In the early stages of life development on Earth biocoenosis has created conditions which promoted Metazoa organisms to complicate their structure. On some stage of the evolution this made the species mortal. Apart from that, loss of ancestral immortality and further differentiation into r- and K-strategists was influenced by natural selection of biocoenoses. During its course, biocoenoses containing mortal species and those with higher share of them gained the evolutionary advantage.