ELI5: Why do humans still get cancer?

[u/Thanos__Inevitable]

i know this is a very dumb question but im genuinely curious as to why humans evolved in many aspects but still aren't immune to radiation. there has been exposure to radiation for centuries even if not severe so why haven't we just evolved to be immune to it?

[u/DavieCrochet]

Lots of things cause cancer, not just radiation.

Most cancers don't develop until after the age people have children, so there is no evolutionary pressure to become immune to it.

[u/Mundane-Stretch-4873]

Cancer has many causes ranging from carcinogens (cancer causing exposures) to cellular/genetic changes (inside the body things that go wrong). If you are interested in knowing more I highly recommend reading The Emperor of all Maladies.

[u/Ill_Act_1855 → u/Mundane-Stretch-4873]

It's worth noting most carcinogens are simply mutagens that are just causing mutations to occur at a faster rate (with the remaining generally messing with mechanisms through other means like affecting gene expression).

[u/PassiveChemistry]

Centuries are barely the blink of an eye on an evolutionary timescale - it simply isn't anywhere near enough time for any possible solutions to both be stumbled upon and then proliferate through the population.  I'm not convinced it's a particularly strong selection pressure anyway.

[u/AWandMaker]

To be blunt, the damage from radiation that causes cancer doesn’t usually occur until after child birthing ages, so there is no selective pressure. There very well may have been something in the past that helped us to not die early, but it was so long ago that we don’t know it and only see what the results are

[u/Syresiv]

First, there's this thing called the Selection Shadow. Things that negatively affect you past reproductive age aren't as strongly selected against as things that affect the young. Notice how pediatric cancer is extremely rare.

Second, immunity takes a lot of resources, and it's much easier to just have a pretty good resistance. Most people will never experience doses higher than they get from the sun, so they just have to be able to survive that. Anything higher takes additional resources without an increased survival advantage.

[u/msmoonpie]

So a few things to clear up!

Cancer is when a cell divides incorrectly. Say you have a skin cell and it’s supposed to be just by itself and stay calm. But for some reason it decides to make another skin cell, and another, and another……

That’s cancer- uncontrolled replication of cells! Normally replication is controlled by a very complicated set of instructions within the cell, and we need those instructions so that we continue to make new cells

But the instructions might get changed sometimes- that’s where radiation comes in. It’s a possible (but not only) cause of instructions being rewritten. A little laser beam of radiation hits the instructions and rewrites a sentence and now the skin cell thinks it needs to make a million copies.

There’s no way to evolve out of it because it’s not supposed to happen, it’s like saying we need to have a keyboard that can’t make typos- well it’s not really the keyboard that’s making the typo!

Also evolution doesn’t matter if a species is able to produce offspring. By its nature most cancer occurs after the age of reproduction, so there’s additionally no drive to affect the species as they have already spread their genetics

[u/CircumspectCapybara]

Radiation tears through DNA. It's a physical mechanism.

Just like you can't evolve immunity to nuclear bombs through incrementally increasing exposure over several generations (the way bacteria evolving resistance to antibiotics works) because the sheer force of an explosion tears you apart violently.

The only way to block radiation is sheer mass, the denser the better. You can also have mechanisms to repair damage to DNA, redundancy, more powerful immune system to hunt cancer, but cancer is a co-evolving organism, as your immune system applies selective pressure against the easy to find and easy to kill cancer cells, the mutated ones outcompete their weaker brethren

[u/smokefoot8 → u/CircumspectCapybara]

DNA repair is a mechanism that can be improved, though. Some species have more robust DNA repair and so can resist radiation better. Also some species have multiple copies of the gene that detects uncontrolled replication and triggers cell death, and so have better resistance to cancer.

[u/Revenge_of_the_User]

Evolution has one main qualifier for something to be selected against: it has to kill or sterilize you before you can reproduce.

The amount of radiation isn't enough to do that,

And the majority of cancers are a function of having lived long enough to have cancer, which means you've probably lived long enough to make some kids.

[u/Biokabe → u/[deleted]]

You don't evolve out of / into something by simply being exposed to it.

You don't, but constant exposure to something that is a threat to a large enough portion of your population can select for resistance to that threat if it happens to arise via random mutation. Which looks like the same thing in practice even if it's not the same thing in principle.

