Usually when we talk about evolution we talk about cool organisms and about how they have acquired traits over time that have given them a better chance of survival. But, not this time—nope, we are going microscopic again. This time we will talk about your body as its own ecosystem. Unlike the ecosystem outside of our bodies, our internal ecosystem isn't about competition or survival of the fittest. Instead, pretty much all of the cells in our body cooperate, one way or another. Many cell types never reproduce and other cells dedicate their cellular lives to the production of other cell types.
Cancer disrupts this society of cells where balance and collaboration are the rule. A cancer cell develops the ability to survive and divide better than its neighbors. The invasive cancer cell can quickly become a cancer clone, which in turn out-competes adjacent cells in the human body. Cancer cells therefore transform a collaborative environment into an environment where only the fittest survive, the same principles that govern the evolution of organisms over time.
Cancers can usually be attributed to one, single founder cell that experienced some sort of heritable change that gave it a selective advantage over other cells. This change allows the cancer cell to grow and divide under conditions where a normal cell does not, and allows it to colonize areas of the human body usually reserved for other cells. Does this sound familiar? A genetic change, which is passed onto offspring, enables the colonization of a unique niche. We could just as well be talking about a change in a complete organism.
It is estimated that 1016 cell division events occur in a human during his lifetime (Alberts et al, 2003). Over time, mutations, which sometimes happen when the cell makes a mistake during DNA replication or that can be induced by special agents, like cigarettes, that can cause damage to your DNA, accumulate in the dividing cells. Sometimes the sequence of the cancer cell's DNA itself may not be changed, but instead the structure of the DNA is altered in a heritable way, and these types of changes are called epigenetic. It is believed that what makes a cell cancerous is usually more than one heritable change. This is why cancer is largely thought to be an old age disease, because mutations that accumulate over time allow the cancer cell to outcompete other cells in the body. As cancerous cells reproduce, they acquire even more heritable changes that allow them to better invade surrounding tissues. A highly progressed cancer cell usually has many, many genetic changes, including gross changes in the number of chromosomes they contain.