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Abstract

How did biological organisms become so complex? Endosymbiosis is the prototypical example of an egalitarian evolutionary transition and is central to the origins of many complex biological systems. Why do only some symbioses undergo evolutionary transitions, and how does the host-symbiont relationship change during this process? Here, we characterise endosymbiosis by two emergent collective-level properties: the relationship between the host and symbiont (mutual dependence) and reproduction (reproductive cohesion). Using methods from adaptive dynamics, we study the co-evolution of the traits underlying these properties. Our central result is a robust demonstration that even when investments in dependence and cohesion are uncorrelated, host-symbiont mutual dependence arises faster than reproductive cohesion. Further, we show that the collective's formation and shared fate, coupled with different generation times of the host and symbiont, leads to an emergent asymmetry in how much they invest in the collective. Lastly, we account for biological realism that previous models have ignored and show that this can preclude a successful evolutionary transition towards stable endosymbiosis. Together, this work uncovers a fundamental property of endosymbioses, highlighting the immense effects of simple ecological factors and providing a clear way forward for theoretical and empirical investigations.