Extinction
Events1原文
The Earth was formed about 4.6 billion yeas ago.
The first life appeared about a billion yeas later.
The first plants didn’t appear until around 2 billion years
ago.
Until about 580 million years ago, life forms were small and
microscopic.
Since that time, life has evolved into many different types or
species.
However, scientist estimated that more than 99% 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.
Scientist think that some of them were started by asteroid
impacts or volcano eruptions.
Events of these can change the global conditions that life
depends on.
One major extinction event happened about 66 millions years
ago.
That event may have been started by a large asteroid that hit
the Earth.
As a result, the dinosaurs and about a 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
month 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 the enough sunlight, plants and plant-eating animals
quickly died.
With the death of so many plants and plant-eating animals, the
food chain were collapsed.
This lead to death some more spices.
Mass extinction events have played an important part of in the
evolution of life.
In fact, some extinction events have helped
to accelerate evolution.
For example, some mammals lived through the age of the
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.
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