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.
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. There have been several of these mass extinctions, the earliest notable one at the end of the Ordovician period 440 million years ago; 85 percent of all species died out. Those that survived may have simply been lucky; their legacy is the species alive today.
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. Extinction stems from habitat loss.
What causes extinction events is the subject of debate broadly between catastrophists, who argue the sudden impact of an extraterrestrial body, perhaps producing a huge tsunami or sufficient dust to completely block the sun for a few seasons and trigger irrecoverable change, and uniformitarians, who believe that developments, such as changes in sea level and volcanic activity, in the perpetual evolution of the earth are the cause. More credence is now given to a gradual (over at least 100,000 years) process of relentless environmental deterioration.
Popular scenarios include climatic shifts caused by impacts of meteorites or comets which generate dust and smoke in the atmosphere so bringing on a devastating freeze, extreme climatic swings, major changes in sea level, and prolonged periods of volcanism caused by roving tectonic plates. It is also possible that a source of extraterrestrial gravitation, such as an asteroid belt, has come close to the solar system and upset its geomagnetic stability or some other stable state. In India, there is evidence of massive outpouring of basaltic lava from huge volcanoes at the end of the Cretaceous.
Debris from explosive disintegration of comet fragments high in the atmosphere is said to cause damage such as occurred in Siberia in 1908. Far greater damage would be caused by the return of large showers of comets. Another theory suggests a 'killer star' that was, in fact, a binary star with the sun. This star, a small, cool dwarf, has an elliptical orbit that takes it as far as 2.4 light years from the sun, where a comet cloud containing 100 billion comets is known to exist. As it passes through the cloud, it drains some of the comets, which it flings into the inner solar system as it passes by every 28 million years.
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 percent of Earth's species disappeared abruptly from the fossil record. In the worst of these ecological crises, the end-Permian event some 252 million years ago, at least 96 percent 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.
Since then, there have been at least six major ecological catastrophes. The most well known is the mass extinction that 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. The impact sent tonnes of gases and dust into the atmosphere, blacking out the Sun. Temperature plummeted around the globe. Dark clouds blanketed the Earth for months, perhaps even years, preventing plants for trapping the Sun's energy through photosynthesis. Marine algae and whole forests died, wiping out the base of the world's food chains. Everywhere animals succumbed to cold and hunger.