Bigger brains, more cancer?

Rachael Rettner reports on a hypothesis that human cancer risk may be a side-effect of brain evolution. The hypothesis emerges from studies of gene expression, which show that regulated cell death (apoptosis) is downregulated in some human tissues:

The researchers are tying these two hypotheses together. They think that reduced apoptosis may have helped people acquire their large brains. But it may also have made us more prone to cancer.
"It's kind of hard to explain why we could have evolved to have a less efficient apoptotic system," says [Georgia Tech researcher John] McDonald. "So the hypothesis we came up with was that maybe selection to increase brain size was what put the selective pressure on the system to reduce apoptosis." And even though less apoptosis may have meant more cancer, there would not have been selective evolutionary pressure against it since most cancers don't appear until after reproductive age, McDonald adds.

The article leads with the assertion that humans have more cancer than chimpanzees. Undoubtedly that’s true, but I think it’s a tough comparison to make. Chimpanzees in the wild only live into the bottom range of ages where humans start to have a high cancer risk. Neither captive nor wild chimpanzees have been observed in sufficient numbers to have accurate estimates of the rates of rare diseases, and captive chimpns Some cancer risk alleles, like BRCA1, hit earlier in life, but they are special cases – rare, recent, and possibly selected for pleiotropic effects on other traits.

No question, some species must have adaptations to reduce the incidence of cancer per cell: whales, for example, live as long or longer than humans and have vastly larger cell numbers. If they had the same cancer risk per cell, every whale would be half tumor. But humans didn’t just add brain tissue, we added mass and reduced the proportion of other tissue types including muscle and gut. I’m guessing this is a more complicated issue than the simple hypothesis would suggest. Still, there’s nothing impossible about it….