Before we explain how seeds came to be, it’s important to note one basic fact about plants. Land plants evolved from ocean plants. That is, from algae.
Plants are thought to have made the leap from the oceans onto dry land about 450 million years ago. And, as seeds are thought to have developed to keep plant embryos from drying out, it makes sense that the first seeds we know of are at least 360 million years old.
But what happened between 450 million years ago and 360 million years ago? This gap of about 100 million years indicates that some method of (land) plant reproduction predated seeds.
In fact, during this period, many plants used spores to reproduce. Some plants, like ferns, still produce spores. You might have even seen some spores shake out of those brown dots on the undersides of ferns – the brown dots are not the spores but the structures that produce them (the spores themselves are even smaller). A single plant can produce thousands or millions of spores at once.
During Earth’s Devonian period, a group of plants called progymnosperms started manufacturing two sets of specialized spores: male spores, and female spores – the living tissues inside these spores produced eggs and sperm. Both types of spores were shed from the plant. But, if they landed close together, fertilization could take place. The new embryo could make a new plant.
Spores have some obvious vulnerabilities, though — they’re very tiny, and have to survive and travel without built-in nutrition or physical protection. They also need water for fertilization to occur. In other words, spores aren’t necessarily the most reliable means of plant reproduction—especially for plants living on dry land.
So, over time, some progymnosperms evolved into seed ferns. These seed ferns began producing female sex cells that stayed with the parent plant, and waited for male sex cells to arrive. These seed ferns slowly evolved into what are called seed plants.
In seed plants, the female sex cells (no longer numbering in the millions, by the way) still remained with the plant. But these plants developed special structures to “catch” male sex cells, otherwise known as pollen. This fertilization process was novel in that it did not depend on water; pollen was – and still is – able to be carried to a female cell by wind or flying insects.
Once fertilized, a seed plant embryo would grow inside an ovule, the part of the plant that housed a female sex cell. Tissue from the ovule developed the capacity to serve as a protective coating (a/k/a seed coating) for this embryo. This coating not only protected the embryo, but also protected the densely packed nutrition surrounding the embryo, once the embryo left the plant. (It bears mentioning here that another important function of a seed, beyond keeping a plant embryo moist, is to provide a developing embryo with nutrition. This stored food helps the embryo grow in the early stages of its development, and increases the young plant’s chance of survival.)
And that’s how seeds evolved!
In many modern plants, seeds are contained within a fruit. This fruit is useful in attracting birds, mammals, and insects, all of which help spread seeds around. On the other side of the spectrum, there are many “seedless” plants alive today, including ferns and horsetails – found in damp environments – as well as many aquatic plants.
Our thanks to:
Sir Peter Crane, Dean
Yale School of Forestry and Environmental Studies