Life cycles are central to all living organisms on Earth -- every living being is born, grows, and eventually reproduces. There is a great diversity of life cycles even among species we consider ``simple'': unicellular and primitive multicellular life forms. They reproduce in a number of ways: by fragmentation of the colony in multicellular pieces, by emission of unicellular propagules, and by dissolution of the organism into solitary cells. The choice of life cycle has a strong impact on the evolution of complex traits on the way to advanced multicellularity. But what drives the evolution of life cycles in the first place? The proposed research project will develop theoretical foundations for the evolution of multicellular life cycles. We will investigate the evolution of reproduction via a single-cell bottleneck (WP1), the impact of cell colony structure on the evolution of reproduction modes (WP2), the eco-evolutionary dynamics of multicellular life cycles (WP3), and generalise over previous findings to reveal the interplay of factors driving the evolution of multicellular life cycles (WP4). To do so, we will combine methods and insights from mathematics, physics, demography and classical biology. The proposed research program will cast light on the fundamental principles behind one of the most fascinating events in the history of life - the transition from simple solitary cells to complex multicellular existence. As such, the research results will be of interest not only to academics but also the general public. In addition, the Researcher will further enhance his knowledge and skills by working in an internationally renowned institution alongside experts in the field.