Reality is amazing!!
Stars can function as "breeder reactors", where the natural flow of neutrons produced as intermediate / side effect of the various fusion reactions going on in a later-stage-life star get absorbed by nuclei and then (by beta decay) produce beyond-iron elements. This is called the "s-Process" (slow), and responsible for a large range of elements into the Lanthanides or so. And stellar winds, or the planetary nebula stage at the end, will return some of this to the interstellar medium.
It's correct that the heaviest "naturally present" elements require the so-called "r-Process", heavy overabundance of neutrons / extremely high neutron flow as in supernovae, or direct fusion of beyond-iron nuclei as in neutron star collisions.
There is rather active research happening here, both astronomical (trying to detect various nuclei from x ray spectroscopy of cataclysmic events) and theoretical (because r-Process cannot be simulated in labs since the neutron fluxes needed are not within our reach).
That said, it remains true that many "transition groups" elements are bred via s-Process in relatively-ordinary stars.
Iron 56 is the isotope with the highest nuclear binding energy per nucleon, so it can form from the fusion of any lighter nuclei.
After iron 56, the binding energy decreases slowly, so the next heavier nuclei can still form from the fusion of certain lighter nuclei, but not from any of them, but only from pairs with a lower average binding energy.
So the following heavier nuclei after iron 56 can still form through fusion, but with increasing atomic mass the probability of their formation decreases quickly, until it becomes negligible.
Relatively large amounts of cobalt, nickel, copper, zinc, gallium and germanium still form through the fusion of lighter elements, but after germanium the amount of chemical elements formed through fusion becomes extremely low. Already the amount of germanium formed through fusion is almost ten thousand times less than the amount of iron.
The binding energy per nucleon decreases very slowly, so even uranium has a higher binding energy per nucleon than helium, so energetically it could form through the fusion of hydrogen or helium, but such an event has a completely negligible probability (because there is a negligible chance for so many hydrogen or helium nuclei to collide simultaneously and if they fuse into nuclei of intermediate mass those block the propagation of the fusion reaction by having higher binding energies than the heavier nuclei).
The elements heavier than germanium form almost only through neutron capture, with the exception of some proton-rich isotopes, which form through collisions with protons. There are several kinds of environments with abundant neutrons where heavy elements can form, where the concentrations of neutrons and their energy distributions are different, so in any of these environments there are different classes of isotopes that form preferentially there.
In a relatively young stellar system like ours, the matter from which the star and the planets have condensed is a mixture of chemical elements coming from different sources.
In environments with extremely high neutron abundances (which include the nuclear explosions on Earth, not only supernova explosions, neutron star collisions and the like), all chemical elements up to fermium (Z = 100) are formed. Nonetheless, while the matter composed of these elements travels through space until the formation of a new stellar system, most of the trans-uranium elements, except the plutonium, decay. When the Solar System was formed, it still contained relatively large amounts of plutonium, not only thorium and uranium, as the heaviest elements, but since then until now the plutonium has decayed, like also most of the uranium 235 that is used now in nuclear reactors.
The amount used in a nuclear clock would be very small, but even so, its availability would be a problem.
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.
It is quite hard to say, how much radiation (natural, or man-made) is worrisome, because people have been living in locations with high level of background radiation for centuries, or even longer.
Ramsar (in Iran), Guarapari (in Brazil), Orissa and Kerala (in India) and Yangjiang (in China)
https://www.sciencedirect.com/science/article/abs/pii/S13504...
Alsa indoor radon levels in thermal spas, is quite high.
https://www.researchgate.net/publication/319898073_Indoor_ra...
Nonetheless, I think that I have seen recently right here on HN comments about an article showing that there is a higher incidence of cancer for the flight personnel of airlines, which is likely to be caused by the long time spent during their careers at high altitudes with increased radiation level.
In the case of high altitude, exactly like for the weakly radioactive elements that stay inside our body, the exposure to radiation is certain and permanent for everyone.
For the people who live in geographic areas with higher radiation levels, the difference in individual radiation exposures can be very great, because most of the extra radiation exposure would come from ingestion or inhalation of dust, as otherwise the alpha and beta radiation from the soil or rocks would not penetrate the body.
https://en.wikipedia.org/wiki/Effective_dose_(radiation)
In the end you get the stochastic health risk to the whole body, which is the probability of cancer induction and genetic effects in Sieverts. 1 sievert (Sv) corresponds to a 5.5% chance of developing cancer.
In most cases you deal with mili Sievert amounts of dose, which correspond to very small increases in probability of cancer and they are hard to prove in medical studies because the base rate of cancer in humans is large and you have many factors that affect cancer rate: sex, weight, age, smoking, life style, environmental factors, food.
I read the article about higher incidence of cancer for the flight personnel of airlines, it would be interesting to compare it to risks of thermal spa workers. Or miners, there can be a lot of radon underground, especially with insufficient ventilation.
I have no idea which of these articles is more correct.
So the exposure to potassium radiation is constant for any living being on Earth.
Exposure to thorium and radon varies greatly depending on the location, i.e. it can be significant in places with granitic rocks or with sediments whose origin is in the erosion of granitic rocks.
So some people, animals etc. may be exposed to more thorium/radon radiation than from other sources, especially if inhaling dust or radon gas, but for most the greatest exposure is from the content of potassium 40 and carbon 14 that is inside their bodies (which produce only weak beta radiation).
Averaged over the entire Earth, thorium is about one hundred thousand times less abundant than potassium, while potassium 40 is only ten thousand times less abundant than all potassium, so there is more potassium 40 than thorium. When the Earth condensed, there was much more K40 than today, so in the early Earth potassium produced more heat than thorium and uranium.
In the continental crust, more of the thorium of the entire Earth has been concentrated than the corresponding fraction of potassium, so thorium is only about 2300 times less than potassium by mass, so in the continental crust there is about 4 times more thorium than potassium 40 by mass.
However, rocks with potassium, like feldspars, are everywhere, while phosphates, which are the main minerals with abundant thorium, are more rarely encountered at the surface. So there are plenty of places where there is much more K40 than thorium or uranium.
Moreover, at equal mass potassium 40 is a few times more radioactive than either thorium or natural uranium, due to having both a higher probability to decay and to having more atoms per unit of mass. So even if potassium 40 is 4 times less abundant by mass than thorium in the continental crust, it generates several times more radioactive decay events per second. The energy of the radiation from K40 is lower, so overall thorium produces somewhat more heat in the crust.
While potassium generates more radiation than thorium, it is dispersed, there will never be in nature a place with concentrated K40, without being diluted with non-radioactive potassium. So any living being or any potassium mineral will never be as radioactive as a piece of rock that contains thorium minerals.
Moreover, if thorium remains enclosed in some mineral for thousands of years, that mineral accumulates the decay products of thorium, e.g. radium, polonium etc., becoming several times more radioactive than pure thorium. If such a thorium mineral is broken, the dust from it would be many times more radioactive than pure thorium.
So there is a risk that one may encounter a piece of something containing thorium with relatively high radioactivity, while there is no such risk with potassium 40, outside of some laboratory that would try to separate it from natural potassium (for which no good reason exists and such a purified K40 would be highly dangerous, because unlike with thorium or uranium, any living cell would attempt to incorporate such potassium).
In conclusion, while some animals, or more likely some humans, might encounter some thorium minerals and be irradiated by them, the radiation doses averaged over all living beings consist overwhelmingly of radiation from K40 and C14, augmented for marine living beings with a lower dose from the uranium dissolved in sea water. The radiation dose coming from thorium or from other weakly radioactive elements that are abundant, e.g. calcium and vanadium, is pretty negligible in comparison with those.