There is a growing amount of research into the evolution of cancer. This helps explain why the human immune system cannot defeat the disease and why treating cancer is difficult.

Carl Zimmer summarized the research in Scientific America. One of the findings is that there is a tradeoff between aging and cancer.

Cancer is like micro-evolution. The cells mutate and compete with healthy cells for survival. This explains why cancer cells adapt to pressure from the immune system and chemotherapy.

Cancer, in other words, re-creates within our own bodies the evolutionary process that enables animals to adapt to their environment. At the level of organisms, natural selection operates when genetic mutations cause some organisms to have more reproductive success than others; the mutations get “selected” in the sense that they persist and become more common in future generations. In cancer, cells play the role of organisms. Cancer- causing changes to DNA cause some cells to reproduce more effectively than ordinary ones. And even within a single tumor, more adapted cells may outcompete less successful ones. “It’s like Darwinian evolution, except that it happens within one organ,” explains Natalia Komarova of the University of California, Irvine.

Every time a cell divides, there is a risk of mutations. The mutations, if they survive, continue to reproduce. The immune system evolved to slow mutations and development of cancer so the individual could reproduce. There are tradeoffs between the competing systems.

The immune system produces cancer suppressing proteins that stop mutant cell reproduction and starve the growing tumors. One of these proteins, p16, has an adverse side effect. It causes aging.

Without p16, cells do not age at the same rate. They behave like young cells despite old age. The more anti-cancer proteins a person has, the faster they age and die. The fewer anti-cancer proteins they have, the faster they get cancer and die. Evolution selected a moderate amount of p16.

The cancer suppressing proteins are most effective when organs are rapidly growing in childhood. This protects the individual until they reach reproductive age. Here, there is a second tradeoff. As it prevents cancer in youth, it sets the biological conditions to allow cancer growth in old age. The body’s defenses gradually weaken as it is overcome by aging and cancer.

Since evolution does not care what happens to individuals post-reproduction, genes never evolved to fully defeat cancers or to stop aging. The Genes created a defensive mechanism that delays the inevitable just long enough to allow individuals to reproduce.

“Fitness” in evolution has nothing to do with strength or survival. It has more to do with reproduction and genetics. Evolution favors those with the most grandchildren, but it does not really care if the grandfather lives to see them.