In The Beginning

In the Beginning there was nothing. Then, 10 million years ago, at the request of a race of hyper-intelligent pan-dimensional beings called "Mice", the planet building people of Magrathea constructed the greatest computer ever to exist "The Earth" for the sole purpose of finding the ultimate Question of Life The Universe And Everything. This Planet sized computer was then placed into the orbit of a small, unregarded yellow Sun in the uncharted backwaters of the unfashionable end of the western spiral arm of the galaxy. This new planet was populated with many living creatures including dolphins and a creature that had been derived from the chimpanzee and didn't live in caves. The planet was filled with fossils, layers of rock and various chemical element to make it seem like it had existed for approximately 4.5 billion years.

The computers program continued for 8 million years until the rejected third of a race of people called Golgafrinchans (mostly telephone sanitizers and management consultants) crashed into the Earth with the intention of colonising it. These people wiped out the native ape descendants and assumed their position as future dominate creatures on earth thus ruining the program to find The Question. These people went on to become humans.
Like your self.
Unless your a hitchhiker stranded on earth.

And that is the origin of The Earth and Humanity

So long and thanks for all the fish