I'm positing a world where a group of humans have become stranded. They discover that something in the environment mutates human DNA. They develop a means to slow the drift toward no-longer-human. My question is: could mutations in human DNA change an individual in his lifetime as opposed to across generations? Also, how rapid could this change be?
In an organism as large and complex as a human, no. The results of somatic mutations during one's lifetime are either "nothing", "cancer", or "radiation sickness", and the results of germline mutations are either "nothing, because the embryo is non-viable" or "changes appear in the next generation".
The answer is yes and no
Can you change your arm and hand into a flipper? No. Can't do that in real life (that hasn't stopped people from using it in their stories, though). But, let's get back to that in a moment and focus on what you can do.
The answer is yes, if you consider the answers to Is it possible to genetically modify a being after birth?. But you need to understand that such modifications are NOT changing an arm and hand into a flipper. The result of changes we can do today is still very much human.
Modern gene therapy isn't even changing (to my knowledge) things like eye color. They're modifying genes that lead to disease or disability. If anything, what we're doing today is making a human, more human.
But, from this perspective, you could believably make simple cosmetic changes to humanity that's already represented by the human genome. Do you want a roman nose? Or blue eyes rather than brown? Do you want your hair to have the tendency to grow longer? All are within the realm of what it is to be human, and I could believe that such changes could be made and the body would go about making the changes.
Well... the more I think about changing my nose to a roman nose... that's a lot of cartilage to move around and reshape. I'll be honest with you, I'm having trouble believing that changing the "your nose looks like X" part of my genome as an adult would do anything at all to my nose. In a real, practical sense, the job of that DNA strand is done. It's like changing a piece of startup software after the program is running. You could do it, but what would be the point? (Unless you had children....)
And here's the "no..."
The answer is no (remember, in real life) if what you're thinking are things like changing my arm and hand into a flipper — something decidedly not human. The adult body isn't clay that can be molded into a new shape. Remember, I've concluded that the DNA for what (e.g. my arm) looks like has been used and is now meaningless. Even with the use of stem cells, you'd need to replace what you want changed. In other words, your environment could make the change to your DNA, but nothing would happen until you chopped the arm off and then let it sit for a month or two in a vat of stem cells. (I'm not even sure that would work, but we're inside the world of suspension of disbelief.)
The idea that changing DNA on the fly results in a new-you on the fly is ripe SciFi fodder used and consumed over and over in nearly every independent-episode show or comic book. It might have started with Spider-Man, but it ran amok with Star Trek:TNG. (I'm looking at you "Identity Crisis" and "Genisis"!). And from that perspective, there's nothing at all stopping you from using the trope. It's been used so many times that people don't really question it, they just accept it as entertainment.
But could it really happen? I'm going to go out on a limb and say, "nope." Not in your lifetime. Not ever.
OK, but could it work at all?
We could merge a couple of tropes and come up with the idea that radiation has changed my DNA and I then married a true-blood human and the result was hybrid children that look a bit too much like the creatures from the Tremors movie franchise. Great idea!
But if you're asking, "is that realistic?" the answer is always, No. Radiation does not change DNA in productive ways (and even if you don't want your arm looking like a flipper, that's still a productive change). Radiation kills things. The mutations are always damaging if not deadly.
But reality is overrated.
The other answers all confirm that a traditional mutagen such as radiation or chemicals in the environment could not cause thesort of gradual mutation in living organisms that you are seeking - in the short term they would only produce cancers and other tumerous growths. But how about the McGuffin of tiny AI nano-bots (seeded by some hyper-intelligent beings in the past), that are programmed to infiltrate the bodies of any suitable species they come across and adjust/reprogramme the genetic code of all their cells.
In that way you can propose any wierd or wonderful mutation you like. Want the ability to run faster and jump higher - then minor changes in the genetic code could induce the growth of stronger muscles. Want flippers - then a more significant rewrite of the code could result in arms/legs slowly metamorphosing into them. Want big changes - then after a complete re-write of the geneic code the whole body might 'pupate' and emerge with 6-legs and compound eyes.
And with the right amount of intelligent re-writing you could have significant changes occur over timescales as short as months.
No, aside from tumors, cancers and growths it's simply not possible.
Firstly you're environmental mutagen would have to inflict exactly the same DNA mutation on, at least, every single cell the new feature was comprised of, at massively impossible odds.
Secondly even if it did do that it still won't matter because we finished developing as an organism before we were born, and as adults we've even stopped growing.
In short any new mutations can pretty much only possibly express in an organism as it develops in the womb from a single fertilised egg cell that had that mutation.
The only change your environmental mutagen, whatever it is, can possibly cause to an already adult organism are cancers of various sorts.
CRISPR would like to have a word. Your planets ecosystem has evolved a natural CRISPR-like virus that floods every nook and cranny, and is afloat everywhere in the air and water. It's not only a single type of virus, but plenty of variations - all waging war on the macro-fauna constantly, to change them in interesting and horrifying ways. These virii not only make their hosts create more virii, but also use their CRISPR-like tools to modify the DNA thus that their hosts, once their immune system is bypassed and they are fully flooded, create tools keeping them alive against other fauna. Claws, hard-boned carapaces, extra eyes or hearts and so on and forth. They also subtly modify the adrenaline glands and other hormonal processes to make their hosts aggressive, fearless and in constant search for prey - some of which they only cut a little bit, to inject the virus there.
This virus uses features found in fauna that is often dormant (happily so), like the natural propensity for developing cancers (which are guided by the virus to give them the rapid growth rates they require for extra appendages); forced CRISPR-mutations in stem cells, which injects body parts that are not existing in the base animal, and others. Also, once a virus is deeply embedded, it helps the immune system to fight off all other similar virii later on, to keep control of this host for as long as possible.
