Extremely long lives and background radiation

In my setting, there's a species that can live for about five thousand years. They are just as vulnerable to physical damage as humans. I want this setting to be very realistic (supernatural elements notwithstanding), so eventually, I thought, how would background radiation start affecting them long-term? And how long would it take for symptoms to start showing? As far as I know, radiation poisoning is cumulative, so doses from background radiation can't just "go away", but I will freely admit that I know little about radiation poisoning.

Using the xkcd radiation chart (which may or may not be a bad idea), I found that the average background radiation dose is about 4 millisievert (mSv) per year, and since usually fatal radiation poisoning is 4 Sv, I came up with something called the "Millennium Plague"; as a fatal dose is received every thousand years from background radiation, the individuals of the species start dying when they reach about a thousand years old (give or take fifty years). This is healed with magic, assuming it flushes all traces of radiation from the body, but it starts up again in another thousand years.

However, this still feels somewhat incorrect to me. Please give me your thoughts and/or predictions, and correct my science or numbers if I'm wrong in any way.

Seems like something that would live that long would have a very different biology to the average human, in particular how their cells regenerate. Hence, while a gunshot might kill them as easily as a human, as perhaps might poison, but radiation might not be quite as much a problem for them.

I'll hide your name inside a word and paint your eyes with false perception.

Long-lived species likely have evolved effective ways to repair some DNA damage. This would probably include background radiation.

Keep in mind that human cells can repair themselves following radiation exposure - the effects aren't permanent. For example, one common form of radiation damage is UV light, which causes a thymine bond to form in DNA (basically, instead of sticking to the adenine that it is supposed to, when exposed to UV light thymine will instead stick to any nearby thymine). Skin cells have an enzyme that repairs this damage by forcing the thymine to stick back to its adenine base-pair.

In a very long-lived species, its likely that they have an enzyme present that repairs DNA, or perhaps they use a different chromosome structure that helps prevent the degradation over time of their genetic code. For instance, bacteria are effectively immortal because they have a circular chromosome structure, but eukaryotes are not because their chromosomes are linear. Now, a circular chromosome structure probably wouldn't work for a multi-cellular organism, but perhaps some of their genetic information could exist as plasmids - free-floating circular bits of DNA that aren't a part of the chromosome proper. These already do exist in our cells in the form of mitochondrial DNA, which is effectively immortal in humans, so simply take the central chromosome but divide its duties up amongst plasmids.

the downside? There's a higher risk of errors during the copying process (higher mutation rate), and any organism without a central nucleus would be very slow growing as it requires a lot of time in between divisions for all the plasmids to replicate.

(My first paragraph was going to completely repeat what MGIFOS said.)

They won't have to worry about any millennium curse, because long-term and short-term radiation exposure have different effects. Deadly radiation exposure works by killing so many cells of a certain type that the body is unable to recover; with long-term exposure, the cells have time to replicate and replace the dead ones, so there isn't much of an effect at all. Consider:

Say a dose of size N kills 10% of your bone marrow, and that your body can replace 10% of its bone marrow every year (these are made up numbers, obviously). If you are exposed to a dose of 10N all at once, you'll lose 100%ish

of your bone marrow and you'll die. But if you get a dose of N/year, over the course of the first year you'll lose 10%, they'll grow back, you'll lose another 10%, they'll grow back, and so on. At the end of 10 years, you will have lost an amount equal to 100% of your bone marrow—but you'll be fine because it's all been replaced.

The real dangers of radiation would be mutations causing cancer and similar problems. Like all risk factors, the effect would be cumulative: If you have a 0.01% chance of getting cancer every year, then by the time 5000 years have passed, the odds that you would have gotten cancer one of those times gets pretty high.

So, if I'm hearing you correctly, the "cumulative" part is solely in risks for mutations, cancer, and so on, right?

Incidentally, to prevent nerve degradation over time, their nerves can re-grow, which I later realized meant they could get nerve cancer (assuming I'm not screwing up there, too).

If you can live for 5000 years, you've got some mechanism to repair DNA damage. I wouldn't be surprised if they were essentially immune to cancer; usually a cancer forms in several stages, and there's identifiable genetic damage in each stage. If you can repair genetic damage to the extent that you'd need to in order to live that long, you'd probably be immune. Cumulative radiation exposure would probably be caught by the same mechanism.

Shinigan (Naruto fanfic)

I would say they would have to be incredibly durable, fantastic healers and be susceptible to very few diseases, and have no natural predators. The problem with, say, humans living past 150, even if we could counteract the issues with DNA degeneration and whatnot, is that the longer you're alive the more likely you are going to die from a car crash, or a fatal disease like cancer, or a random drive by shooting, or a house break in that goes bad, or just get depression and kill yourself, or get involved in a plane crash. Basically after a certain point your chance of dying from something random like that is about 100%. It gets to the point that a polar bear randomly breaking into your house to eat you isn't all that unlikely.