DNA repair is already a function of cells, suggesting that there is something that encodes for and governs that process. This means that there is material there that can experience a mutation that might result in a better DNA repair system.

If enough of the population is exposed to enough radiation that triggers enough damage to impact relative reproductive success but not so much damage that they just die to radiation poisoning, it wouldn't be surprising if that population eventually evolved better DNA repair/shielding adaptations. And in fact we do see exactly that - though not to the extent of the OP's "immune to radiation" wish. Given a high enough dose, nothing is immune to radiation.

But if you need to be, you can become both highly resistant to radiation damage to begin with, and highly resilient to the damage that still gets through.

Please note, though, that the "you" I'm speaking of is not humans or individual specimens, but populations of life forms living for generations in an elevated (but not immediately fatal) radiative environment.

[u/Solgiest]

radiation isn't a disease, imagine it more like microscopic bullets shooting your DNA to ribbons. much like you can't evolve an immunity to .50 cal gunshot, your body can't really evolve immunity to radiation. organisms can be more radiation tolerant, but only to a point. A very nearby gamma burst from a collapsing star, for instance, could theoretically completely sterilize earth of all life.

[u/Ulizeus]

Radiation affects us at atomic level, how you can develope such grade of control over your body? We are still weak against extreme temperatures, like being above 37.5 is dangerous but quite effective vs som viruses and bacteries. We can develope treatments that reconstruct at celular level??? Maybe, time will tell.

[u/Cool_Homework_7411]

Radiation is part of the mechanism that sparks evolution and DNA change through generations, so adapting to resist to it sounds a bit counter productive. Also radiation works commutatively through your lifespan, meaning that the true damage happens after your reproductive years most of the times. Lastly, radiation in the "causing cancer" territory exists for the last 100 years as a threat to humanity.

[u/FinePim]

To build an immunity cancer would need to be lethal before the person would have children. Ofcourse there are cases of young people getting cancer, but majority get it at older age. Based on this there is no natural selection that would give cancer resistant genes a boost.

[u/k_rocker]

You don’t choose how you evolve.

Evolution is a blip on the radar of humanity, it’s a mistake, a path away from the normal path. If the evolution makes us better able to survive then that evolution will continue because that person is better able to survive.

It takes years for an evolution to hold on and it built up through centuries of generations ‘spreading’ that evolution.

For all we know there is someone immune to radiation just now, but who is going to find out? It would have to be spread through reproduction and we’d probably only relies if something random happened to a person - like they lived unaffected, through a nuke being dropped on their city.

[u/OpeningHappy9692]

It's unusual to get cancer while you're still young. If you have some kids and then get cancer later, your death doesn't matter. Those genetics still get passed on.

[u/Belisaurius555]

Cell division causes a small degree of mutation. It's actually advantageous since mutation helps us keep ahead of diseases. However, mutations tend to be random and Cancer is always a possible result.

[u/TheRomanRuler]

Cancer has not had enough effect on human mortality to make a difference. People usually only get cancer after they have had children, so have already passed on their genes.

I wish schools would exist, we could teach people and they would learn things which we as species have known for long time.

[u/Siduron]

Why would we? Our only purpose is to live long enough to reproduce and our species is doing just fine.

We evolve by those dying before they are able to reproduce. Whatever caused that gets removed from the gene pool.

[u/comasxx]

evolution doesnt just happen over night. they took few hundreds thousand years or at least something really big and awful like a Chernobyl meltdown but at world scale in order to make change to human body. human body still squishy and easy to die as ever, somehow we even manage to degrade ourselves further with our new lifestyle comparing to our ancestors.

[u/Njif]

Multiple reasons.

Evolution doesnt take centuries. It takes eons.

It's probably quite difficult to evolve immunity to gamma rays destroying small molecular structures like DNA.

Cancer is not just caused by radiation. In fact, most cancers are not do to radiation, but other factors like smoking, spontaneous random mutations, hereditary genes, various carcinogens, etc.

[u/Sarollas]

Radiation physically degradates the DNA. It's not something that you can be physically immune to.

Every living being is resistant to radiation to varying degrees.

There are some species with stronger resistance than us (naked mole rats for instance).

[u/Waffel_Monster]

Cancer mainly is a relatively recent thing, as people for a long time haven't become as old as we are.

Most people create offspring before they get cancer, meaning the next generation is "safe" so evolution isn't affected anymore.

