What's the opposite of a silver lining? A hypothesis floating around in the scientific community, and published in PLoS One, argues that our big brain is the reason that humans are so prone to cancer.

One could argue that humans are the smartest animal alive, and it would be because of our larger brains. But, in order for our minds to expand to their current size, our cells had to become less willing to kill themselves off, increasing our risk for cancer.

When cells become damaged or unnecessary, they destroy themselves in a process called apoptosis. Apoptosis is also important for developing animals. When animals are developing, their brain quickly develops a huge amount of neurons before pruning them out. At a certain age, animals' brains simply stop making new neurons.

Two of the three researchers behind the study in PLoS One, Gaurav Arora and John McDonald, had previously found that the genes that promote apoptosis are essentially suppressed in humans, while those that restrict apoptosis are promoted.

The more recent study, from researchers at the Georgia Institute of Technology, compared skin cells from humans and two primates, chimpanzees and macaques. The researchers found the human cells are reluctant to commit apoptosis. When subjected to apoptosis-triggering chemicals, the human cells responded less significantly than those of the primates.

Fewer human cells died, and when they did, they required higher concentrations of the chemicals. By the time that the study ended, the human cells did not even change their shape in the way that cells are supposed to when they are about to die.

That lack of apoptosis may explain why humans' brains are so large compared to their body size. Because human fetuses' brains prune cells less often, it may cause their brains to swell. A study found that, when the proteins responsible for apoptosis were turned off in mice, they grew huge brains.

The huge brains in humans are responsible for humans' long lives, which is why we are able to spend so much time lavishing attention on our children and learning new things.

But the downside is that the lack of apoptosis may put humans at risk for tumors, since the destruction of malfunctioning cells would lower the risk of cancer. "Reduced apoptotic function is well known to be associated with cancer onset," John McDonald says.