The findings will provide insights into how choanoflagellates evolve complex interactions with their environment and microbial partners, shedding light on early host-microbe relationships that may have influenced the origin of animals. Additionally, this work has broader implications for conservation and biotechnology, with potential applications in microbial stress resistance and bioremediation. This research will examine how environmental stressors drive adaptation and symbiosis in B. monosierra, focusing on high salinity, arsenic, and cyanide. To achieve this, the fellow will use an interdisciplinary approach, investigating B. monosierra in both its natural habitat and cont