The Earth was formed about 4.6 billion years ago.
The first life appeared about 1 billion years later.
The first plants didn't appear until around 2 billion years ago.
Until around 580 million years ago, life forms were small and microscopic.
Since that time, life has evolved into many different types or species.
However, scientists estimate that more than 99 percent of them are now extinct.
There have been several extinction events in the history of the Earth.
In an extinction event, many life forms or species of life die out.
The causes of mass extinction events are unsolved mysteries.
These events are not predictable and can happen at any time.
Scientists think that some of them were started by asteroid impacts, or volcanic eruptions.
Events like these can change the global conditions that life depends on.
One major extinction event happened about 66 million years ago.
That event may have been started by a large asteroid that hit the Earth.
As a result, the dinosaurs and about half of all species on the planet died out.
When the asteroid hit the Earth, it started a chain of events.
A chain of events, is a series of events linked together, like a chain.
This particular chain of events, happened over a period of months and years.
When the asteroid hit the Earth, it caused a huge cloud of dust to enter the atmosphere.
This cloud of dust blocked the sun and darkened the Earth for many months.
Without enough sun light, plants and plant-eating animals quickly died.
With the deaths of so many plants and animals, the food chain collapsed.
This led to the deaths of many more species.
Mass extinction events have played an important part in the evolution of life.
In fact, some extinction events have helped to accelerate evolution.
For example, some mammals lived through the age of dinosaurs, but they couldn't compete.
When the dinosaurs were removed, the mammals were able to compete more successfully.
Once they could compete, mammals evolved to a higher state.
Without extinction events, perhaps we wouldn't be here today.
