Humans are the result of billions of years of evolution. In the last couple of thousand years, we've evolved into something that is recognizable as a human. But are we still evolving? Are there traits being selected for that make us stronger, more intelligent, or more energy efficient. I would argue that we stopped evolving at this level a long time ago.
Medication was a great invention. When someone gets hurt or sick, they are much, much less likely to die. By all account this is a great thing, but the other side of the coin is that people who are more likely to get hurt or sick, do not leave the gene pool. Thus, for the most part, no matter your fitness in society, you still have an excellent chance of having kids if you want.
Obviously there are some people who never quite find someone to marry, and that could be because they were not "fit" enough to find someone. But it could also be that they just didn't want to. The point is that evolution has a lot less power in making the decision than it used to.
Thus, I don't think that humans have gotten much smarter either; our systems have. A human from 2000 years ago raised today would be just fine and a human from today sent back 2000 years would fit in somewhat well. The individual human hasn't changed that much, the systems that we built have. These systems are what have been evolving. Instead of better immune systems, our society has evolved modern medicine. Instead of bigger brains, education systems have been created. Instead of bigger muscles to fight with, we have democracy. Humans are just cells in these larger organisms that are changing all the time.
Human's could slowly start to be replaced in our modern society with much dumber / weaker versions of themselves and the world would still go round. If the system caries them more and more, then they just need to be able to follow the path in front of them. Go to work, buy things, go to war if your government tells you.
So this raises the question? Could we design a society for being that are much dumber and weaker than us? Would it be possible to build a squirrel utopia where they have similar technology and accomplishments that we do?
This might sound absurd, but you had complete control of how the systems should work, you could try to account for all the faults of squirrels. Make sure that they all get educated, build factories where squirrel mech-suits are created, so they can have more precise control over the world, setup political systems that keep them inline, so society doesn't collapse. If done well, maybe the squirrels get "upgraded" like we have and might be able to do similar things to us.
This is to point out that the individual's intelligence and strength is not what is important, so it could be much, much lower. And there is a case to be made that there are qualities of humans that are getting less and less because we offload the responsibility of something to the systems we rely on. I no longer need to be able to hunt since I have a phone that orders me food. The squirrel no longer needs to worry about food because an infrastructure was built around it that provides for it.
The difference between squirrels and humans is that we bootstrapped our own society, where it doesn't seem like squirrels could create their own by themselves. We are special in that way. But we might slowly lose that as time goes on. If we truly have stopped evolving and offload more and more to the surrounding systems, then there might come a point where the average human does not need to be nearly as intelligent as they are today.