Humans are the result of billions of years of evolution. In the last couple of thousand years, we've evolved into something 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 accounts, 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 2,000 years ago raised today would be just fine, and a human from today sent back 2,000 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, we have created education systems. Instead of bigger muscles to fight with, we have democracy. Humans are just cells in these larger organisms that are changing all the time.
Humans could slowly start to be replaced in our modern society with much dumber or weaker versions of themselves, and the world would still go round. If the system carries them more and more, then they just need to be able to follow the path in front of them: go to work, buy things, and go to war if their government tells them to.
So this raises the question: Could we design a society for beings that are much dumber and weaker than us? Would it be possible to build a squirrel utopia where they have technology and accomplishments similar to ours?
This might sound absurd, but if you had complete control over how the systems worked, 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; and set up political systems that keep them in line so society doesn't collapse. If done well, maybe the squirrels would get "upgraded" like we have and might be able to do things similar to what we do.
The point is that the individual's intelligence and strength are not what is important, so they could be much, much lower. And there is a case to be made that some human qualities are diminishing because we offload more responsibilities 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, whereas 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 continue to 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 the average human is today.