Cancer is the ultimate betrayal. A body’s own cells become enemy combatants, smothering their brethren as they multiply out of control. It’s a utilitarian outrage — but cancer is also amazingly prevalent. In the U.S., men have a 45 percent chance of being diagnosed with cancer in their lifetimes, while women have a 39 percent chance.

So why haven’t we evolved resistance to the disease? Surely something that wipes out so many members of our species would get fast-tracked out of the gene pool!

Turns out it’s not that simple. A story in the January issue of “Scientific American” reports on how natural selection is losing the battle with cancer.

It’s not for lack of effort. Tumor suppressor proteins are natural selection’s most powerful weapons against cancer, keeping it in check by looking for abnormally dividing cells and stamping them out. Certainly, this should be unequivocally good.

Or maybe not. Recent studies in mice make the picture a little murkier. Scientists at UNC Chapel Hill engineered mice that were missing p16, an important tumor suppressor gene. The researchers expected the mice to quickly drop dead, their bodies wracked with tumors. Some mice did just that. But the survivors showed something rather strange. As they grew chronologically older, their cells still acted biologically young. In one experiment, the scientists destroyed insulin-producing cells in the animals. The mice with p16 were unable to recover, developing fatal diabetes and quickly expiring. The mice without p16, on the other hand, bounced right back. The progenitors to their insulin-producing cells, uninhibited by p16, were able to multiply fast enough to restore normal pancreatic function. So even tumor suppressor genes are an evolutionary trade-off. Natural selection can’t aggressively ward off tumors without hurting healthy tissue.

Another way cancer can thwart natural selection is by co-opting essential bits of human biology. Fatty acid synthase, or FAS, is a protein that scientists believe was critical to the supersizing of human brains. But cancer cells use the enzyme, too — FAS is so important to the malignancy that inhibiting it can kill a tumor. That puts natural selection between a rock and a hard place. It is helpless to defend against FAS-exploiting tumors without stunting human brain development.

Reproduction is another essential biological function that is hijacked by cancer cells. For example, a fetus forms a placenta to draw vital nutrients out of its mother’s bloodstream. This process is not unlike the way a tumor forms a network of blood vessels to siphon away the nutrients that it needs. Genes that code for aggressive placenta building would be favored by natural selection if they grew a healthier baby. But these same genes could be co-opted by tumor cells when that baby grew up. And natural selection might increase the prevalence of these cancer genes over time, since they also make more viable babies.

Another example in the reproduction department comes from sperm cells. Human men spend decades as sperm factories. If these little soldiers are dividing furiously, a man’s sperm count goes up, and so do his chances for offspring. Genes for faster-dividing sperm will be prized by natural selection. But the warp-speed cell division so prized in sperm production would make a tumor extra-deadly. Natural selection again has its hands tied. When choosing between more babies or less cancer, natural selection will go for the babies.

The whole thing is sort of depressing — humans have essentially evolved to get cancer. In my opinion, this should give pause to any Intelligent Design yahoos. But even diehard Darwinians can learn a lesson from the cancer story.

Darwin dealt a tremendous blow to the human ego. Not only was our planet not the center of the universe, but our species was not even specially created.

But even those who know better cling to the idea that humans are special. Evolution is seen as a directed process, beginning with lowly amoebae and culminating in humans, “the paragon of animals.” It is a salve for our self-esteem to imagine evolution selecting, perfecting and finally revealing its human masterpiece.

Cancer biology shows that evolution is as bumbling as we are. It is a tinkerer, not an architect. Humans are riddled with evolutionary throwbacks. We yank out wisdom teeth that evolved to suit our larger-jawed ancestors. We complain about aching backs that worked better when we were quadrupeds. And we harbor infection-prone appendixes that were useful only in our cud-chewing relatives. A human is, as my CS pal likes to put it, “a buggy program.”

The interplay of natural selection and cancer paints a bleak picture of human evolution. Natural selection often takes one step forward and two steps back. We can’t count on it to wipe out cancer, much less perfect our species.

Still, I find this perspective inspiring. We are humans, alone and unaided, clawing our way out of the evolutionary muck. Natural selection hasn’t beaten cancer? We’ll just have to do it ourselves.

Contact Shelby Martin at samartin@stanford.edu.