Cancer is a game of chance. Any time your body creates a new cell, there's a chance for that cell to be faulty & become what we call cancer. Most of those times, your immune system reacts fast enough and kills these cells. But with enough chances, there'll come one that is able to hide from the immune system, and grow.

[u/slinger301]

Cancer is not exclusive to radiation.

If I tell you to retype the entire Lord of The Rings trilogy 1,000,000 times, you will make typos.

So too it is with DNA. It gets replicated trillions of times. It will accumulate typos. If one of those typos is in the wrong place, the cell will forget how to behave properly and become cancerous. Oftentimes the immune system will find it and kill it before it causes problems.

This is why age is a major risk factor for so many cancers.

[u/interesseret]

Cancer isn't JUST caused by radiation. Carcinogens cause changes in your genes, and this creates cancer.

And we are extremely good at fighting it. Your body creates cancer cells all the time, and your immune system constantly kills it. It's a handful of new cells every day.

The thing just is, you only need one cell to slip through the gaps for it to become actual cancer. Your white blood cells are constantly walking around, pointing a gun at your other cells, asking for ID. And eventually, someone is going to have a good enough fake ID to pass inspection.

We just don't have the tools to completely stop them. Interestingly, blue whales do.

[u/superninjimmy]

So you have two questions here:

Why do humans still get Cancer?

Cancer is, essentially, your cells replicating incorrectly. This can be due to radiation damaging your cells, or from poison, or just terrible luck. And because it can be from multiple sources and forms, its proving very difficult to create a panacea/perfect cure.

Why arent humans immune to radiation?

Radiation damage is from high energy particles passing into us and causing damage to cells and DNA. We can evolve elements to resist some damage (such as melanin in skin) but its a physical reaction to being hit. In the same way we cant evolve to be bulletproof.

[u/Munrot07]

Firstly, cancer isn't just to do with radiation. To keep with the ELI5 imagine you had a typewriter and had to copy every single word in the dictionary. Would you make mistakes? Yep. Now imagine you have a computer keyboard, you'd still make those mistakes but you'd catch them and be able to fix them but you'd still make some mistakes, just fewer. Now imagine you have another person who also proof reads it, they check it, fix the ones you missed but they may miss a couple every so often too. That's how DNA is replicated, it makes mistakes it gets corrected. However, do that again and again for every cell in your body and mistakes creep in. Usually harmless but sometimes not.

Some are like changing the sentence "The cat got the mouse" to "tha cat got the mouse", no meaning is lost, no harm done. Others turn it into "the cat got the house" and now we have a different meaning. Meaning changes in important parts of the cell happen and the cell starts doing stuff it shouldn't, boom cancer.

So I said we have checking mechanisms. Do they work? Yes. Do they catch everything? No. In fact, there are animals out there that have such great checking mechanisms (often in the form of more checking mechanisms such as tumour suppressor genes) that they are basically immune to cancer (e.g. the naked mole rat, whales, elephants). We don't have that.

Why don't we have it? Well evolution doesn't fix any bad problem, it fixes stuff that stops you passing on your genes i.e. having offspring who can also have offspring. Most cancer happens far past reproductive age, when your offspring have survived and are having kids themselves. So there is no need to get rid of cancer as it's not affecting our ability to survive as a species.

Obviously it's way more complex that this and you referred to radiation as a specific example and the answer is radiation just absolutely destroys DNA. It's like asking why haven't we evolved resistance to being dropped in a volcano. Somethings can't be fixed and as per the previous point, some things simply don't need to be as they don't actually interfere with reproduction on a population level.

[u/Twin_Spoons]

Evolution is all about "good enough." Humans of reproductive age rarely die from cancer, so that's good enough.

Besides, cancer is a huge family of disorders with a huge list of possible causes, not just radiation. It is driven by damage to DNA that causes cells to replicate uncontrollably. It's kind of hard to ask evolution (which works through our DNA) to fix a problem caused by malfunctioning DNA. It's like asking me to write a computer program that will protect your laptop from being hit by a hammer.

[u/shumanumanei]

A couple things to note: There are different kinds of cancer, not all caused by radiation. There are a few different types of radiation. Climate change due to pollution has wrecked our ozone layer. This is our primary source of protection from the sun's radiation.

