It is thought that life first began on the Earth around four billion years ago. It is impossible to consider the evolution of organisms without also considering the evolution of the Earth that gave rise to them. For example, around 2.7 billion years ago, the Earth developed a magnetic field, enabling it to protect itself from radiation from outer space. Because of this, organisms were able to colonize shallow stretches of ocean, and eventually photosynthetic organisms emerged that breathed in carbon dioxide from the air and produced oxygen as a waste product. Between 700 and 600 million years ago, the oxygen that gradually accumulated in the atmosphere over a period of some two billion years due to the presence of these photosynthetic organisms formed the ozone layer. Because the ozone layer cuts out ultraviolet radiation that is harmful to living organisms, for the first time land became a safe environment for these creatures. The story of how some 400 million years ago plants colonized the land and how life on land diversified was recounted in Part 5: Deforestation.
So, just what is the connection between the formation and breaking up of the supercontinents and the history of life on Earth? Well, here, too, there is a close connection. Some 700 to 800 million years ago when the Rodinia supercontinent existed, there was a great ice age in which the entire surface of the Earth froze. At this time, many species of organism became extinct, but the ones that survived were able to do so by waiting patiently beneath the surface of the ice for the Earth to warm up again. Eventually, Rodinia began to break up due to heat from below the Earth's surface, creating lots of shoals. It was on these shoals that an explosive burst of evolution then took place, as if life had been waiting for just such an opportunity. Such was the sheer number of organisms that came into being during this short period of the history of life on Earth that it came to be referred to as an "explosion" of life (the Cambrian explosion).
Due to the formation of the ozone layer, plants were able to colonize land some 400 million years ago, eventually forming forests that spread across the surface of the Earth turning the entire planet green (on the globe at left, the continents are shown as green landmasses starting from 370 million years ago). These forests, which gave rise to swamps and shaded areas, became the cradle in which a multiplicity of land-based organisms were able to grow, eventually resulting in the evolution of mammals and insects.
Although on the one hand life on Earth continued to diversify, on the other hand the same Earth's energy that led to the formation and breaking up of the supercontinents also led to the mass extinction of species. Large-scale extinction events are known to have occurred on five occasions in the last 600 millions years. The largest of these, which occurred 250 million years ago, is thought to have been related to continental drift. So what exactly happened?
Source: J. John Sepkoski Jr. "A Compendium of Fossil Marine Animal Genera" (Bulletins of American Paleontology, No. 363, 2002)Around the time the Pangaea supercontinent was forming some 300 million years ago, our ancestors in the form of mammals and insects were thriving on land. At that time, there were many different species of organisms living both in the sea and on land in what was without doubt a natural paradise. However, 250 million years ago, Pangaea started to break up, and in less than no time this paradise was transformed into a hell on Earth. It is thought that giant lumps of magma called super blooms, each measuring some 1000km in diameter, rose from underneath the continent as it broke up, breaking through the surface to form pillars of fire several hundred kilometers long and up to three kilometers tall. These huge volcanic eruptions continued for as long as one million years, the smoke reaching as high as the stratosphere and covering the entire planet. Sunlight was no longer able to reach the plants, which died because photosynthesis could not take place, and the entire planet lapsed into a state of oxygen deficiency (superanoxia). In this harsh environment, it is thought that as much as 95% of all species became extinct.
Among those species that survived were the dinosaurs, who found a way of breathing in an atmosphere with low levels of oxygen, and later went on to thrive. Mammals also found a way of using oxygen more efficiently by coming up with a method (viviparity) of developing their fetuses and providing them with oxygen inside the mother's womb instead of laying them inside eggs. In other words, the harsh environment that came about as a result of the continental drift mechanism also laid the foundations upon which new species were able to flourish. This dynamic relationship between the Earth and life formed the backdrop to the evolution of humans.