Abstract | Phylostratigraphic tracking of cancer genes suggests a link to the emergence of multicellularity in metazoa
It seems from this study, our genes specifically the cancer genes where in fact already part of the uni-cellular organisms. And as cell evolved to multicellular organism, we also have multiple inherent "gatekeepers" to keep these genes in check.
So what triggers these genes to become active again? hmm I bet Dr. Bruce Lipton can be a good reference to this.