In the very beginning of earth's history, this planet was a giant, red hot, roiling, boiling sea of molten rock, a magma ocean.
The heat had been generated by the repeated high speed collisions of much smaller bodies of space rocks that continually clumped together as they collided to form this planet.
As the collisions tapered off the earth began to cool, forming a thin crust on its surface.
As the cooling continued, water vapor began to escape and condense in the earth's early atmosphere.
Clouds formed and storms raged, raining more and more water down on the primitive earth, cooling the surface further until it was flooded with water, forming the seas.
It is theorized that the true age of the earth is about 4.6 billion years old, formed at about the same time as the rest of our solar system.
The oldest rocks geologists have been able to find are 3.9 billion years old. Using radiometric dating methods to determine the age of rocks means scientists have to rely on when the rock was initially formed (as in - when its internal minerals first cooled). In the infancy of our home planet the entire earth was molten rock - a magma ocean.
Since we can only measure as far back in time as we had solid rock on this planet, we are limited in how we can measure the real age of the earth. Due to the forces of plate tectonics, our planet is also a very dynamic one; new mountains forming, old ones wearing down, volcanoes melting and reshaping new crust.
The continual changing and reshaping of the earth's surface that involves the melting down and reconstructing of old rock has pretty much eliminated most of the original rocks that came with earth when it was newly formed. So the age is a theoretical age.
When Did Life on Earth Begin?
Scientists are still trying to unravel one of the greatest mysteries of earth: When did "life" first appear and how did it happen? It is estimated that the first life forms on earth were primitive, one-celled creatures that appeared about 3 billion years ago. That's pretty much all there was for about the next two billion years.
Then suddenly those single celled organisms began to evolve into multicellular organisms. Then an unprecedented profusion of life in incredibly complex forms began to fill the oceans. Some crawled from the seas and took residence on land, perhaps to escape predators in the ocean.
A cascading chain of new and increasingly differentiated forms of life appeared all over the planet, only to be virtually annihilated by an unexplained mass extinction. It would be the first of several mass extinctions in Earth's history.
Scientists have been looking increasingly to space to explain these mass extinctions that have been happening almost like clockwork since the beginning of "living" time. Perhaps we've been getting periodically belted by more space rocks (ie. asteroids), or the collision of neutron stars happening too close for comfort?
Each time a mass extinction occurred, life found a way to come back from the brink. Life has tenaciously clung to this small blue planet for the last three billion years. Scientists are finding new cues as to how life first began on earth in some really interesting places - the deep ocean.