Graham Hancock has some interesting theories as to why human knowledge and memory 'only' dates back around 10,000 years or so. Hancock points to evidence that this could be due to a catastrophe, a major world disaster, that brought down civilisation and left only the 'lucky' survivors to continue the human race. Fascinating reading!
http://grahamhancock.com/phorum/read.php?1,76819,76819
http://grahamhancock.com/phorum/read.php?1,76819,76819
