There is one animal that will be familiar to all of us, whoever we are, wherever we live. Even if we've never had direct contact with one, we will have seen one, or at the very least, heard one. It is more than likely that one sleeps in your house, possibly even on your bed. I'm talking of course, of the dog. Yet, this animal, that lives alongside us and has inspired and intrigued artists and scientists, become an invaluable partner to farmers, policemen, soldiers and a trusted confidant to over five hundred million of us all over the world, presents us with one great unsolved mystery: how did this relationship - the most complex and enduring of any between human and animal - start in the first place? Here Kate Humble explores this question.