Entropy
I took an AP chemistry class in high school. The teacher once said that, if you don't know the answer to a question in chemistry, you should guess either "hydrogen bonding" or "entropy". This was apparently funny.
I never came to appreciate the importance of hydrogen bonding, but I have come to appreciate the profound importance of entropy. The move from lower to higher entropy is a ubiquitous driving force in nature. Everything is moving toward disorder, of its own accord; the forces of entropy are constantly working to destroy everything that we care about. It is because of this drive toward entropy that everything has to be constantly maintained.
It's why we have to eat regularly or else our bodies stop working. It's why they will eventually stop working anyway. It's why old buildings have to be demolished, because they've deteriorated dangerously. It's why if you leave your car alone for long enough, when you come back, it won't work. If you leave a book alone for long enough (in this case, a very long time), it will eventually become unreadable. Longer, and it will become a pile of dust.
Human beings, cars, books, and everything else that we care about is a form of order. Each of these things requires a very specific organization in order to have the properties we care about, an organization that usually comprises only an incredibly tiny fraction of all the possible ways its parts could be arranged. Hence, a random reorganization of its parts destroys everything that matters about it. And that random reorganization is always in progress, sometimes quickly, sometimes slowly, but always heading toward destruction. This isn't a metaphysically necessary truth, but it's something close to it.
[Side point: this means that the possibility of everything that we care about -- life, books, cats, Game of Thrones, and all the rest -- is driven by the incredibly low-entropy starting point of our universe. Which makes it a question of the most fundamental import why that starting point was like that. But that is not our topic now.]
Dysgenic Pressure
Once you appreciate the forces of entropy, you'll see that they are implicated in almost everything that is happening. For example:
https://reader.elsevier.com/reader/sd/pii/S0160289616300198. This paper reviews evidence that IQ's are dropping in some developed countries. Previously, IQ has been rising -- the Flynn Effect -- but it appears to have peaked in some places and started downward.
Why is this happening? I don't know all of the story, but I think entropy is part of it. [Note: This paper gives counter-evidence, suggesting the cause is environmental: https://www.pnas.org/content/pnas/115/26/6674.full.pdf.]
I was saying above that everything we care about is a form of order. The human genome is one of those things. If you think about the amount of DNA in a human being, and then consider all the possible sequences of base-pairs that could exist with that length of DNA, the overwhelming majority of possible sequences would be worthless. They would not code for any viable organism. Everything we value about ourselves depends on having a DNA code falling within a minuscule portion of the space of possibilities. Some rearrangements of our DNA would be improvements, but the overwhelming majority would be detriments. Most large rearrangements would be fatal.
The forces of entropy are always working to destroy everything; everything has to be maintained, or it will disintegrate. That includes the human genome. If nothing counteracts it, random rearrangements will happen. If nothing stops it, our DNA will slowly get scrambled. Of course, there is something stopping living species from developing randomly scrambled DNA sequences. The thing is natural selection. Harmful changes get selected out, and that constant pressure of selection constitutes the required maintenance.
But natural selection is a cruel maintenance mechanism. It keeps living things in a constant struggle, lest they be destroyed. The competition among organisms ensures permanent scarcity of food and resources.
Luckily, our intelligence has enabled us largely to insulate ourselves from the cruelty of natural selection. We figure out how to protect human life from most of the forces of nature -- from starvation, from sickness, from predators, from death due to injury. We even do a fair job protecting ourselves from each other.
But one of the things this means is that we are disabling the force that maintains our genetic fitness. Harmful genes are much less likely to kill a human being in a modern, civilized society. Those genes are therefore more likely to spread. Modern life is far more tolerant of a wider range of attributes than primitive life; more tolerant, then, of the random reorganization that the forces of entropy are constantly pushing toward.
In our evolutionary past, it must have been that higher-intelligence humans were more likely to survive and reproduce than lower-intelligence humans. That's why we developed high intelligence. But, plausibly, that is no longer true in modern society. Humans of lower than average intelligence can survive and reproduce just as well as those of higher intelligence. And of course, they may choose to reproduce at higher rates.
But intelligence is only one example. The most straightforward example is genetic diseases. Many people with genetic disorders who would have died young in centuries past can today survive to reproduce.
A minor example: I have moderate asthma. Some people have more severe cases; some even die of it. It also has a genetic component. In times past, severe cases would prevent one from surviving and reproducing. Today, though, there is an excellent chance that modern medicine (drugs) will keep one alive long enough to reproduce. So genetic susceptibility to asthma must be growing more common in the population.
Over the generations, as medicine becomes more powerful, our genes will progressively deteriorate. Detrimental changes will continue to happen, spread, and work to destroy the order that makes human bodies work. Once the pressure of selection is taken off, entropy will push toward as much disorder as is compatible with our still surviving. Perhaps one day, all human beings will require constant support by artificial medical devices to survive. Perhaps our descendants will be cyborgs, not by choice but by necessity.
Of course, at the same time as our bodies were deteriorating, our minds would deteriorate, for the same reason. Perhaps we will grow increasingly dependent on computers to do our mental tasks, just as we become dependent on machines to do our physical tasks. When the robots break, they'll have to be repaired by other robots because no humans will be smart enough to do it.
Perhaps this is how the species will eventually end. Perhaps one day, the species faces a new challenge that requires intelligence to solve. Maybe it will be a change in the natural environment (again, almost all large changes are destructive), or maybe a change in our society. Our computers can't solve it because they were developed and programmed for past challenges, and they lack any genuine intelligence; humans can't solve it because there isn't anyone smart enough any longer. Perhaps the challenge causes our fertility to decline (as, of course, is already happening).
Perhaps it causes our machines to start failing at a greater rate, and there are no longer enough people who know how to repair them. (Note that as a rule, the more advanced a piece of technology is, the more specific and complex order it requires, so the more it is susceptible to entropy if not constantly maintained.) Maybe our technology will have become so intertwined that once enough of our machines fail, everything collapses. And perhaps we will be so dependent on technology that the species dies out soon after.
Solutions
All this isn't inevitable, though. (I mean, it's inevitable that the species will go extinct, of course. But it's not inevitable that it happen in this way.) How could the triumph of disorder be staved off? I can think of three answers:
Restoring natural selection. But, as noted, this is a cruel solution. It's also almost unimaginable that people would accept this.
Deliberate programs of eugenics. Someone -- presumably the state (who else?) -- could start deciding who may reproduce, and stop the "unfit" from reproducing. More modestly, they could encourage reproduction for the "fit". E.g., they could pay smart people to have children. Problem: if you've seen any of the government's other programs, you know they can't be trusted with this one.
Genetic engineering. Scientists and doctors figure out how to directly change the genes of your offspring before they are born. You can edit out any genetic disorders. While you're at it, you can make them smarter, healthier, and better looking. If you can, why wouldn't you?
I don't like 1 or 2, but I also don't like the scenario of progressive deterioration of the human genome until we're entirely dependent on technology. So I really hope we get #3 working, and that we (i.e., our descendants) take full advantage of it.
I know some people are worried about the prospect of genetic enhancement, for "ethical" reasons. (I use scare quotes because I suspect that the reasons are more emotional.) It is thus entirely possible that we humans will shoot ourselves in the foot by banning genetic enhancement. But we have to consider the alternatives. It's either 1, 2, 3, or the triumph of genetic entropy.