Evolution has given humans much larger penises than our primate relatives, but it's taken thousands to milloins of years for these changes to take effect. If you're interested in data of the evolution of the human penis, check out this thread:
http://www.lpsg.org/116182-evolution-and-the-penis.html
The latest data shows that average testosterone levels are
decreasing likely due to pollutants in the environment. We're likely to see average penis sizes decreasing over time or maybe even widespread sterility and/or impotence if this trend continues. Needless to say that this is a very scary trend that does not bode well for the human race <
"Studies in the 1940's showed the average testosterone level to be 300 ng/ml higher than for men today."
http://www.usdoctor.com/testost.htm
Men's testosterone levels dropping, study says
Testosterone levels down in last 20 yrs
American Testosterone levels dropping like rain