So to really live that long they'd have to be able to survive all kinds of random accidents, not catch any diseases or be eaten by something.

edited 9th Dec '11 6:48:31 PM by NoirGrimoir

SPATULA, Supporters of Page Altering To Urgently Lead to Amelioration (supports not going through TRS for tweaks and minor improvements.)

True, but once you've lived to X years old it's given that you've survived to X years. (Duh.) What this means, statistically, is that you always have a 50% chance of dying of $RANDOM_THING within some number of years from the moment of calculation, but that doesn't change over time. If, on average, without aging you would die of an accident within 150 years, that number will still hold once you've actually lived to 150—meaning that at that point you'd have a life expectancy of 300 or so.

Shinigan (Naruto fanfic)

They can use magic. Would that count for stuff like that?

Incidentally, one of the minor themes of the series is that, because they can use magic to solve their problems, their technological innovation has stagnated.

edited 9th Dec '11 6:52:05 PM by TeraChimera

Huh... I've done long-lived races all the time, but I've never considered things like background radiation.

Touche, my friend. You have outdone my own levels of OCD in worldbuilding.

~slow clap~

I am now known as Flyboy.

I'm weird like that.

@alethiophile Yes, I'm just saying that the vast majority of things from this species won't live to be 5000. They probably wouldn't even live to be 2,500, or hell, 1,000. Their bodies might be capable of it, but nature probably wouldn't allow it. Even with magic, I think anything over about 400 is a crapshoot. Generally I don't care about this in stories, but if you're getting so detailed as to consider background radiation build up, then it's something to be considered.

Edit: Also, I don't know if humans and modern technology are in this world as well, but things like Xrays and CTS cans are huge factors even for humans. People who work around these machines can tick off about ten years off their life, or something, same if you get too many. This is why Doctors don't like to give you scans unless you really need one because there is a very real possibility of getting cancer from having too many. And too many is something like 20. (dunno the actual number but it's around here I'm pretty sure).

edited 9th Dec '11 7:16:10 PM by NoirGrimoir

SPATULA, Supporters of Page Altering To Urgently Lead to Amelioration (supports not going through TRS for tweaks and minor improvements.)

Okay. I'll think it over.

My understanding of the current theory of aging involves telomeres. They're a sort of genetic accessory, a cap at the end of each allele. Over time, as they are copied, the telomeres lose cohesion or wear out. The strands of DNA stop copying properly, so the cells start to break down, the organs break down, and eventually the whole system fails, i.e. death by natural causes.

Becoming immortal would mean preserving the telomeres. Some mechanism would repair any deviation from the original layout of genes, including the telomeres, so there'd be no possibility of aging or mutation or cancer. It might be an expansion of the immune system or the reproductive system.

Some implications of immortality—the body doesn't change. At all. Scars would heal over. It's possible that the brain wouldn't change either, that is, you could learn new things, but you would have difficulty breaking old patterns of thinking. Without being able to adapt to new circumstances, you'd become obsolete. The world would leave you behind, unless you and others like you tried to keep the world from changing (which would cause other problems).

If your genes don't allow any deviation, you can't reproduce. When the egg is fertilized, the immortal genes try to "correct" their nonimmortal counterpart. In effect an egg would treat sperm as a mutagen and heal itself, while sperm would erase an egg.

An accident is usually a series of mistakes or unlikely events, rather than just one. If you are alert enough, and know what to look for, you can avoid more accidents as you learn what causes them. An immortal, I believe, would develop skills to predict these patterns and avoid their effects. Over time they'd become like instincts. You could still get caught up and killed, but not as often as mortals.

Under World. It rocks!

Cells with preserved telomeres are cancerous. Cell aging and death is a necessary mechanism for overall health.

It's super hard messing with this sort of stuff—there's just a good reason everything works the way it does.

I don't believe it's a necessary truth that cells without their built-in replication limits are cancerous; it's just that cells without those limits have no defense against cancer forming, and the law of averages means that's more likely the more unlimited replication you let happen. If you've already got a total-genetic-protection ability, you can probably generalize that to protect against cancer.

And there's no reason why total preservation of genetic structure equates to total preservation of macroscopic structure. Immortal beings would not necessarily regenerate anything, whether scars or limbs, and they shouldn't have any more problems with mental flexibility than anyone else.

Shinigan (Naruto fanfic)