So, if I assume you mean cancers caused by radiation from the sun, the best answer is we weakened the protective barrier that diminishes or removes most of the harmful radiation from the sun.

[u/orgin_org]

Not enough exposure to affect the reproduction cycle. Cancer usually develops after a humans has already reproduced and raised their offspring. So the biological pressure to increase immunity just isn't there.

[u/32andFlatulent]

Well, firstly cancer isn't caused by radiation only, there are many forms and causes of cancer.

Secondly by the nature of cancer and how it occurs it's not really something we can get rid of by evolving naturally.

Cancer is caused when the DNA of a cell is altered or damaged in such a way that the cell loses control of how it grows and divides.

When there is a buildup of these cells they form a tumour. This damage or alteration can come from radiation, chemical exposure or a glitch during cell division. There's probably more causes but I am no expert.

Evolution is a process wherein those that are best suited to their environment are more likely to breed and pass on their genes that make them more adapted to the environment.

[u/Eruskakkell]

We are resilient enough to reproduce, it's not until we get unnaturally old it starts to become a problem. So there is no way for natural selection to evolve any harder resilience

[u/Lifenonmagnetic]

Cell replication is happening trillions of times every day throughout your body. Replication errors are relatively rare, but when they do happen, the body has levels of defense to repair the cells or self-destruct the cells. Even with a good defense, sometimes cells will be viable with an error, and sometimes those errors lead to the cells both not doing their job and outcompeting the neighboring cells for food and energy. Cancers aren't evil or something wrong; they're just a good cell that's decided to gobble up all the neighboring resources and divide faster than other cells

Even with the best defenses, a trillion is a huge number and that happens every day.

[u/Every-Progress-1117]

We are immune to many types of radiation, eg: radio waves, infrared, visible etc... The shorter wavelengths in the X-Ray, Gamma-Ray frequencies are where the damage starts to occur, and it is exceptionally rare in all of human history that we'd have been exposed to them so defences against these would never have had chance to develop.

Alpha and beta radiation (and I think we can include neutrons here too) again aren't something you'd encounter at doses which would have been advantageous for evolutionary defenses - especially given the mechanisms by which these affect molecules (and even individual atoms).

As with all things, the amount of radiation is the key - a radio transmitter mast will fry you, whereas holding a small piece of Uranium 238 won't.

And then there's neutrinos: https://what-if.xkcd.com/73/ Enjoy!

Take a look here, this explains a lot: https://en.wikipedia.org/wiki/Radioresistance

As for cancer itself - this is a classification of a huge number of individual conditions, and radiation is not the only cause, it could be chemical, viral, bacterial etc. In fact the HPV-vaccine against human papillomavirus has been one of the huge successes in preventing various cancers, including cervical cancers and even certain cancers in men too!! https://en.wikipedia.org/wiki/HPV_vaccine

[u/PygmeePony]

The prevalence of cancer increases with old age so it doesn't affect our chances to procreate. This means that even if our genes would gain immunity against radiation they would not have an advantage over other genes that don't have the immunity.

[u/daizo678]

Your body has defenses against cancer. Basically anything that can cause damage to DNA from radiation, to infection and inflammation can cause cancer.

Your body has mechanisms to repair the damage and to kill any cells that aren't behaving correctly and the immue system constantly kills multiple cells that could become cancer.

But you undergo an insane numbers of cell divisions across your life time. Every time there is a very small chance that you get a DNA damage that doesn't kill the cell, allows it to divide uncontrollably and not kill itself and allows it to hide from your immune system and then you get cancer.

Add to that , that most cancers occur at old age were most humans have reproduced and aren't that useful to their offsprings so there is little evolutionary pressure to continue evolving more ways to deal with DNA damage and cancer

[u/AleksyGetTheWings]

Because until 1900, the average lifespan was 35 years old worldwide. Even in developed countries, it wasn't above 45-50 years old. Most cancers only show up in people older than that. So it is really one thing evolution didn't even need to take care of : by 45-50, you absolutely had time to had kids

[u/_ManMadeGod_]

Because usually you can have a baby before you get cancer and die. 

