Carbon-14 is unstable. One of its protons eventually decays into a neutron and an electron, spitting out the electron. That transforms the carbon-14 back into nitrogen. If carbon-14 weren't continually created, it would disappear. This unstable carbon disperses throughout the environment.
Plants photosynthesize it.
Animals breathe it. Bigger animals eat plants and the smaller animals. Thus carbon-14 works its way into every living organism. In 1947, chemist Willard Libby realized this provided a way to figure out how long something has been dead. That innovation won Libby a Nobel Prize and has enabled archaeologists and paleontologists to date the Dead Sea Scrolls, Egyptian mummies, bones of mammoths and other organic objects up to about 50,000 years old.