How old is the Earth?

The earth is about 4.54 billion years old. The entire solar system is not much older. It is about 4.567 billion years old. The oldest rocks on earth that we have found are small zircon chrystals from Jack Hills, in Western Australia. These rocks are 4.404 billion years old.



You can find out how this date was arrived at if you go to http://en.wikipedia.org/wiki/Age_of_the_Earth

What happened next?

At first the earth was a molten ball of gas and liquid rock. Gradually, it began to cool and the crust hardened. We do not know exactly when the crust solidified, but the best guess is in 10's of millions of years.

Where did the oceans come from?

Water comes from two sources: From molten rock within the earth, and from meteorites outside the earth. We do know that the oceans formed about 3.8 billion years ago. By the time the oceans formed, the crust was cool enough that the water did not boil off.

When did life appear?

We don\'t know exactly when life first developed on earth. The oldest fossil microorganisms we have found are 3.5 billion years old.

Did life come from outer space?

Mars is smaller than the earth, and further away from the sun. It cooled off faster than the earth. Early in its history, Mars had liquid water and was probably a hospitable place for life. It is possible that life originated there before conditions were suitable for life on earth. But could life have hitched ride on a meteor to earth? In the early solar system, there were huge numbers of meteorites striking all the planets. Many meteorites from Mars fell on earth. Go to
http://discovermagazine.com/2001/aug/featmars
to learn more.

Recently a bacterium survived six years in outer space. No one knows how much longer it could have survived in outer space. So it is possible that life started on Mars.

When did life arrive on land?

The first life lived only in the sea. The land was sterile. The next step was surviving along sea coasts, where organisms had to endure drying out when the tides went out. This early life was mounds of bacteria called stromolites. The stromolites may have been capable of photosynthesis.

These are modern day stromatolites:

http://commons.wikimedia.org/wiki/File:Stromatolites_in_Sharkbay.jpg


Photosynthesis

Photosynthesis produced a revolution on earth. For the first time, life could get energy from the sun, instead of depending on minerals from the earth. Photosynthesis had a down side. It produced oxygen, which was poisonous to many organisms that lived at that time. Only organisms that could cope with oxygen would thrive in this new world. On the plus side, oxygen produced ozone, which is essential for protection again solar and cosmic radiation. We now believe that bacteria grew on land in "organic mats" as long as 2.6-2.7 billion years ago.
http://www.space.com/searchforlife/life_origins_001205.html

The first eukaryotic organisms appeared 1-2 billion years ago. Eukaryotic organisms are one-celled like bacteria, but have a more complex internal structure than bacteria. Eukaryotic organisms have a separate nucleus. They also have mitochondria, which can use oxygen to produce energy. Mitochondria were probably separate organisms that developed a symbiotic relationship with the host microorganism. This relationship was so successful that it is found in all higher organisms today. The first highly organized multicellullar plants and animals first appeared about 600 million years ago.These trilobites lived about 300-500 million years ago.



Here is a video that explains in another way how old the earth is: (there is a short ad at the end of the video)

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