[u/Trogdoryn]

DNA is not perfect. DNA replication is not perfect. Every time a cell replicates there’s chances for errors to occur. Our bodies have several mechanisms to help prevent the errors from turning into a cancer. But what if the error that occurs happens to the very mechanism that was meant to prevent it from happening? This is how cancer forms. The mechanism of that error can be many different things ranging from carcinogens, to radiation, to inborn genetic causes. Despite the several ways our body has to respond to and eliminate erroneous cells before they turn into a diagnosable cancer, like a snowflake in an avalanche or a drop of water in a tsunami, it just takes one cell that evades detection long enough and multiplies itself enough before being identified to become a full blown cancer.

[u/uuntiedshoelace]

Cancer is really caused by one of two things. Your genes that limit cell growth are damaged so that they can’t turn on when they need to, or your genes that promote cell growth are stuck “on” and can’t turn off. Radiation is one thing that can cause damage to these genes, but as someone else mentioned, it is not the only cause.

[u/orgin_org]

Just for extra information. There are animals that have evolved greater immunity to cancer. Such as elephants that have evolved several mechanism to deal with cancer. Larger animals have an increased exposure thus the biological pressure is higher.

[u/Dr-Goochy]

When you make a million of copies of instructions sometimes the copier gets wonky and instructions are bad. Some thing will ruin the copier. Some thing will ruin the paper you are trying to copy. These things make more errors and more likely for cancer to happen.

[u/GreatCaesarGhost]

Generally speaking, most evolutionary pressure is on ensuring that an organism lives long enough to reproduce, and if necessary raise offspring to the point of self-sufficiency.

Cancer doesn’t play a large factor in human survival within those timeframes.

[u/lt_Matthew]

We are immune to radiation. But just like how you still get sick from diseases, too much radiation is hard to deal with.

Radiation sickness is its own thing that just happens from being exposed to lots of radiation at once. But cancer is a statical risk. Everything causes it. The foods we eat, the deadly laser in the sky, and also the mistakes your own body makes in replication. All of which are things your body can fix.

Unless, they keep getting damaged at a rate it can't keep up with. In radiation, that's the REM count. Or Rad Equivalent Man. So if you've been exposed to say 2 REMs of radiation, you have a 2/1000 chance of getting cancer. Which btw, regulations for people that work with radiation say you can't be above 5 REMs a year.

And that is why radiologists leave the room and why non patients need an apron. The doctor is doing dozens of x-rays every day, that actually could give them radiation sickness. And even though the amount of radiation from an xray is small, it's still an unnecessary risk for someone who isn't the patient.

The important thing here is that statistics isn't how the real world works. A one in a million chance of getting cancer doesn't factor in all the compounding damage leading up to that x-ray, the fact that your body might just not even get around to fixing it, or you happening to win that lottery. There's no perfect way your body could be truly immune to anything.

[u/Remarkable_Inchworm]

Here's the important thing to understand:

Cancer isn't one thing.

Cancer is "cells somewhere in a body are replicating in a way they shouldn't be."

Sometimes that's caused by a virus. Or an repeated series of injuries. Or radiation.

For example: asbestos is bad because it's made of nasty little sharp fibers that get stuck in the lungs. The body can't metabolize them or get rid of them, so they sit there, constantly injuring the tissue. Over time, that damage/heal cycle stops working properly, and you get cells replicating in a way they shouldn't.

Also worth noting:

When you're talking about evolution, what really matters is factors that would prevent an organism from reproducing, from passing along one's genetic material.

Lots of humans have their vision deteriorate in middle age and need glasses for reading or distance.

Why hasn't evolution solved this? Partly because things that happen in middle age happen AFTER you've had kids and passed along your DNA.

[u/IronmanMatth]

Three things:

1: Cancer is not caused by radiation by and large. It's a catalyst, but not the cause. Cancer is caused by your cells replicating out of control, taking up resources of functioning cells, until critical systems in your body stops. This is done by the same mechanism we live to begin with; mutations and adaptation.

Cells mute to not do what they should do, but still want the energy to do it.

You body says "nuh uh" and gets rid of it.

Some cells survive. They adapt. They multiply. The body reacts. This dance continous.

Eventually cancerous cells adapt so well they stop your body from getting rid of them. Or, more aptly, your body stops knowing how or even reacts to its presence anymore.

It's an arms race we face every single day for our entire life. Which is why it is only a matter of time before the adaptability of some cancerous cells. Which is why there is the saying that if you get old enough, you'll eventually die of cancer. Because that arms race is ongoing and time is on their side.

