Because of its ancient origins, little is known about the first steps in the transition to multicellularity. Perhaps the largest gap in our knowledge concerns the origin of cellular differentiation. Our proposed research will examine how aging (a nearly-universal feature of cellular life) can be co-opted by simple multicellular organisms to drive the emergence of novel multicellular development. The proposal combines the PIs’ complementary expertise in aging and protein homeostasis (Saarikangas) and evolutionary biology (Ratcliff). Our research uses a unique approach: we evolve multicellularity from a single-celled yeast in the lab, examining how, over thousands of generations of evolution, they become more complex. Following our preliminary results, we focus on the role of aging and the chaperone protein Hsp90, mechanistically examining their role in the origin of simple cellular differentiation in our model system. We contextualize these results with mathematical modeling, examining the potential for age-dependent development to arise in different lineages of multicellular organisms. By showing how multicellular development can arise de novo, our research illuminates a problem of profound biological importance, and removes one of the last conceptual hurdles remining in our understanding of the origin of multicellular life.