The textbook definition for extinction is defined as the dying out of a species. Ninety nine point nine percent of the species that ever lived on Earth are now extinct. After analysis of data of 9,250 extinct life forms, there is evidence to indicate that massive extinctions of species have occurred once every 26 million years over the last 250 million years. On this basis the next mass extinction is due in 14.7 million years, more or less. Some people believe we are currently in such a period.
All mass extinctions have been the result of a tangled web of environmental changes. Among these changes, global climate change -- usually global cooling -- may have been the most consistently damaging factor. The most likely causes are gradual atmospheric deterioration or the drifting of continents, or perhaps abrupt catastrophes like gigantic volcanic eruptions or asteroids hitting the Earth. But just as important, many believe, is the dramatic direct loss of habitats as inland seas dry up and coastlines shrink or are flooded. Earth has experienced five mass extinction events that have claimed the lives of billions of species over the last 3.5 billion years. The sixth-largest extinction event is currently taking place and many scientists believe that humans are responsible for it; it is called the Holocene Extinction or Anthropocene Extinction.
Life on Earth has suffered mass extinctions many times in its 4.5 billion year history. Scientists talk about the big five extinctions in the distant past, when at least 75% of Earth's species disappeared abruptly from the fossil record. Each extinction event affected and changed the world, making it almost unrecognizable while opening up spaces for the evolution of new forms.
The first event was known as the Ordovician-Silurian Extinction some 440 million years ago. Millions of small marine organisms died due to intense global cooling and a reduction of sea levels. 85% of all species died out.
The second event was the Devonian Extinction about 365 million years ago, which was yet another global cooling which brought about the the formation of glaciers and the further reduction of the sea level.
The third event was the Permian-Triassic Extinction some 252 million years ago and believed to be the largest mass extinction of vertebrate species caused by global warming. At least 96% of species became extinct. Hardest hit were the mammal-like reptiles that had ruled life on land for 80 million years. These were replaced by the dinosaurs.
The fourth extinction was known as the Triassic-Jurassic Extinction which took place 210 million years ago. Massive and widespread volcanic eruptions that increased atmospheric CO2 and acidified oceans resulted in the death of 76% of all life on Earth, including many vertebrate species.
The fifth extinction occurred at the boundary of the Cretaceous and Tertiary periods, 65 million years ago (known as the K/T boundary), and facilitated the evolution of mammals by ending the domination of dinosaurs. Ammonites, which had thrived in the seas for hundreds of millions of years, also disappeared. The cause of this catastrophe was collision of a massive comet with earth. Upon impact, the meteorite pushed over a million tons of ash and debris that blotted out the sun for several years and led to a seemingly endless winter. Marine algae and entire forest systems died, wiping out the base of the world's food chains.
No evolutionary process could anticipate ecosystem collapse. Most extinctions did not occur because species were somehow "unfit"; instead they were inadvertently caught up in huge disasters that destroyed millions of species. Those that survived may have simply been lucky; their legacy is the species alive today.