Soma as transmission control in multicellular evolution: a population-genetic framework for germline restriction and cellular altruism

Juan Rivas-Santisteban , Juan F. Poyatos 

A central question in the evolution of multicellularity is why lineages repeatedly transition from germ-dominated unicellular states to organizations with extensive somatic investment. If somatic cells are largely excluded from future generations, why are they produced at all, and why do soma-free multicellular alternatives appear limited in stability, persistence, or attainable complexity? Here, we propose a population-genetic framework that distinguishes germline and soma by their differential capacity to transmit genetic variation across generations. Cellular altruism emerges when replication within an individual is decoupled from heritable contribution, allowing cells to adopt transient or permanent somatic roles. This review presents an updated evolutionary synthesis incorporating recent discoveries that expand our understanding of germ-soma separation archetypes. We argue that somatic organization may repeatedly evolve because it mediates cellular conflict by allocating labour, buffering mutational damage, and restricting the lineages through which mutations reach future generations. Facultative altruism can serve as an evolutionary bridge to obligate soma, while creating asymmetries in mutation propagation that favour progressively tighter restrictions on heritable transmission. Under this view, soma is not merely sterile tissue but a recurrent evolutionary filter that governs the amount and composition of genetic variation reaching subsequent generations.