Heart tumors are incredibly rare, whereas other types of cancerous growths, such as those in the breast or colon, are so common they’ve nearly become the norm. But why does cancer prefer certain body parts over others? A new opinion piece suggests an interesting theory to this ever-pressing question: natural selection.

The theory suggests that natural selection has favored strong anti-cancer protection for small organs that are critical to human survival and reproduction. Other organs that are larger or come in pairs could potentially accumulate larger numbers of tumors without being as impaired, whereas other small and important organs, such as the pancreas or the heart, would be easily compromised with only a few tumors. So small, important organs would be better at defending against cancer than organs such as the kidneys, the theory goes.

“Most investigations into the differential vulnerability of organs to cancer have focused on a distinction between intrinsic and extrinsic causes,” Frédéric Thomas, an evolutionary biologist at the Center for Ecological and Evolutionary Cancer Research in France and co-author of the study told Medical Daily in a recent email. “We put forward that evolutionary selection pressures for cancer suppression are unevenly distributed in organs based on the organ’s relative contribution to the host’s survival and Darwinian fitness.”

According to the paper, cancer biologists should start to think of individual organs as specialized islands with their own environmental conditions — each with its own level of oxygen, acidity, and water. Using this model, it’s easier to understand how the survival of cancer cells would depend on the hospitality of the local environment, or the organ, in which it manifests.

Thomas told Medical Daily that his team has already begun to test this hypothesis by using mice to test the speed and extent in which cancer growth occurs in different organs. Results are expected to arrive by next year, and may even help us better understand how to treat certain cancers.

“If we are right, it means that certain organs then have a “secret” to avoid cancer,” Thomas told Medical Daily. “In the same way that we are aware of this among animals (e.g. naked mole rats have no cancer while mice have a lot of cancers) and that we try to understand why, it could be promising to understand the proximate reasons behind the differential vulnerability of our organs to cancer.”

Source: Thomas et al. Evolutionary Ecology of Organs: a Missing Link in Cancer Development. Trends in Cancer . 2016