Where (or When) Does Human History Begin?

Where did the human race begin? When did the human race begin? When I was 4 years old, my dad presented me with a book on dinosaurs. This was a subject which totally fascinated me. He went on to give me a university level textbook on palaeontology. I found it incredible to think that a fossil record could show us things which were no longer with us in this world. Of course, the dinosaurs died out around 65 million years ago at the end of the Cretaceous period due to an asteroid impact on the Yucatán Peninsula. I never supposed that we existed alongside them, but I was always curious to know when exactly we as Homo sapiens emerged.
Apparently, I was not the only one. Years ago, I came across a book (fiction?) by David Mitchell called Cloud Atlas. In his story, he tells the tales of a number of characters over the span of a number of centuries. While I do not want to spoil it for anyone wanting to read this book, I found his way of connecting his characters very interesting. It started me thinking, "how do myth and legend fit into our historical record?" For example, was the lost city of Atlantis simply the deluded daydream of some Greek in his cups, or was there something more to this recollection?
Plato wrote about Atlantis in what is considered an allegory, "Timaeus and Critias". Many scholars would say that this is his answer to his depiction of the "ideal state", Athens, as he wrote about in "the Republic". From what I can tell, Plato was not widely known for writing self-indulgent fantasy pieces. He was known as a scholar and a philosopher, so why would he create a metaphoric society as a foil for his Greek utopia? Enter, Randall Carlson – a paleogeologist. He came up with an interesting theory. He believes he has found the lost city of Atlantis in the North Atlantic, and what is more, he believes he can explain how it met its end. The Younger Dryas impact event. A cataclysmic strike of multiple pieces of a comet that happened at the end of the last ice age. He contends that it was this that was responsible for the extinction of the megafauna, the Clovis culture in North America, and the demise of the Atlantean civilization.