A seed consists of an embryo with a packet of nutrition (usually), all housed in a protective coat. They range in size from the dust-like seeds of orchids to coconuts. And they all trace their origin to spores.
The first land plants evolved from green algae, perhaps five hundred million years ago or more, and dispersed their offspring as spores; modern mosses and ferns still do so. A long time passed before seeds evolved (and of course they have not stopped evolving). Botanists have deduced that spores evolved to become seeds by several major steps and conjectured about what innovative features made such steps successful, such that the next generations maintained and continued those traits. I thought it might be interesting to visualize those steps, to begin to understand what was involved with the process.
Start with spores: Spores germinate to form tiny organisms, called gametophytes because they produce gametes (sperm and eggs) that have one set of genetic chromosomes (one set is termed 1N). Sperm swim around, looking for eggs to fertilize, sometimes joining with an egg from the same gametophyte. The fertilized egg and then an embryo with two sets of chromosomes (termed 2N) is held by the gametophyte as it grows into a recognizable moss or fern that will mature and produce spores (therefore called a sporophyte). So the life cycle is complete—alternating a sporophyte generation with a gametophyte generation (see diagram).
In most spore-bearing species, the spores are all alike, making gametophytes that produce both sperm and eggs. However, in a number of lineages, the sexes became separated such that distinct male and female spores were produced, and these became much smaller. Botanists have suggested that, in one of these lineages, the separation of sexes was one of the important early steps on the evolutionary way that led to seed plants. What might have been the advantages to this arrangement? The two sexes would produce two kinds of gametophytes that could now evolve different characteristics and exploit their habitats in different ways. Females could invest all their energies in producing eggs and rearing embryos, no longer investing in producing sperm, and males could invest all of their energies in sperm. Separation of the sexes also helped reduce the rate of self-fertilization, thus reducing the risks of inbreeding.
The next major innovation was the retention of female gametophytes with their embryos on the sporophyte. These gametophytes became even smaller; fossils show step-by-step examples of how maternal tissues eventually formed integuments surrounding a gametophyte and embryo, thus providing a protective covering that we call a seed coat. The whole package is called an ovule, which matures into a seed. That happened about three hundred and fifty million years ago or so.
Presumably those primitive seeds were simply shed, as spores were, to be dispersed on breezes. Having a protective coat may have allowed seeds to remain dormant until conditions were right for germination—something that most spores cannot do.
Then, about a hundred and sixty million years ago, a major division occurred. Some plants, called gymnosperms (naked seed), kept their ovules and seeds exposed on the surface of the spore-bearing structure. Another set of plants, called angiosperms (enclosed seed), began to put additional layers of maternal tissue around the seed, probably by folding a leaf-like ovule-bearing structure to enclose and protect the maturing seeds from desiccation and consumers such as beetles. (The gymnosperms would have had to solve such problems differently). This structure evolved in many different directions in various lineages of angiosperms, forming the pistils in our familiar flowers. The lower part of the pistil became the ovary, housing the seeds, while the upper part became the receptive surface (the stigma) for pollen.
The evolution of pollen is complex, but it broke the sperm cells’ dependence on water. Instead of swimming, wind-or animal-carried pollen became the sperm delivery system. Somehow the many sperm produced by the male gametophytes of seed-plant ancestors were reduced in number, such that each pollen grain contained one very tiny male gametophyte that produced two sperm.
In gymnosperms, one sperm fertilizes the egg, and the other one just degenerates. Nutrition for the embryo and seedling are provided by the female gametophyte, as was true for spores but now the gametophyte and embryo are inside the seed.
A new invention arose, somehow, in the angiosperms: one sperm fertilizes the ovule and the non-fertilizing sperm unites with certain other nuclei in the ovule to create essential food material (commonly starches or oils) for the developing embryo and eventually the early seedling. (The angiosperm ancestors of orchids did this too, but orchid seeds have lost that stored nutrition and depend on mycorrhizal fungi for food.)
After those major steps, natural selection in angiosperms led to the evolution of flowers and a great variety of pollination techniques and fruit types. Much of that diversification was related to interactions with animals.