The evolution of a head is associated with the cephalization that occurred in Bilateria some 555 million years ago.

What's inside a human head?

The head contains sensory organs: two eyes, two ears, a nose and tongue inside of the mouth. It also houses the brain. Together, these organs function as a processing center for the body by relaying sensory information to the brain. Humans can process information faster by having this central nerve cluster.

How big is a human head?

According to Wikipedia, one study in the United States shows, the average adult head circumference to be 55cm (21 3⁄4 ) in females and 57cm (22 1⁄2 in) in males. Another study conducted in UK shows a slightly different result: with the average female head size being 55.2 cm and the average male head size being 57.2 cm.