That big brain of ours? Yeah, the one that you're using to read this Neatorama post. It's also the reason us humans get cancer.
That's what Georgia Institute of Technology researcher John McDonald discovered when comparing our cells with those of other primates:
When cells become damaged or just aren't needed, they self-destruct in a process called apoptosis. In developing organisms, apoptosis is just as important as cell growth for generating organs and appendages – it helps "prune" structures to their final form. [...]
McDonald compared skin cells from humans, chimpanzees and macaques and found that, compared to cells from other primates, our cells are reluctant to undergo apoptosis. When exposed to apoptosis-triggering chemicals, human cells responded significantly less than the chimp and macaque cells. [...]
McDonald suggests that humans' reduced capacity for apoptosis could help explain why our brains are so much bigger, relative to body size, than those of chimpanzees and other animals. When a baby animal starts developing, it quickly grows a great many neurons, and then trims some of them back. Beyond a certain point, no new brain cells are created.
Human fetuses may prune less than other animals, allowing their brains to swell.
The downside to this is that we're more susceptible to developing cancer cells, which do not respond to apoptotic signals: Link