Don’t they prove evolution? Not exactly. They don’t prove that humans were once apes. They prove that humans once looked different.
Let’s look at the brain as a computer. Except as our computer speeds get faster and faster, the processors get smaller and smaller. So a decent computer now is hundreds of times faster than those decades ago, but it’s also much, much smaller.
But the brain does not offer us that opportunity. The substance of the brain has not changed in millions of years. But the speed of life and number of decisions that we make every day has increased exponentially. Why does that matter? Because if we didn’t make more efficient computer chips, if we just used the materials that they had in the 1970s to create computers now, computers today would be behemoths. There would be no portable smartphone, especially at this speed.
So if we need better processing power, with the same materials, the computers get bigger. So the heads would be smaller years ago, when the demands were so much lower.
If you look at entropy in the brain, and assume that stress causes higher entropy. We also assume that the brain has a stress or entropy limit. As the demands on the brains grew, the brain had to get bigger. Otherwise, it would overheat.
So, of course, any ancient skulls you find will probably be smaller. This is more evidence of how well the human body can adapt to stress than proof that we evolved from apes.
But not all ancient skulls are smaller. Neandrathals are roughly the same size as humans today. So we have some more exploring to do.
Update: Exploring done.