Why is there cancer?

Evolutionary biologists are constantly wondering why things are the way they are. I’ve bored you all with a rather esoteric example: Why would a slug lose its ability to learn?

Here’s another one, a little closer to home: Why wouldn’t the human body evolve mechanisms to fight cancer? Why hasn’t natural selection eliminated cancer long ago? Surely a mutation that prevented cancer would confer an advantage. This is pretty puzzling.

First, I have to hasten to say that our bodies DO have cancer fighting mechanisms. Lots of them. They are found at many different levels (molecular, cellular, immune system). These mechanisms do a really good job. It is actually quite amazing that we live so many decades. Such longevity is very rare in nature.

But why do OLD people get cancer more than young ones do? In fact why do old people get feeble at all? In short, why do we senesce? Surely individuals that don’t senesce should be better at surviving that those that do! Makes sense right? If you don’t senesce, you live longer cause you don't die of old age. But if you are smart (which I’m not), you realize that ,,, it depends!

One of the greats of evolutionary biology, George C. Williams realized this back in the 50’s! Williams proposed the idea of antagonistic pleiotropy.

This is a mouthful, but the idea is quite simple. These are genetic “deals with the devil.” If my actuarial table says I am on average going to be eaten by a saber-toothed cat sometime before my 30th birthday, then genes that improve my chances of fighting cancer when I’m in my 80s don’t do my lineage much good. They will not spread very quickly by natural selection.

On the other hand, if a mutation comes along that actually INCREASES my chances of dying of cancer in my 80’s, but, at the same time, ENHANCES my ability to fight saber-tooth tigers during my first 3 decades, this mutation it will have a big advantage. Cause the bad part of the deal (cancer in my 80s) is hardly ever seen (cause I’m usually already eaten by a saber-toothed tiger long before I’m 80), while the good side of the deal is ALWAYS realized. With my new mutation, I, and my descendents, are able to better avoid saber-toothed cats, thereby living on average 5 years longer till the grand ole age of 35 (and produce a couple of more kids). If we happen to be lucky and live to be 80, the devil takes us and we die of cancer. But overall, we have more babies than the lineages that didn’t get to take the deal, so we have more babies. Now each time one of these "deal-with-the-devil" mutations arises, natural selection takes the bait. Thus, lousy genes that kill us when we are old build up. Viola, senescence, i,e., ower fitness when we are old.

Williams posed this possibility as a very logical, very powerful hypothesis. Of course, in science, this doesn’t make it right.
More later.