I fear this question may be somewhat premature as I’m suddenly skipping forward to a period in the species’ history that is comparable to our own, a far cry from my last question about the building blocks of life.

The short of it is; over countless generations the life expectancy of the species in question has shown a steady and unending rise, going from the poor expectations our own ancestors were familiar with and progressing towards lives of centuries (and after the scenario I’m about to propose a lot longer).

At some point things are going to go awry, life expectancy will peak before dropping and then very suddenly death tolls will rise and it will become apparent to all that these things are connected that something is desperately wrong.

Now a while ago in other questions I spoke about symbiotic relationships and portrayed the species as very complex and almost deliberate microbiome and my rough plan was to have something like...

“the longer they live the greater the buildup of waste chemicals from these symbiotic parasites (insert appropriate word) and that it’s not a problem until it becomes too great and subsequently it’s resulted in some sort of genetic deficiency that it now rearing its ugly head en masse”.

...that was a wild moment of imagination whilst first establishing the history.

Question: Are there existing examples of what I’m describing and is there almost a list of ‘usual suspects’ for this sort of thing? Or are we delving too far into fantasy? I don’t know enough about biology to seek analogies in medicine. I know it’s nothing like cancer, that much is sure.

It is a bit of a plot sensitive moment, a lot of key decisions are made as a result of the tragedy. It’s the greatest test they’ve faced since the end of the Stone Age.

  • $\begingroup$ Just realised I misspelled ‘too’ twice. Bad writer! Very bad! $\endgroup$ Dec 10, 2019 at 23:29
  • $\begingroup$ I'm not clear on what you're asking.You describing some species of humanoid that has an increasingly long lifespan then some crisis happens and life expectancy drops really quickly...due to some parasite(?). There are plenty of ways to design a biological system with some threshold, that if passed causes a failure cascade. That's easy to do. $\endgroup$
    – Green
    Dec 11, 2019 at 4:56
  • 1
    $\begingroup$ You provide no details on why life expectancy is increasing and no details of why it would suddenly stop increasing. And without details of the symbiotic parasite (oxymoron?) relationship to the host, we can't really work out what's going on. Descriptions of these relationships and their evolution overtime would be crazy handy. $\endgroup$
    – Green
    Dec 11, 2019 at 4:58
  • $\begingroup$ The build up of toxic chemicles actually happens with heavy metal posionings. Generally, the body cannot expell heavy metals so they never leave and accumulate until they are poisioning the system. There is actualy some evidence that there is a certain threshhold of age for humans, where if you make it to this age, the only thing that will kill you is your old age (I believe it's above 90 years). Though it's not known if this is because of the rarity of people of this age dying from other reasons or something at play. $\endgroup$
    – hszmv
    Dec 11, 2019 at 12:24

2 Answers 2


I'm thinking ammonia buildup.

In high concentrations, ammonia is toxic to humans and I suspect that it is toxic to humans in some lower doses if it is present in a chronic or continuous state. The thing is, ammonia is a natural by-product of the organic processes in the body.

Now, for fish and other vertebrate sea dwellers, this is not so much of a problem because as soon as the ammonia is created, it can be excreted. Fish (more or less) constantly urinate, constantly vacating the ammonia directly into the water around them. Land dwelling animals don't constantly urinate, so instead of just excreting the ammonia they actually convert it to urea.

Urea is another waste product but with far less toxic side effects than ammonia. So, the body takes the ammonia, converts it to urea, and stores it in the bladder until it is ready to be excreted.

Trouble is, there are organisms that can convert it back to ammonia because in that form, it is useful as a fertiliser in the soil. It is not all that hard to imagine a creature that as a symbiont may excrete ammonia directly - Any internal symbiont would have to 'plug in' to the waste processing systems of the host somehow to ensure that the symbiont waste isn't just excreted directly into the host, likely causing toxic shock. But, if you're going to do that, it's simpler to just take advantage of the host's ammonia to urea conversion processes rather than double up the process. The idea would be that the human body processes all the ammonia from both creatures and manages the eventual excretion via the bladder. Sure, it would mean more work, but the human body would likely adapt and be up to the challenge of increased ammonia levels.

If however the extended lifespan made the urea production less efficient, or the symbiont somehow stared producing more ammonia over time for some reason, it's possible that the human body would not be able to keep up with the volume of ammonia inside it and that would lead to an eventual ammonia buildup and therefore a toxic effect.

Assuming that the symbiont isn't immortal, this could even be as simple as a necrotic phase at the end of the symbiont's life, releasing a spike in ammonia levels into the host as a consequence of its own impending death.

Bottom line is that if the human host body can't keep up with the ammonia, or the symbiont just starts releasing a lot more of it for some reason, you could easily end up with a lot of humans starting to get toxic shock and dying from the ammonia buildup.

  • $\begingroup$ Thank you Tim, some really useful points for me to consider. $\endgroup$ Dec 11, 2019 at 15:09

I don’t think it would work as you expect. One key element is the level of technology. If the species is advanced then they could potentially extend the life span of the species by a range of medical interventions and genetic modifications. If it’s not an advanced species then they would be at the whim of nature. Life expectancy could reasonably extend if death through accident and predation were to decrease for some reason.

A genetic mutation that gives the body it occupies a big advantage in youth, but at the same time triggers some deleterious effects in later life might well spread and could bring the average life expectancy down. I believe such genes exist and would be selected for by evolution.

“the buildup of waste chemicals from these symbiotic parasites” Note a symbiont is not a parasite.

  • $\begingroup$ Technology is an issue, cultural evolution and socio-religious beliefs have hindered advancements in medicine. This is the key event that forces them to face facts and go back on 2 decisions previously made not to pursue certain fields or research. I’m looking for something that has been building up over thousands of years and is suddenly effecting everyone on such a scale that desperate action has to be taken. Am I over complicating things? Does a life threatening epidemic need to be so convoluted? $\endgroup$ Dec 11, 2019 at 4:26

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .