Multicellularity is often interpreted through ecological or spatial mechanisms that allow early aggregates to persist without direct fitness benefits. While such models illuminate how multicellular forms can be maintained and diversified, they presuppose that early cells already possessed the energetic capacity required for differentiation, adhesion, and the maintenance of non-reproductive cell types. I argue that complex differentiated multicellularity could not have arisen at all without the energetic preconditions established by endosymbiosis. The acquisition of mitochondria and plastids fundamentally altered the geometry of energy generation, enabling orders‑of‑magnitude increases in ATP availability per gene and per unit volume. This energetic revolution lifted the constraints that had limited prokaryotic complexity for billions of years, making sustained division of labor feasible for the first time. Once this threshold was crossed, large‑scale geochemical regimes—not classical ecological structure—provided the initial directionality for functional divergence. Recognizing endosymbiosis as the primary enabling condition, and geochemical structure as the subsequent organizing force, yields a hierarchical causal framework for understanding how multicellularity—and ultimately complex life—became possible.
S. Kato (Thu,) studied this question.