Torn,
If man evolved over hundreds of thousands of years ago, why were there only about 200-300 million people alive 2,000 years ago? Surely there'd have been many billions by then?
As already mentioned population growth was kept low because of higher mortality rates, as our technology and understanding of the world increased mortality rate lowered to where we have the human population mushrooming in size very quickly.
As our technology increased along with better medical treatment, increase supply of food availibilty, the building of great cities, indoor plumbing, new vacines, mass transportation, better nutritional understanding, safety laws, United Nations, and the like have all contributed towards this exponential growth. It looks like a future of mandatory population control may be our next option to slow down growth unless some catastrophy does it first: