Figure I. Estimated probability of colorectal cancer by age 90 based on the number of cells in the colon.

The probability of getting colorectal cancer at a certain age was calculated with the equation p = 1-(1-(1-(1-u)^d)^k)^Nm [71] where u is the mutation rate per gene per division, d is the number of stem cell divisions since birth, k is the number of rate limiting mutations required for cancer to occur, N is the number of effective stem cells per crypt and m is the number of crypts per colon [71]. Parameter values are listed in Table S1. This shows that assuming all other parameters are equal, larger animals should have a much greater lifetime risk of cancer when compared to smaller organisms. Blue dots for mouse, human and whale indicate the estimated risk of colon cancer occurring within 90 years of life given the approximate number of cells in a human colon, 1,000 times fewer cells to represent the mouse, and 1,000 times more cells to represent the whale. The estimate for 1,000 times smaller than a human (e.g. a mouse) is still barely above zero even after 90 years. In reality, a mouse only lives a maximum of 4 years [35], so based on this equation they should never get colorectal cancer. The red dot indicates the lifetime risk of colon cancer according to the American Cancer Society which is about 5.3% for men and women averaged together [10].