The origin and early diversification of land plants: Insights into the nature of evolution
David R. Pettersson
The biological history of the planet Earth is characterized by a prolonged period of evolutionary advances (about 3 billion years) followed by an incredibly brief period (roughly amounting to 100 million years, or 3.3% of life's time on Earth) of rapid diversification and proliferation of the basic life forms developed during the previous 3 billion years. The evolution of photosynthetic life and, more specifically, that which would eventually colonize the land, is no exception. During the three-billion year-long organizational phase, photosynthetic prokaryotes and microbial mitochondrian, in addition to other primitive life forms, would come together to form a more efficient, eukaryotic organism. By the Ordovician period, it is believed that eukaryotes (in the form of Charophycean algae) made the transition to existence upon the land. The first photosynthetic land forms evolved the basic structures and behaviors required for life on land. Once these evolutionary requirements had been met (by the Early Devonian), a rapid diversification and proliferation of forms followed. The sudden spread of these land plants had a significant impact upon the biological, climatic, and chemical properties of the Earth. An analysis of the evolutionary trends of the marine-terrestrial transition of plants exhibits striking similarity with the trends associated with the Precambrian-Cambrian transition of the animal kingdom, commonly refered to as the "Cambrian explosion."