Why haven't we evolved a way to defeat cancer?

Chris Smith put this question to geneticist Diana Alexander, who started by explaining the process of cancer...

Diana - Cancer is uncontrolled cell division and it leads to the formation of tumours which can cause losses in tissue function, which can eventually result in death, so this is bad for our bodies. So we kind of would expect it to be selected against and, potentially, that we would have evolved not to get cancer, but we still do see people get cancer. I think the answer to this largely lies in how natural selection works to maximise reproduction.

We have evolved ways to prevent cancer at an early age. We have very efficient DNA repair mechanisms which repair DNA damage that can lead to mutations which drive cancer and these work really well. You see cancer occurs at a very low rate in younger people but, in older people, the rate of cancer increases because there are more mutations that accumulate because the repair mechanisms aren’t 100 percent effective.

So the theory is that we haven’t evolved to prevent it completely because we want to maximise reproduction, so you have selection to prevent cancer at a young age. But it’s not selection to prevent cancer indefinitely because the force of natural selection decreases with increased age because there is a lower chance of reproduction. So what we get is kind of a balance where it’s unlikely to get cancer at a young age, but we haven’t evolved to prevent it indefinitely.

Chris - So we’ve evolved not to get cancer up until the time by which we would have reproduced but after that, your body no longer has as much of an ability to select because it won’t harm your ability to pass your genes onto the next generation?

Diana - Exactly.