While numerous innovations took place in the sea during the Silurian and Devonian periods, it is on land that the most important events—at least to us land-living animals—took place. The conquest of land, first by plants and then by a succession of animal phyla, began in the Silurian and Devonian. Why did the conquest of land occur then? The standard view of the history of life is that these events took place because animals had finally evolved to a point where land conquest was possible. In other words, the evolutionary advances in arthropods, mollusks, annelids, and eventually vertebrates—the major animal phyla involved in the conquest of land—had finally and coincidentally arrived at levels of organization that allowed them to climb out from water and conquer the land. An alternative view is that the first conquest of land took place as soon as atmospheric oxygen rose to levels allowing land animal life. Let’s first look at what was required of both plants and animals to allow terrestrialization, the adaptations needed to permit life on land. Let’s begin with plants, for without a food source on land, no animals would have made the effort to gain a terrestrial foothold.
By 600 million years ago, plant evolution had resulted in the diversification of many lineages of multicellular plants, some of which are familiar to us still—the green, brown, and red algas, which are members of any seashore in our world. But these were plants that had evolved in seawater. The needs of life—carbon dioxide and nutrients—were easily and readily available to them in the surrounding seawater. Reproduction was also mediated by the liquid environment. The move to land required substantial evolutionary change in the areas of carbon dioxide acquisition, nutrient acquisition, body support, and reproduction. Each required extensive modification to the existing body plans of the fully aquatic plant taxa.
It was the green algal group, the Charophyceae, which ultimately gave rise to photosynthetic land plants. Many obstacles had to be overcome. First was the problem of desiccation. Green algae washing ashore from their underwater habitat quickly degenerate in air, for they have no protective coating. But these green algae produce reproductive zygotes that have a resistant cuticle, and this same cuticle may have been