Humans are pretty optimized for intelligence. It's not the only evolutionary pressure we face but it's a pretty important one, and it's notable that we haven't

Humans are pretty optimized for intelligence. It's not the only evolutionary pressure we face but it's a pretty important one, and it's notable that we haven't seemed to get much bigger brains in hundreds of thousands of years. Given how much evolutionary pressure there is to be smarter, evolution could probably find a solution bipedalism/baby head size problem. Moreover, brains grow in size substaintially after birth. This is weak but real evidence that there's some other constraint in the mix. The most obvious candidate is that 3 dimensions is a pretty serious constraint for a physical computer. There are a number of different ways to operate wthin this constraint, but it always ends up being a wall. I've got to fit the computing elements in a volume, those computing elements take power, and generate waste, both of which must enter and exit through the surface. Technology can maybe do a better job at solving these problems than evolution, but since evolution has a much more energy efficient way of computing, I think it is way too soon to say definitively. The other candidate reason is more subtle. That what actually matters is the collective intelligence of humanity. Our greatest invention is the ability to communicate our knowledge, so that we don't have to start from scratch with each new human. The collective intelligence of humanity then, actually lives across 8 billion computers. Each intelligence is responsible not only for a section of the knowledge space, but for a unique representation of that knowledge space. Those unique representations correspond to unique sets of "natural" inferences, and allow humanity itself to be many many times more intelligent than our "smartest member". In this sense, what it means to be as smart as possible is similar to the question of how many proccesor cores I should put on a CPU. I believe it is not that controversial to suggest an upper limit to how complex a single CPU core should be.