“ Death is a fact of life, extinction is a fact of evolution” (Leakey)
Thirty billion species are estimated to have lived since multicellular creatures first evolved in the Cambrian explosion
And about thirty million alive today, so 99.9% of all species that have ever lived are extinct
extinction is the ultimate fate of all species
Extinctions are happening all the time
The continual rate of extinction is called the background rate of extinction. It is the number of extinctions that would occur naturally.
The normal background rate of extinctions is about two to five families of marine invertebrates and vertebrates per million years.
And one to ten species per year
But sometimes there are periods when the extinction rate far exceeds the normal background extinction rate. The background rate has been punctuated by about 15 episodes of mass extinction in last 500 million years.
But six of these mass extinction events were so severe that around half of the life on earth was affected.
These are great mass extinctions.
Three of the five major mass extinctions occurred during the Paleozoic era:
At the end of the Ordovician period,
during the late Devonian period, and
at the end of the Permian period.
The graph shows when the five extinctions occurred. As you can see, the Permian extinction was the most severe.
251 million years ago
Greatest mass extinction in Earth’s history
90-95% of marine species became extinct
70 percent of vertebrate animal species on land.
Huge die off of plants caused a huge erosion that clogged rivers
http://www.earth.rochester.edu/ees207/Mass_Ext/higgins_mass2.html The Cyclical Nature of Extinction Events It has been suggested that mass extinction events occur in a roughly cyclical manner--that is, about every 26 million years. The following graph is a product of a 1984 study by Raup and Sepkoski and shows clearly that major extinction events seem to be quite regularly distributed over time. (WARNING! This claim is still very controversial!)
http://freerepublic.com/focus/f-news/1050165/posts?page=353 1980 630,000 years ago 2 million years ago http://www.icr.org/pubs/imp/imp-302.htm,
Fig. 21.16, p. 347
Jon Burnett, Oct 1/03, Wales
The Earth is continuously bombarded by debris. But most of this simply burns up in the atmosphere (which can be seen by watching shooting stars). However, if an object is big enough it can survive passage through the atmosphere. The damage done by a meteorite depends upon its initial size.
Meteorites striking the Earth can cause catastrophic events if they are big enough.
Last Few Seconds of the Cretaceous
Location of the Chicxulub crater (circle) on Mexico's Yucatan Peninsula. http://www.icr.org/pubs/imp/imp-334.htm
65 million years ago
Probably caused by an asteroid 4-9 miles (6-15 km) in diameter that hit the Earth about 65 million years ago, creating the Chicxulub crater at the tip of Mexico's Yucatán Peninsula
The extinction killed 16 percent of marine families, 47 percent of marine genera, and 18 percent of land vertebrate families, including the dinosaurs.
Triassic-Jurassic Manicouagan, Canada (214 Myr) 100 km
Are all mass extinctions caused by impacts?
This would be a very extreme view…
If true, impacts would be one of the most important forces in evolution.
Still very very controversial and vigorously debated.
Impact theory would be boosted if we could find craters corresponding to the other mass extinctions.
But remember how hard (sometimes impossible) it is to find impact craters on Earth… and we must accurately date them.
Animals are fragile and easily killed , unlike microbes, so mass extinctions tend to be more damaging to animals families than to microbial life forms
Microbes inhabit a wider range of environments, so less chance of their habitat being destroyed by a single event
This means that the chance that a mass extinction wipes out all life on a planet is quite small. Because once a deep microbial biosphere is established, like it is on Earth, life is difficult to eradicate completely short of complete sterilization of the planet by a major impact or nearby supernovae etc.
This also explains why there were few or no mass extinctions before the generation of complex life