In this verdant month of May, a diverse assembly of trees, shrubs, vines, flowers and grasses carpet the landscape. Since our species evolved well after land plants graced the Earth, this sea of green has become part of the collective human consciousness, an environment that we take for granted.
But back in the early Silurian Period, some 440 million years ago, all plant and animal life was limited to the sea, dependent upon the primordial ocean for water, nutrition and protection from the destructive effect of ultraviolet radiation. With the development of the protective ozone layer (high in the atmosphere), marine plants and animals were free to inhabit shallow, coastal regions; nevertheless, lacking transport tubules, plants remained dependent upon an aquatic environment to secure the water and nutrients that they needed.
Then, by the middle of the Silurian (about 420 million years ago) vascular plants evolved, equipped with stems. Able to grow across coastal rocks and beaches while maintaining a foothold in the sea, these pioneer plants eventually developed root systems that eliminated their dependence on the ocean waters and allowed them to colonize the land. Four hundred million years later, they have diversified into an amazing variety of terrestrial plants, from mosses to redwoods. And, of course, the evolution of land plants allowed animals to follow, setting the stage for the rise of amphibians, terrestrial insects, reptiles, dinosaurs, birds and mammals.
