The evolution of the sporophyte in terrestrial plants is a controversial topic in evolutionary biology, due to two opposing theories that are equally supported. The “homologous theory” proposes that sporophytes descended directly from gametophytes in an aquatic algal ancestor that was retained in terrestrial descendants. This theory would then define a sporophyte as a “transformed gametophyte with the specific function of spore production”.
The “antithetic theory” proposes that the sporophyte is essentially an exaggerated zygote, originating when a zygote in an archegonium delayed meiosis and instead divided mitotically to form a blob of diploid cells. This theory indicates that the sporophyte evolved simultaneously in algae and bryophyte ancestors more or less at the same time they were transitioning onto land. This theory is also called the interpolation theory because it involves another step being “inserted” into the life cycle of plants.
There is evidence for both theories, and no real solution in sight yet, but it is still an interesting question worth pondering.