My setting takes place on a planet that has a very high level of radiation because of reasons unknown to the population. This factor or group of factors are not man-made, but due to some change in planetary conditions. As a result, 50% - 70% of fetuses either die in miscarriage, or are stillborn. The remaining percentage make it to full term. The human beings who survive suffer no other ill effects that go along with radiation, such as cancer, infertility, etc.

I want to make this reality possible as part of a natural occurrence that happened without human intervention. How can a planet become radioactive enough to cause most fetuses to die, but not affect the remaining population in a harmful way?

  • "70% of fetuses die in utero" sounds like infertility to me. – Willk Dec 4 at 22:17
  • Have it turn out that cancer is caused by a virus plus radiation, not radiation itself, and this society has long since cured the virus. – Joe Dec 4 at 22:35
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    A fetus that dies in utero is the very definition of stillborn. Perhaps you mean miscarriage? Which is an embryo that dies in uterio. – Cyn Dec 5 at 0:51
  • What is your definition of "human beings?" Are we assuming the precise DNA sequence that we have today on Earth, or are we assuming that this sequence has changed in response to the high level of radiation on this planet for many generations? – Cort Ammon Dec 5 at 1:39
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    Wouldn't natural selection work? The foetuses that survive are the ones most resistant to radiation, and in turn those of their children that are most resistant to radiation survive, so that in a few generations, virtually everyone is resistant to radiation and the infertility problem disappears – nzaman Dec 5 at 13:40

This seems highly unlikely. "kills fetuses in utero" is a highly specific effect, of reasonable power. "Causes cancer" is the sort of thing that happens to people just because you futz with things too much. (That's why there are so many kinds of cancer, and so many contributing factors).

Having this be the effect of a particular disease, of endemic toxins, or of a specific kind of genetic damage/mutation seems much more likely than as the result of ongoing radiation. Indeed, you could still have radiation be the cause, if peripherally - there was an initial burst of radiation that messed people up a lot, and killed a bunch of folks. Once that passed, the group that came out of the genetic bottleneck had some serious issues with miscarriages. Now, that's the sort of thing that tends to fix itself over a few generations, as "good at having kids" has really blatant benefits as far as getting your genetic payload to the next generation, but it'd persist for a while, at least.

Inducible DNA damage repair genes.

Your humans have robust and inducible DNA damage repair genes. Just as UV exposure causes melanin production in the skin (among other things) to protect from UV so too ionizing radiation on your planet induces increased DNA damage repair enzymes.

The problem is that fetuses start with no DNA damage repair enzymes. As soon as their radiation sensing cellular mechanisms kick in they start upregulating their DNA damage repair genes as fast as they can. But that ability to sense radiation and produce repair enzymes does not become possible until the fetus is several weeks along. If the fetus can upregulate its repair abilities before too much damage is done, it is fine for the rest of its life. Most of the time that is not what happens - the cells are too heavily damaged before the repair mechanisms are up to speed and so the fetus dies.

The radiation actually poses no direct ill effect to humans

But the radiation triggers a virus that normally lies dormant. This virus is similar to viruses we have on earth, like Rubella or Chickenpox, in that it has little to no lasting effect on children or adults but it wrecks havoc on fetuses.

In this case, the virus has such a small effect on people that they may not even realize they have it; it feels like a slight cold or maybe a rash like they ran into a mosquito or two. And, in this case, the effects of the virus on a fetus are worse than any earthly virus. 50-70% of pregnancies end in miscarriage* or stillbirth.

[* Note: You can decide if the virus affects all pregnancies or only ones that have progressed into the second trimester where the baby is called a fetus. Either way, just adjust your numbers to come out how you want them.]

Radiation activated chemicals

During pregnancy, some chemicals are needed for the fetus to develop correctly. Unfortunately, these chemicals suffer rapid degradation when exposed to radiation and if the mother does not produce them in high amounts, the fetus dies. Once the fetus stops needing them, it is in the safe zone and develops without problem. You can choose how long this period is to help in the narrative.

To solve the problem of why this species has not gone extinct, this chemical could be the one that grants the specimens radiation resistance and DNA repair capabilities.

Also if so many miscarriages happen you would need one of these circumstances to keep the species alive:

  • High rate of pregnancies.
  • Long lifespan.
  • Lots of specimens solely dedicated to breeding.

And with time, evolution would select those who produce more of this chemical and the effect would mitigate. There are also protections against this, like moving all pregnant women to living quarters made of lead or things like that.

High activity nebula

Imagine a planet near a high activity stellar zone, where a lot of supernovas happen regularly (in cosmic time scale). This would mean that the planets formed in these nebulas would have a high percentage of heavy elements, some of which are radioactive.

Also, the comets flying around these zones would have a high quantity of radioactive metals. If one or some of these comets fell into the planet and burned out in the atmosphere, all of these metals would be spread out through the atmosphere and around the world, which would make the surface of the planet quite radioactive, plus activating some of the elements already present in the planet.

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