"...for most of our species’ time on Earth—including most of the agricultural era—humanity’s natural propensity to expand has been held in check by negative feedback, e.g., food and other resource shortages, disease, and inter-group conflict. Circumstances changed with the scientific/industrial revolution, particularly the increasingly widespread use of fossil fuels. It took 200,000 – 350,000 years for human numbers to reach one billion early in the 19th Century, but only 200 years (as little as 1/1750th as much time!) to balloon another seven-fold by early in the 21st Century. Improvements in medicine, public sanitation, and population health contributed to this expansion, but coal, oil, and gas made it possible. Fossil fuels are the energetic means by which humans extract, transport, and transform the prodigious quantities of food and other material resources into the products needed to support our burgeoning billions. More than any other factor, fossil fuels enabled H. sapiens to eliminate or reduce normal negative feedbacks. Freed from historic constraints, our species was, at last, able to exhibit its full potential for geometric growth."