Many hosts excrete virii through when exhaling, or when shedding dead skin particles, and of course with their feces.
Don't take off your respiration mask when visiting this place. On the other hand, it would not matter much, as virus variants have developed ways to enter your body in many ways - through the air your breathe, through the skin, through water, or of course when eating infected flesh from other animals.
Imagine genes as building blocks, complex creatures like humans have so damn many building blocks that to express some significant mutation many building blocks have to mutate in a really specifc and coordinate way
Its possible for someone to have many of these building blocks mutate, but in a uncordenate way, and its usually is expressed like some form disease (disruption of an intricate system)
First, random mutation almost always leads to
Cancer; the multicellular organism individual cells start acting more like unicellular (or non-cooperating multicellular) organisms.
Ineffectiveness. Some metabolic pathway stops working correctly.
In germ-line, developmental failure. The "program" that builds the fetus into an organism doesn't produce something that survives.
Mammals and similar animals don't really rearchitecture themselves after initial development. They grow in a few ways, and that is pretty much it.
A mutation sufficient to cause macro-scale shape changes would first have to mutate the human into having that kind of capability! And that kind of evolutionary change is on the level of a new genus.
As random mutations are, well, random, any kind of mutational effect will result in mostly dead humans. This is much of how radiation doses kill you; the radiation causes enough DNA damage (and other cellular biological processes) that most of your body's cells are no longer doing their job.
Our body's cells have a failsafe system where, when damaged, they commit suicide. Radiation (and most likely other kinds of mutation) does enough damage that it triggers this en-mass. One of the key mutations that result in cancer is that system being disabled. While a good chunk of it might be non-DNA based damage, the damage to the DNA also causes cell suicide and also can block cell reproduction itself.
A mutation mechanism capable of actually reliably producing something other than death would require that mechanism to co-evolve with humans (or similar) for many (mechanism) generations (to build up information), and significant selective pressure (to generate some particular effect), and would most likely involve a lot of human death long before it became effective.
This might be possible with a guiding intelligence, or a very long time period (megayears? Maybe many kiloyears). But short of that, not really plausible.
Yes, the thought emporium did it and showed the results on his youtube channel. He used a virus to change the DNA in his guts so that his guts would start producing lactase and dampen his body's genetic disposition towards lactose intolerance: https://www.youtube.com/watch?v=J3FcbFqSoQY
The answer is going to be pretty complex depending on how large the changes well be, how well-suited the virus is for infecting humans in particular, and how much sickness and death you're willing to risk in your populace.
First, the risks:
- Our bodies and immune systems are structured to fight off changes to our DNA. Triggered cell death, killer T cells that assault anything which seems foreign, error-correction mechanisms, and the fact that "random" mutations may often yield non-viable cells will all conspire to limit changes to a living organism's DNA. "Flu-like symptoms" will cover most of that immune response, and will probably continue until / unless the thyroid and other key portions of the immune system are infected & rewritten enough to change the immune system's concept of what qualifies as "you"... Then your original body is going to be attacked until it's gone. If the proteins on a cell's surface (and the body's other IFF mechanisms are similar enough for the viral and original cells, the immune system response might be less severe.
- Triggered cell death is itself genetically regulated, as one of the ways to limit genetic changes (above). When those regulations break down, you either get autoimmune disorders or cancer.
- Having a single virus (or class of viruses) which can reproducibly infect every cell in a human without triggering massive immune responses is a stretch. You'll want a virus which triggers a minimal immune response, spreads rapidly, and which generates changes compatible with all blood, marrow and tissue-donation types ... or you're going to kill off large segments of the target population.
Then, the possibilities:
- First, there are cases of viruses changing human DNA, but usually those have involved infections that spread to the gametes (and presumably gonads), changing the species on generational time scales.
- Some portions of the human body that naturally grow and restructure themselves over time: Skin naturally regrows to cover wounds, the outer layer of skin (and inner layer of intestines, iirc) constantly get recycled, and blood cells (esp. red ones) get fully flushed and rebuilt on a regular basis. That's pretty far from the kinds of mutations commonly seen in SciFi, but I could believe that skin color / patterning and blood/digestive chemistry might shift on the timescale of a year or two.
- DNA changes (and related changes to cellular chemistry) might also be able to "reactivate" previous growth stages for other cells, but that would probably present as cancer-like and/or reabsorption of the altered portions of the body and change it on normal growth timescales, i.e. a decade or two. If the new DNA is making something which grows up faster than a human, that might get shortened... or the former-human might just die quick because of alterations in how telomeres act.
- Microchimerism is a known side effect of pregnancy: you likely have a few of your mother's cells running around inside your body, and she likely has some of yours. Sometimes these cell-sharing effects generate autoimmune disorders, but may also be helpful or neutral. Only a small portion of the body's cells get swapped this way, and the changes are subtle, but genetic incompatibilities between mother and child are among the dangerous potential complications of pregnancy. Maybe these changes could be hooked as a way of sneaking a new colony of mutated cells into an adult body to provide longer-term effects with a suppressed immune response: Essentially, I'm thinking of a sequence like: 1) mother gets infected during pregnancy, 2) fetus is infected, 3) mother's immune system wipes out the disease locally but the fetal DNA is completely overwritten, 4) infected fetal cells colonize the mother's body and are close enough to slip by the immune system, 5) mother's body continues to change slowly on decadal time scales, even after pregnancy ends.