Looking back to the pre-Cambrian period, we find the protists, or first organisms, of the plant kingdom. These first plants may be as old as 2 billion years. Green algae evolved from these protists. Today, we recognize about 7,000 different species of green algae. During the Cambrian era, which started approximately 542 million years ago, some algae evolved to live on land, rather than water. About 100 million years later, mosses and liverworts evolved from the algae. Mosses and liverworts lack any sort of vascular structure that restricts their size to small plants. This lack of ability to move nutrients and water throughout the plant kept these plants small.
Ferns, club mosses and horsetails were the first vascular plants to evolve from mosses and liverworts. The first of these primitive species had elongated tubular structures that allowed water and nutrients to move through the plant. Likely, the club mosses were the first vascular plants, followed by horsetails and ferns.
It may have taken some 1.5 billion years for plants to evolve to this point. However, it was at this stage of evolution that plants began to change quickly, at least on a geological scale. By 350 million years ago, the first ferns and horsetails were present. They quickly dominated the plant kingdom, forming large forests of trees that grew up to 100 feet tall and 5 feet in diameter. These were not the trees we know today, but large versions of ferns and horsetails.
From the ferns and horsetails evolved the gymnosperms during the Carboniferous period of the Paleozoic era. This evolution occurred just 50 million years after the first ferns and horsetails. The name gymnosperm means "naked seed." Gymnosperms produce their seeds on cone-like structures without flowers.
The male and female cones produce microspores and macrospores, respectively. These join on the female cones and grow into seeds. As the gymnosperms quickly grew into dominance over the next 200 million years, the ferns and horsetails lost footing and size, gradually evolving into the smaller plants we know today. Gymnosperms make up the conifers and some other cone-bearing plants we know today.
All things come to an end and the evolution of flowering plants, the angiosperms, meant the end of dominance for the gymnosperms. The first flowering plants appeared 150 million years ago in the Jurassic period. This late major evolution gave plants a way to change and adapt to the environment as never before.
Today, angiosperms make up the vast majority of the plants on earth with more than 240,000 distinct species and many more, mostly in rain forests, yet to be discovered and classified. By contrast, all other plants make up only 70,000 distinct species with few, if any, yet to be classified. Flowering plants may be the latest major evolution of plants. Given past history, however, it is unlikely to be the last.