What historical factors were important in affecting "D" (death rates)?
Very early in human history, increases in brain size and the development of increasingly sophisticated culture and communication allowed death rates to decrease. There is no reason to suspect that birth rates changed much during this time, but the drop in death rates allowed the population growth rate to increase slightly (remember, G = B - D).
Beginning about 8,000 B.C., the human population growth rate began to increase more and more rapidly. This was the time of the agricultural revolution, which allowed people to settle in one place with the security of raising their own food, rather than having to live in a more nomadic style as hunters and gatherers. Major decreases in death rates ("D") resulted, which led to increases in "G" (again, we assume that "B" did not change significantly).
The mid 17th century saw advances in technology, medicine and sanitation, all of which fostered further decreases in "D" and consequent increases in "G."
So, part of the reason that the human population grew slowly for so long and then faster and faster with time is that death rates ("D") decreased.
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