Natural selection can be defined a process of differential survival and reproduction of members of the species as some members of the species have genes which when express are responsible for traits that can aid in the survival of the organism in its environment and also promotes its reproduction. The beneficial traits are passed from one generation to another this is responsible for sustenance of the species. It gives the survival advantage to the members of the species.
This is because muscles of human cells are made up of proteins filaments of actin and myosin, which contain nitrogen molecule. Definitely this should contain the radioactive nitrogen incorporated.
