Genes in the
DNA of
living things which cause the
body to
degrade and
die after a set amount of
time. Most living things have these genes, which is the lifespans of a creature tend to be similar.
Humans have the genes also, though until recently, disease and violence killed people early enough to keep us from dying because of the genes. Science is working on a way to turn them off, and coupled with a true cure for cancer, the possiblity of physical immortality is close.