Though organisms change and evolve, the most dramatic changes occur during mass extinction events, defined as a period of time when the rate of species going extinct is increasing with respect to the rate at which new species arise. Throughout the Phanerozoic era, there were 15 documented major extinction events. Five of these were major mass extinction events where more than half of all species on Earth at the time were lost.
Mass extinctions can be either gradual or sudden. The process of extinction follows five different phases:
- First is the extinction phase, which features a rapid decrease in biotic diversity.
- The second phase is the survival phase. Diversity is at a minimum here, but there is little further extinction. Organisms that survive past the second phase are known as the holdover taxa.
- Third is the rebound phase. Diversity begins to slowly increase again as ecosystems recover from the extinction event. The term progenitor taxa is used to describe species that have survived and now provide the evolutionary seeds for future organisms.
- The fourth phase, the expansion phase, is marked by a rapid increase in diversity due to the evolution of new species. The increase in diversity is helped in part by the opportunities and voids left by the many species lost during the first phase.
- The last phase, a recovery interval, is marked by a long period of environmental stability as diversity continues to rise. Disaster taxa describe species that proliferate most successfully following an extinction event. They are typically small and simple, hardy rather than complex and therefor capable of surviving extinction events.
Evidence for extinction events come from sweeping changes in the fossil records at different geological changes. However, fossils provide an incomplete and often confused record of past life. Roughly one third of organisms feature calcified parts that could be turned into fossils. The erosion of sedimentary rock may also obscure how old a fossil really is. Our understanding of mass extinctions is therefore inversely proportional to their age (older events have less certain evidence), and is proportional to the extent of the extinction. Extinction events that caused a greater loss in diversity are easier to categorize.
While mass extinction events threaten the very presence of life on Earth, they also lead to enhanced evolutionary diversity. It is important to understand these ubiquitous pressures on life as similar events are likely to occur on every planet. If life is easily and regularly eliminated through frequent extinction events, then it may make finding surviving life in the Universe much harder.
The Ordovician-Silurian (O-S) Extinction
The Ordovician-Silurian (O-S) extinction, which occurred 450-440 Mya, describes two events that together resulted in the loss of nearly 70% of the worlds species at the time, making this the second-largest mass extinction event. The extinction event had a global affect and particularly affected marine life, where it is estimated that nearly 85% of species were lost.
The event was most likely caused through sudden climate changes. The timeline matches up with a shift in the then super-continent, Gondwana, into the south pole. As Gondwana passed over the south pole, it began to form ice caps across its surface. This shift also exposed more land, causing a drop in CO2 levels through weathering and cooling the planet.The cooling of the Earth led to the formation of glaciers, which locked up waters from the ocean. This, in turn, caused sea levels to drop, exposing and destroying shallow-water habitats along continental shelves. Evidence for glaciation during this time period has been found in the Sahara Desert. The combination of glaciation and cooling is thought to be the main causes of the O-S extinction.
The event ended when volcanic out gassing of greenhouse gases increased the atmospheric temperatures enough to melt the glaciers and stabilize sea levels. The rebound phase of the O-S extinction resulted in increased biodiversity on the re-flooded continental shelves. However, recovery species in the ocean and land plants of the time, were initially less complex.
The Late Devonian Extinction
At least 70% of all species were also lost during the Late Devonian extinction, which lasted from 375-360 Mya. There is some evidence that this extinction event was actually a series of seven, distinct extinction pulses. As with the O-S extinction, marine life was particularly hard hit.
The Late Devonian extinction is thought to have been caused by changes in the sea level, triggered by global cooling combined with acidification and oxygen depletion in the oceans. Similar to the O-S extinction, changing sea levels or oceanic volcanism.
