英语时差：智齿的演变（下）

But, if these extra teeth were such an advantage for our ancestors, then why do most people today need to have their wisdom teeth removed? Again, the answer lies in evolution. As we humans have evolved, our skulls have become slightly smaller, while our brains have actually become a bit larger. To make room for these changes, our jaws have moved lower and further back on our skulls, and in the process, our jaws have become shorter. Early humans had longer jawbones which provided enough room for all 32 teeth. Most humans today have smaller jaws that can’t accommodate the four extra teeth. When there isn’t enough room for the teeth, they become “impacted” stuck, in other words and many of us have to have them surgically removed.