Any cell of any living being, including humans, contains potassium ions, which are required to neutralize the excess negative charge of proteins, otherwise the interior of the cells would become acidic and it would self-destroy.
No other positive ion can replace potassium, because any other abundant positive ion has a much greater tendency of forming solid precipitates, which would also destroy the cell. That is why any living cell expels the abundant ions of sodium and calcium outside it, while pumping inside any potassium ions from the environment. The moment when a living cell stops pumping potassium and magnesium inside and sodium and calcium outside, is when the cell dies.
Among the primordial chemical elements, which already existed at the formation of the Solar System, there are many which are weakly radioactive, i.e. they have some isotopes with half lives that are of at least many hundred million years, but of many billion years for most of them.
Among these weakly radioactive elements, the human body contains not only potassium 40, but also calcium 48, but the latter is much more weakly radioactive than potassium. Besides primordial radioactive isotopes, there are also radioactive isotopes that are formed continuously by the cosmic radiation, like carbon 14, which is also present in the body of any living being.
Among the primordial radioactive isotopes, the most radioactive are uranium 235 and potassium 40, followed by uranium 238, thorium 232 and platinum 190. The existence of weakly radioactive isotopes is not random, but it is determined by a set of rules of nuclear stability. For instance, potassium 40 is radioactive because any isotope with an even atomic mass of a chemical element with an odd atomic number and heavier than nitrogen is radioactive. Potassium 40 just happens to have an unusually long half life, so it has not decayed yet (the long half life is because potassium 40, like Buridan's ass, cannot decide whether it should decay into argon or into calcium, so it stays in limbo).
All living beings have mechanisms for repairing damages caused by radiation to their nucleic acids, so the very low levels of ubiquitous natural radiation are not worrisome, except in certain locations where they are much higher than normal.