The Tectonic Shift of Life
Tectonic plates colliding and subducting, with long chains of volcanoes spewing gases into the atmosphere…not exactly the sort of environment that sounds conducive to life. However, in a new study by Josh Williams of the University of Exeter, it was just these sorts of tectonic events that were important to the rapid development of some of the first complex animal life over 500 million years ago.
Before about 550 million years ago, Earth’s atmosphere contained less that 10% of the level of oxygen gas that is currently present. This limited life to either single-celled organisms, very simple animals like sea sponges, or soft-bodied, thin, sheet-like animal forms such as Dickinsonia. After this time period, entering into the Cambrian period around 540 million years ago, oxygen levels in the atmosphere increased by about 50%. This enabled a proliferation of animal life with a complexity never before possible, with animal life now evolving such features as central nervous systems, increased mobility, increased size, carnivorous diets, and even eyesight. Some of the more common fossils from this period of proliferating life, called the Cambrian explosion, include the trilobites.
Billions of years after Earth's initial formation, what triggered this increase in levels of oxygen in the atmosphere? According to this latest study, it was an increase in continental arc volcanic activity. These chains of volcanoes, which can extended for thousands of miles, form when continental and oceanic tectonic plates collide, as they did during the formation of the supercontinent Gondwana around 550 million years ago. As this volcanic activity increased, among the gases they released was carbon dioxide, a greenhouse gas. This led to a warming of the planet which acted to increase the weathering of continental rocks releasing phosphorus into the ocean. This phosphorus, in turn, drove increased photosynthesis by cyanobacteria (blue-green algae) releasing increasing levels of oxygen as a by-product.
As detailed in a previous article, the process of tectonic plate collisions has the capability to usher in global ice ages. Coupled with this latest study, the evidence is mounting that plate tectonics has had great implications for the history of life on Earth, both good and bad.