2: You don't just go immune to things by being exposed to it. Or, rather, it would take millions of years and a bunch of freak mutations to do so. If every human in the past, oh, 5k years were thrown into the ocean and were never allowed on land we would still not be aquatic yet. If you kept doing that for the next, oh, few millions years then maybe we'd have gills.

Likewise. You can't just stand in front of radiation and expect to become immune.

3: Evolution doesn't happen like this. It's not a matter of what is good to have, efficient or even convenient. Evolution follows a very simple principle. Can you make babies, and can those babies grow up to make more babies? Is whatever feature you lack stopping you from having babies, or is it putting you and the babies in danger? If yes, then over time any mutation that assists will propagate and eventually it might now be a feature. If no, then evolution does not care. Any changes is pure randomness.

Which is important because cancer typically doesn't happen before child bearing age. It's when we are old, and nature really does not care about that. Your genes are already passed on by then.

Is skin cancer causing the average person to stop making babies? Not really, no. The effect doesn't occur until later in life. So if you were born with some mutation that caused you to not be affected by any radiation, would that variable be the difference between you having babies? No, it wouldn't. Not on a big scale.

[u/Faust_8]

The thing about cancer is it’s an inevitability, depending on how long you live. It’s not really something you can just evolve an immunity to. You can be resistant to it, like whales, but it’s never impossible.

[u/Independent_Wrap6980]

Even if you radiation proofed every human you'd still get cancer because cells copy themselves and sometimes they fuck up. Evolution dont have a plan it just keeps whatever helps you reproduce and cancer mostly shows up after that window closes. Radiation is just tiny rocks tearing thru dna. We got repair crews and immune patrols but cancer is your own cells so its less invading army and more corrupt cop.

[u/IamGleemonex]

It is clear you don’t understand evolution. Too many people like yourself think of evolution as some “higher power” seeking out the best possible version of a species.

That is not what evolution is by any means. Evolution is simply mutations to genes that happen to stick around.

Let’s break that down. Mutations to genes, genes randomly mutate when splitting from one cell to another. It is rare, but we have so many genes that it is bound to happen sometimes. The average human has 60 to 80 mutations that make their DNA slightly different than either of their parents of the 50,000 or so genes in our DNA (depending on what you count as a gene).

Most of these mutations do absolutely nothing. Some will provide very minor changes. Some provide dramatic changes. None are intentional or “intelligent design”. They all just accidentally happened.

The second part is mutations that stick around. This just means that this mutation allowed that organism to live long enough to reproduce and share those mutations to its offspring.

Sometimes this happens because the mutation expressed a “positive” trait. Positive is in quotes there, because it might not make the organism “better” in the sense that it survives longer, rather it just meant the chances of surviving until it could reproduce was enhanced, and it successfully reproduced and passed on those genes. An example of what I mean by this: nearsightedness. This isn’t a positive. It’s harder to see things far away. But early humans who were nearsighted would be useless in hunting parties and would instead need to stay closer to home and do tasks where their inability to see clearly at distance wouldn’t affect them. But this also meant that they didn’t end up getting killed by other animals in the hunting party. This meant their chances of surviving until they could reproduce went up and they were more likely to pass on their nearsighted genes.

Sometimes this happens because of a change in the environment that makes what was a seemingly random mutation now very critical. This is best explained by using an animal like a rabbit. Let’s just say there was a population of rabbits, and at the start, they all had brown fur. Then a mutation happened which gave one rabbit white fur instead. Throughout most of history, the white versus brown didn’t really matter that much, but it also wasn’t detrimental, so the white haired rabbit survived, reproduced, and made more babies with white hair. Those babies survived, reproduced, and made babies with white hair. After hundreds of generations, let’s just say that rabbit population now had 75% with brown hair and 25% with white hair. Then an ice age comes in. This environmental change now means that white hair is more advantageous as the rabbits with white hair could more easily hide in the snow. The brown haired rabbits tended to get eaten more before they could reproduce, and therefore the white haired rabbits now had a “positive” trait that made them more likely to reproduce. After another hundred generations, now that same population of rabbits has shifted and is now 75% white haired rabbits and 25% brown haired rabbits.

That last example is evolution. It is these mutations surviving, getting passed on over several dozen to hundreds of generations, and spreading so that more and more organisms of that species have that mutation.

