A Mass Extinction is when most of the life on a planet, in our case, its the Earth disappears. Its happened before, it'll probably happen again in the far future. Mass Extinctions have happened throughout the history of Earth but when we initially think of a Mass Extinction, we automatically think of the Meteor that wiped out all the Dinosaurs. However, it was not the only time there had been a mass extinction of life on Earth. There have been five extinctions through the lifetime of life on this Earth.
Life has been around on this planet for 3.8 Billion Years but for the majority of that time, the life forms have been very simple Micro-Organisms. The first multi-cellular life form first appeared about 2.1 billion years ago. Although there have been five main extinctions, if life had been around longer, there would have been a lot more extinctions.
This mass extinction was about 443 Million Years ago, it was caused when the world went into an ice age. At that time there were no creatures on the Earth and everything was under water. The Doctor Who race of aliens, the reptilian Silurian are from just after this period. The Silurians are fictional but the period in time is real. It was the third largest mass extinction in history. Because of the ice that covered the planet, it caused the Earth's temperature to drop. It wasn't one actual extinction but two separated by hundreds of thousands of years. 85% of all life at that time was wiped out. It is believed to have been the second most severe of the main extinctions.
It was believed that the cause was that the supercontinent at the time had shifted over the south pole and the water froze and turned to ice, thus depriving the oceans of water. When the continent shifted, the ice began to melt and return to the oceans. No major life groups were wiped out, It is also though a Gamma Ray Burst from an exploding star caused this mass extinction. A gamma ray burst is caused when a giant star comes to the end of its life and supernovas, shooting out streams of gamma rays. An exploding star doesn't have to be close, it just needs to be about 6,500 light years away. The rays would also have to be in the path of the planet to affect it. A star WR104, in the Constellation of Sagittarius is the most likely to go pop but it will take thousands of years for it for the gamma rays to get here.
At the end of the Devonian period, some 375-359 million years ago, the second mass extinction to befall Earth occurred. It is generally thought to be a series of mass extinctions than one particular event that occurred over millions of years. There have been a number of possible causes of why the extinction occurred ranging from meteors to climate change. When we talk about Global Warming / Climate Change / Greenhouse Effect now, we are talking about man-made climate change not the change that occurred over thousands of years.
The extinction caused 75% of all life to become extinct and then it would take millions of years to recover. One of the possible reasons of the extinction was caused by the supercontinent Gondwana drifted back over the south pole. Also blamed is the ecology of the planet, the plants were removing the carbo dioxide from the atmosphere thus causing the planet to cool down.
The period is also nicknamed "The Great Dying". The Permian event occurred just before the start of the reign of dinosaurs in the Triassic period, which wiped out 96% of all life on the planet. It is said that all life now are descended from the 4% of the life that survived. Like all the previous extinctions, it wasn't caused by one single event but by many over the years. Insects which are hardy creatures suffered in this extinction than in any previous and future extinction. This event took place 248 Million years ago. It is believed that there volcanic eruptions in Siberia, (Russia) also had an effect on the environment. Frozen methane which is a green house gas is believed to have been released in large quantities.
After the previous mass extinction, it would take another 10-20 million years for life to recover to what it was before hand. Over 200 million years during the reign of the dinosaurs, there were a number of events that caused a mass extinction, there were a number of causes for this including meteor strikes and climate change. The Jurassic-Triassic extinction is the least understood of all the mass extinctions because of a lack of evidence. It was probably caused by falling sea levels which had an effect on the ecosystem.
65 Million years ago, the most famous mass extinction occurred, the one that wiped out the dinosaurs. The dinosaurs were already in decline but this was the final nail in the coffin as they say. A meteor slammed into the Earth in the Yucatan Peninsula, Mexico, causing dust and rock to spew into the sky and causing the clouds to blacken and prevent sunlight from reaching the life forms that needed it to survive. Some insect, mammal and other life forms did survive.
The Martian Mass Extinction is all speculation and there are no real evidence on it. It has been speculated that life may have existed on the planet Mars once, after all it is in The Sun's Goldilocks zone. The goldilocks zone is the area from the Sun that is not too hot and not too cold but it lost its ability to protect the planet from harmful rays from the Sun. This in turn destroyed the atmosphere causing the planet to die. The protection from the Sun is provided by the magnetic fields that are generated from within the core, its been speculated that the field generator stopped producing any more shielding. If there is any life on the planet, it will be micro-organisms somewhere on the surface.
If Venus ever had any life, there isn't any there any more. Although the theory, there could be microbial life forms in the upper atmosphere of the planet. The planet is also inside the Goldilocks zone, it is about the same size as our planet but the environment is hot and sulphuric. The atmosphere is hotter than Mercury because of the environment. The atmosphere is like that of a greenhouse, the hot air is trapped, unable to escape and keeps getting hotter and hotter. If lets say there had been life on the planet, it would long be extinct, the most likely cause is volcanic eruption spewing out ash and molten lava across the planet.
Mass extinctions could have occurred on Extrasolar Planets (Exoplanets), in other solar systems across the galaxy and further. It would help explain why we've not heard from them, because they weren't sufficiently intelligent enough to protect themselves from the event. Until we get faster than light speed, we will never be in a position to journey and investigate. We shouldn't just think that mass extinctions are limited to Earth but are frequent across the universe.
It can't be ruled out that there won't be another mass extinction in the future, it might not happen for thousands or even millions of years. When our star, the Sun explodes into a Supernova, then all life on Earth will be end. The good news is that the Sun will not supernova for at least five billion years but life will cease one billion years time when the Sun becomes too hot and make the Earth inhospitable. We hope that we will be in a sufficiently advanced state to be able to move off the planet before we get wiped out or in the case of a meteor, be able to prevent it from happening by redirecting the asteroids path.
According to some scientists, the next mass extinction has already begun. The report that came out in June 2015 from scientists from Stanford, Princeton and Berkeley University said that vertebrates (animals with backbones such as us) are disappearing at 114 times faster than normal. Humans would be the first to disappear. Since 1900, 400 species disappeared and shows no sign of abating. If bees disappeared, the whole eco-system of life on this planet would collapse. The reasons for the disappearance range from climate change to pollution.