Today, as global warming threatens the welfare of many species, including humans, we tend to believe that this is a unique, man-induced phenomenon. And though we are surely contributing to the process, many upheavals in Earth's climate have occurred over the past 4.6 billion years, some of which led to mass extinctions.
While the most famous is the Cretaceous-Tertiary extinction, likely caused by an asteroid strike and leading to the demise of the dinosaurs, the most severe mass extinction in the history of our planet occurred near the end of the Permian Period, 225 million years ago. The continents had merged into the giant land mass of Pangea during this Period and, by its end, the era of extensive tropical "coal forests" was fading and the climate had begun to cool. This cooling increased dramatically at the end of the Permian, likely due to massive volcanism in Siberia; altered ocean currents and a change in sea chemistry may have also played a role.
During the Permian Extinction, 70% of land species and 95% of marine species were lost; the latter included trilobites (present for over 300 million years), sea scorpions, placoderms and ancient corals. Whatever the primary cause may have been, this extinction set the stage for the rise of the dinosaurs, one of the most successful and diverse groups in Earth's history.
