Does the human body come equipped with cancer?
What is cancer? Obviously, it's a general term that covers a wide range of often life-threatening diseases, but what is the stuff that cancer is made of? This might be one of those things you sort of knew but never really thought about -- that's how it was for me for a long time. Consider the frightening absurdity of this: Cancer isn't a germ. It isn't a parasite that gets into you from the outside. Cancer is you. It comes from your DNA, from your own organs and tissues. Despite the fact that it can kill you, it's as much a creation of your body as your lungs, kidneys and brain.
The human body grows, maintains and repairs its tissues through the process of cell division. Cells make copies of themselves to fill in the gaps where needed. When a cell's DNA -- the chemical recipe that establishes the local rules governing cell function and reproduction -- gets damaged or altered in a way that makes the cell useless, the cell is supposed to simply die and get swept away by the body's natural cleanup crew. But what if a cell in your body gets damaged, becomes useless, and then refuses to die? What if it not only refuses to die, but makes copies of itself, including its damaged or altered DNA, creating more and more useless cells that do nothing helpful for the body? This is called a tumor. If the relentlessly self-copying cells are of the type that spread and threaten other body tissues, it is malignant cancer.
One might wonder, "Why are our bodies so keen on making cancer cells? Shouldn't millions of years of evolution have selected against this? Shouldn't cancer be less prevalent than it is?"
OK -- interesting question. It looks like cancer interferes very little with the propagation of genes. "How can that be?" you might ask. According to the American Cancer Society, one-third of women and half of men in America get cancer sometime in their lives. Indeed, various forms of cancer, such as those of the lungs, breast and colon, are leading causes of death in the United States (of course if you look at worldwide stats, cancer takes a back seat to things like lower respiratory infections and diarrheal diseases [source: WHO]). However, there's an important factor we need to take into consideration: Cancer tends to strike disproportionately once people are culturally and often biologically past their childbearing years. If you take a look at the stats provided by the U.S. government's Surveillance Epidemiology and End Results (SEER) program, you can see that an overwhelming majority of new cancer cases are recognized in the elderly. In 2009, across all races and sexes, for every 100,000 people over the age of 75, there were about 2,272 cases of cancer. Compare that to people under 20, whose rate was about 17 per 100,000.
Unfortunately, and as you're probably aware, genes are "selfish." They're not very concerned with your happiness or longevity, except insofar as these things might help them reproduce. So it turns out natural selection has very little influence one way or another on diseases that mainly affect the elderly, since these diseases, as destructive as they are, don't keep genes from spreading.
But this is certainly not all there is to be said about the evolutionary interpretation of cancer. After all, it could certainly turn out that, for example, genes that carry a predisposition for cancer also allow the body to activate a process of fast, hyper-efficient cell division to speed up the repair of damaged tissues or keep the body young.
Have you read anything interesting on this subject? Or have you got a theory of your own? Let us know in the comments section!