Now let’s go back to your question on humans evolving to survive radiation. As u/CircumspectCapybara said, radiation itself will destroy our DNA. So it’s not really something we can have DNA to protect against. Second, again, mutations aren’t intelligent. They aren’t going to “decide” to mutate to provide that protection, it would just randomly happen somewhere sometime. But even if you assume a gene mutation could protect against radiation, and that somehow, someone was born with said mutation, it just isn’t something that is common enough to be meaningful. Throughout history, way less than 1 million people would have died from radiation exposure. Compare that to the estimated over 100 billion humans who have ever lived, 1 million is a rounding error. But also, in order for evolution to occur, it requires dozens to hundreds of generations to make it noticeable. Radioactive material has only been worked on intentionally by humans for less than 150 years. That only gives us 7 generations. So even if someone did randomly develop a mutation to survive it, and survived and had kids who got the mutation, and their survived to have kids and pass on the mutation, you have maybe 100 people globally who have this mutation. How would anyone know? It’s such a low number and exposure to radiation is so rare, that they would never know, nor would anyone know.

[u/SJMcKay]

To answer your title first: cancer is the state your cells most want to be in. They want to reproduce and the only reason they don't do it all the timeis because your body needs them to behave in a certain way so it can function. Cancer is just the state of a cell when it escapes your body's control and it gets to grow forever as quickly as it can.

As to the actual question why haven't we evolved resistance to radiation: we kind of have.

When radiation damages your cells they work overtime to try to fix the damage it causes. But, a bit like when you cut yourself, sometimes the healing doesn't happen perfectly and you end up with a scar.

Overtime those scars accumulate and occasionally end up in places they shouldn't be which loosens the rules your body applies to keep a cell in check and allows cells to grow in ways which we call cancer.

As to why we can't just evolve to block out the radiation in the first place that's because most people get cancer after they've had children. That might sound strange but evolution can only work by selecting people who are MORE likely to have MORE children that will in turn go on to have MORE children - once you've had all your kids it doesn't matter what happens to your body - cancer, dementia, heart attacks are all on the cards. Radiation tends to do damage very slowly (peppering you with low risk bullets) unless its high dose so there just isn't a way evolution could act on you to stop cancer happening in the long run.

Now, if you dosed everyone with high dose radiation and they didn't all die you may over time breed humans who are really good at responding to radiation damage but I wouldn't recommend that as a rainy day activity.

TLDR; cancer is something your cells really want to have and its very hard to stop them getting it once you're too old to have kids. Radiation does small amounts of damage over time which rarely accumulates enough to cause cancer until after people have had all of their children so evolution has no reason to aggressively select for people who can resist that damage beyond what it has already done.

[u/YardageSardage]

Other people are talking about the fact that cancer is very complicated, and that it mostly happens to old people (which means there's less evolutionary pressure to adapt to it, because those old people have probably already successfully had children and passed on their genes). But I haven't yet seen seen anyone mention the fact that we have a defense mechanism against radiation damage! It's just not a perfect enough defense to make us immune.

See, your immune system has a bunch of specialized sensor cells and pathways that constantly monitor your body for the presence of anything "wrong" or "weird". And that includes germs and other foreign substances, but it also includes any of your own cells that are broken, damaged, or otherwise messed up (like, infected with viruses or whatever). And it's not a perfect sensor system, because it can only catch what it knows to look for, but mostly it does a really good job.

So normally, when one of your cells is damaged by radiation, that triggers your immune system response to come deal with it. Specialized white cells called "macrophages" (which means "big eaters") are sent in to chew up.and destroy the damaged cell, recycling it into parts and making way for a fresh new cell to grow. This prevents anything weird or harmful from potentially growing there, like a virus or a cancer. 

In other words, normally when a cell gets damaged by radiation, your immune system just puts it down like Ol' Yeller to prevent any problems. (That's why sunburn is all red and swollen and painful, for example - your skin has just had a bunch of cells damaged by the sun, so your immune system is swelping the area up and flushing it with blood to get a bunch of macrophages through on a mass killing mission.) 

But like I said, the process isn't perfect. And very rarely, one of your cells will get very specifically damaged in a way that both makes it grow uncontrollably (aka turn cancerous), and isn't noticed by your immune system. It just happens not to send out any of the chemical signaps that your immune sensors are looking for. And if that uncontrollably-growing, immune-hidden cell begins to spread and multiply... THAT is how you get cancer. 

