BBC News - Why is there only one human species?:
'via Blog this'
An interesting article on what's believed to have been a combination of climate shifts and supervolcano eruptions that led to the global dominance of H. sapiens. Several prominent paleoanthropologists believe that a series of dramatic climate changes that hit the planet over the past three million years also helped to provide an evolutionary selective pressure that drove the adoption of larger brains and more adaptive behavior in our species.