The rapid evolution of more complex plants from 30 cm up to 30 m during this period likely contributed to the oxygen depletion in the ocean. The increase in plant mass required the development of extended root and vascular systems in plants. These roots will have stabilized increasingly deeper layers of soil, causing a change in the chemical composition of which nutrients were run off into rivers, lakes, and eventually the ocean. This affect could have decreased the amount of oxygen in the oceans as well as reduced CO2 levels, which would have contributed to global cooling.
Less oxygen in oceans also caused organic matter to be better preserved, preventing organisms from decomposing and recycling their nutrients. Instead, many organisms formed into oil, which was absorbed by the porous reef rocks common in the area. These oil deposits remain a major source of oil in America today.
The End Permian Extinction
The End Permian extinction (or Permian-Triassic or P-T extinction) occurred about 252 mya and is the single largest mass extinction event ever recorded. It is nicknamed the "The Great Dying." Approximately 96% of all marine life was lost along with over 70% of land species, including everyone's favorite prehistoric creature, the trilobite.
The cause of such a large extinction was most likely the final stages of the break up of Pangea. The shifting of continents caused molten rock to be exposed and increased volcanic activity, releasing volatile carbon, methane, and SO2. The effect is actually quite similar to that of burning fossil fuels today. The Earth began to heat up under this new blanket of greenhouse gasses; it is thought that the temperature of the entire ocean increased by 2°C.
Warm ocean water transported heat to the poles, which in term began to melt and release more greenhouse gases that had been frozen before. As the amount of CO2 increased in the atmosphere, it likewise began to dissolve and accumulate in the oceans, causing oceans to acidify. Furthermore, the warmer ocean water was unable to hold as much oxygen. The combination of increasing acidity and decreasing oxygen likely gave rise to the grievous destruction of marine life during the P-T extinction.
The End Triassic Extinction
The End Triassic extinction occurred 200 Mya with 70-75% of all species becoming extinct. Most of the large amphibians common at the time were eliminated and dinosaurs were left with little competition, allowing them to flourish in the following Jurassic period.
The end Triassic extinction event was sudden, lasting less than 10,000 years. Many causes have been proposed, but none are definitive. Gradual climate change can explain some of the observed properties of the extinction. An asteroid impact could also explain some of the properties of the extinction, but a crater of the appropriate size and age has yet to be found. The extinction may also have been caused by an unlucky sequence of volcanic eruptions increasing the release of carbon dioxide and sulfur and triggering climate change. Increased amounts of volcanic compounds can be found in the rock layers from this period.
The Cretaceous-Paleogene (K-T) Extinction
The Cretaceous-Paleogene (K-T) extinction occurred 66 Mya and 75% of species became extinct, including all land-bound dinosaurs. Dinosaur fossils are only ever present before this time in the rock records. The extinction had a surprisingly variable impact with dinosaurs being greatly affected while mammals and birds survived to ultimately become the predominate life forms on the planet.
The rock record from this time exhibits an unusual amount of iridium with gold, osmium, and platinum. These elements are relatively rare on Earth, but are commonly found in meteorites. The amounts of these rare metals could have been contained in an asteroid that was 10-15 km in diameter. The energy from such an asteroid would be equivalent to the force of 100 million hydrogen bombs. Other evidence includes shocked quartz and spherical rock droplets, which require both high temperatures and great pressure to form. Widespread soot deposit also suggest enormous fires across land masses.
The Chicxulub crater, buried beneath the Yucatán Peninsula in Mexico has both the appropriate age and size that is expected of the asteroid that could have caused the devastation seen during the P-T extinction event. The crater stretches 150 km across and is approximately 20 km deep.
Extinctions and Life
While mass extinctions are devastating, they do serve to encourage diversity. Had the dinosaurs not been eliminated 65 Mya, there may never have been such a successful emergence of mammals. The frequency of life in the Universe is a function of how often it arises, how often it survives and how long it endures after evolving. In this way, the frequency and intensity of mass extinctions control the frequency of life. On Earth, there have been 15 major mass extinctions in the last 500 million years. Five of those 15 eliminated more than half of all species on the planet.