Cancerous tumors are well adapted for survival. The cells within cancerous tumors do not recognize the "stop" points for cell growth. This means that cancerous cells divide long after non-cancerous cells would stop. Furthermore, cancer cells trick the body into fueling them with substantial energy to grow larger. Does this mean that cancerous cells are simply foreign parasites that have well-developed strategies for attacking human bodies? On the contrary, cancer cells are made of our own cells, which gives these cells enigmatic and frightening auras. With that being said, the ever-present question must be asked: why hasn't natural selection eliminated cancer entirely? According to evolutionary biologists, natural selection has favored certain defenses against cancer but, also, has provided some tools for the growth of cancer cells. Of course, understanding the evolution of cancer does not guarantee that this disease will be eliminated. However, revealing cancer's history could help develop new ways to fight it more effectively.
References
Zimmer, C. Evolved for cancer? Scientific American. 18, 14-21 (2008).
Monday, February 16, 2009
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The main reason that natural selection would never be a strong pressure against cancer is that the great majority of cancers occur after reproductive age. If a condition does not impact reproduction, selection has no way to act.
Furthermore, cancer is much more common in industrial countries with generally higher life expectancy, which suggests it is a byproduct of the strange biological phenomenon of living decades after the ability to reproduce is gone (at least for women).
The huge levels of carcinogens that humans have been releasing into the environment is extremely recent, and even if youthful cancers have increased as a result (I have no idea if the data show this), there has not yet been time for populations to respond to any selection pressure this might have imposed.
While it is true that natural selection has only a limited ability to stop cancer, it is also important to realize that some of natural selection's defenses have delayed cancer until late in life (rather than completely eliminating it).
In his "Evolved for cancer?" article, Carl ZImmer states: "Natural selection favors anticancer proteins such as p16, but only in moderation. If these proteins become too aggressive, they can create their own threats to health by making bodies age too quickly."
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