I've got a long post up building on the work of Smil et al in describing what the demographics of the Roman Empire were like. They were horrible: the odds are that most of the people reading this would be dead, likely of one horrible preventable illness or another, in an environment far worse than any now existing. Looking to the future's all well and good, but looking to the past is just as important, I think.
Go, read.
Go, read.