Cancer seems inscrutable. It has been variously described as a disease of the genome, a result of viral infection, a product of misbehaving cells, a change in metabolism, and cell signaling gone wrong. Like the eight blind people touching different parts of an elephant, these all indeed describe different aspects of cancer. But the elephant in the room is that cancer is evolution.
Cancer is a form of evolution within our body, the “soma”—cancer is somatic evolution. Take that spot on your arm as proof that some of your cells are different from others: some darker, some lighter. This difference is also heritable when one of your body’s cells divides into two daughter cells, encoded as a mutation in the cell’s DNA, perhaps caused by sun exposure. Much of somatic evolution is inconsequential. But some of the heritable variation within a human body may be of a kind that makes a more substantial change than color: it produces cells that divide faster, setting in motion a chain of events following from the inescapable logic of Darwin’s natural selection. The cells carrying such a mutation will become more popular in the body over time. This must happen as the criteria of natural selection have been met: heritable change providing an advantage over neighboring cells, in this case in the form of faster growth.
But no single mutation can produce a cell that is cancerous—i.e., one able to mount a threat to the well-being of the body. Similar to the evolution of a species, change occurs upon change, allowing a population to adapt sequentially. As the clones of faster-dividing cells amass, there is power in numbers, and it becomes probable for another random mutation to occur among them which further increases the proliferation. These mutations and their selection allow the cancer to adapt to its environment: to ignore the signaling of its neighbors to stop dividing, to change its metabolism to a quick and dirty form, to secure access to oxygen. Sometimes the process starts with an infection by a virus—consider this just another form of heritable variation as the viral genome becomes a part of the DNA in the cell it attacks. Evolution is rarely fast, and this is also true for somatic evolution. The development of a cancer typically takes many years, as the mutated cells acquire more and more mutational changes, each increasing their ability to outcompete the body’s other cells. When the cancer finally evolves the ability to invade other tissues, it becomes nearly unstoppable.
Evolution is sometimes confused with progress. From the perspective of a cancer patient, somatic evolution certainly isn’t. Rather, as cancer develops, changes in the composition of the body’s gene pool occur: the very definition of evolution. The notion that cancer is evolution is not an analogy, but a matter of fact characterization of the process. It is humbling indeed that evolution is not only an ancient process that explains our existence on this planet, but is also constantly happening within our bodies—within our soma.