When sporangia became seeds: The innovation that rewrote land plant evolution

The origin of the seed represents one of the most transformative evolutionary innovations in land plants, underpinning the ecological success of spermatophytes and fundamentally reshaping terrestrial ecosystems. Rather than arising as a discrete novelty, the seed evolved through a long sequence of reproductive transformations that progressively reorganized the plant life cycle. This review aims to synthesize the evolutionary steps leading from free-sporing vascular plants to the seed habit, emphasizing their functional and developmental integration.

We synthesize palaeobotanical, morphological, developmental, and evolutionary evidence concerning the transition from free-sporing vascular plants to spermatophytes. Particular emphasis is placed on the ecological and evolutionary significance of heterospory, gametophyte sexual specialization, reduction to a single functional megaspore, endosporic development, megaspore retention, integument evolution, pollination, and the progressive internalization of fertilization. We argue that these innovations did not evolve independently but were assembled sequentially thus progressively transferring protection, nutrition, developmental regulation, and reproductive control from the free-living gametophyte to the parental sporophyte. This process transformed the ovule into a highly coordinated reproductive system in which dispersal, nourishment, pollination, embryogenesis, and environmental buffering became developmentally integrated.

The seed should be understood not simply as an integumented megasporangium or a novel reproductive organ, but as an integrated modular biological system that fundamentally reorganized the plant life cycle. The coordinated evolution of heterospory, monomegaspory, endospory, megaspore retention, integumentation, and pollination established a new reproductive strategy that increased parental investment, reduced dependence on external water, enhanced reproductive resilience, and enabled the successful colonization of increasingly complex terrestrial environments. This systemic reorganization ultimately laid the evolutionary foundation for the extraordinary diversification, ecological success, and long-term dominance of seed plants.