4. Passage Eight Wisdom teeth are normally the last teeth to appear. This usually happens when people are in their late teen years or early twenties—in other words， when they are older and wiser. Wisdom teeth are chewing teeth， at the back of the mouth. They can grow into place normally and never cause a problem. But often there is not enough room for them in the mouth. They might crowd other teeth. Sometimes they even push sideways through the gums (牙床). An impacted wisdom tooth is one that fails to completely rise through the gum—the term is 【erupt】. Wisdom teeth that only partially erupt can leave space for bacteria to enter around the tooth. Infection is a risk in these cases. Wisdom teeth that are not well in place and become impacted are often removed. People should have the placement of their wisdom teeth examined between the ages of sixteen and twenty. X-rays can show wisdom teeth that are below the gums. The American Dental Association says removal is generally advised when wisdom teeth only partly break through the gums. Removal is also advised if there is a chance that poorly placed wisdom teeth will damage other teeth. And removal is called for in cases where fluid collects around a wisdom tooth that is partly or fully below the gum. But why do we have wisdom teeth if we often need to get them removed? One theory has to do with our diets. Scientists say the diet of prehistoric humans probably required more chewing teeth. Life was probably alittle rougher on the teeth back then， too. So it was good to have extras. The American Association of Oral and Maxillofacial (上颌面的) Surgeons says young adults are the best candidates for wisdom teeth removal. The group says older patients may be at greater risk for disease in the tissue surrounding the teeth. After surgery， there can be swelling of the gums and face and some pain. Both can be treated with cold wraps and medication (药物治疗). Humans have wisdom teeth probably because ( ) .