Why animals have tumours

From the viewpoint of an evolutionary biologist, carcinogenesis should be looked upon as a protective mechanism against destruction of DNA. Because genes expressed in embryonic cells are covered and protected by heterochromatinization, they are the most appropriate 'alternate genes' compared to genes that are expressed already in somatic cells. When DNA-damage occurs, the embryonic genes can be activated. Some somatic cells exhibit some features of embryonic cells.