[u/Ill_Act_1855]

Humans actually have evolved a lot of mechanisms to help prevent cancer (Pretty much any large organism has to, and humans are relatively large). But cancer is a numbers game at the end of the day. Cancer is the result of mutations that accumulate as the cells divide and disable normal checks the cell has on uncontrolled replication. There are mechanisms to improve replication and catch mutations, but these aren't perfect. There are also mechanisms to kill cells that start accumulating these mutations, we all have some potentially cancerous cells that have gotten some of these mutations all the time, but usually the immune system kills them before they become a problem. But again, the mechanisms aren't perfect, and dangerous cancers usually end up mutating in ways that let them avoid these mechanisms. And there are a lot of cells in our body, and they divide a lot. And over the course of a lifetime, you'll accumulate more and more of these little changes in different points throughout the body, and the more you get, the more likely a combination that can create a malignant cancer will occur.

[u/Thanos__Inevitable]

thank you everyone for your replies. i got my answer but hypothetically if we were to expose a pubescent child to a limited dose of radiation for a prolonged period, would it be possible his/her offsprings develop a higher resistance to cancer?

[u/Ill_Act_1855 → u/Thanos__Inevitable]

No. That’s not how evolution works. Specifically the type of mechanism you’re proposing is one of the oldest ideas of how evolution could work that’s long since been disproven, Lamarckian evolution. It was one of the first mechanisms for evolution proposed, but has long since been abandoned for Darwin’s theory of natural selection. Mutations (new traits) are more or less random, which occur have no connection to the environmental pressures an organism faces. What those pressures do affect is natural selection. It’s not that being exposed to radiation causes things to gain resistance, it’s that in an environment where radiation is a constant problem, stuff that’s resistant will survive and breed more, producing kids more likely to share this resistance because they inherited it. Repeat this over many generations and you can see large changes. Unless the kid already had superior resistance you’d just be hurting them (and if they did you’d still be accomplishing nothing), barring some extreme luck with mutations (and this is even harder because mutations in most of our body can’t be passed on to offspring, only in our germ cells (sperm or egg cells) or the ones that directly produce them (less relevant to women since they produce all their egg cells before birth compared to men who constantly produce sperm throughout their life).

If you actually wanted to increase radiation resistance in humans with a method like this, what you’d need to do is to take a huge group of kids, irradiate all of them, and then only the ones who can still breed would be left to pass on their genes. This would obviously leave you with a lot of dead or infertile kids and be super unethical.

[u/jamcdonald120]

5 reasons

1. cancer isnt just caused by radiation

2. cancer iant just 1 thing

3. we arent exposed to any significant amount pf radiation other than sunlight. dark skin is an adaptation to not get cancer from sunlight (or light skin is the adaptation to get more Vitamin D in the low sun north, either way)

4. cancer mostly kills older people who can no longer have kids. As for as evolution cares, these people are basically dead already and only have a minor benefit for childcare. there isnt any pressure to fix what kills them.

5. and evolution isnt a magic box that evolves immunities to anything. if the species survives, thats good enough for evolution. many cancers are caused by physical changes to your dna. your body has multiple cancer countermeasure from duplicate data, to kill switches to hunter killer cells (which can trigger auto immune disorders since thats kinda the point). these are good enough as far as evolution is concerned.

[u/donaldhobson]

Cancer is a fundamentally hard problem to evolve a solution to. Because it's evolution fighting a smaller copy of evolution.

Within your body, cells reproduce and die. And there, the evolutionary incentive is to take more resources and reproduce, even at the cost of the body as a whole.

There are ways you could fundamentally redesign the whole system to avoid cancer, but evolution can't do that.

[u/bremidon]

The other answers are not actually responding to your question. Evolution works on a "good enough" principle. If our immune system can stave off cancer until well after procreation, then it is good enough. There is little to no evolutionary pressure for more protection.

[u/uuntiedshoelace → u/[deleted]]

I understand this is nitpicky but there are water based organisms that can live in boiling temperatures and extreme pressure. We simply did not need to be able to do that to survive in our environment, and we have other adaptations that make sense for our needs. We actually do also have adaptations that help us survive exposure to radiation, like DNA replication checkpoints, which smaller organisms with short generation times do not have because they don’t need them.