Thousands of years ago, people decided to stop roaming around and killing stuff and set up farms. And somehow our heads stopped growing because we invented cheese.
According to researchers, humans began to have smaller heads and sleeker jaws around the time that we started relying on dairy foods, like milk and cheese, and farmed plants, for sustenance. The argument is that humans started eating softer foods, rather than crunching and grinding away on whatever we could rustle up in the forest. This meant that we didn’t need huge, powerful jaws anymore.
But it’s important to keep in mind that the whole reason we started farming was to make beer. Society as we know it today was founded on beer and cheese.