Teeth actually begin their lives about two months after a baby is conceived, as tiny buds from the mouth lining grow downward into the jaw that is developing beneath it. Ten buds grow on top and ten on the bottom, one for each baby tooth. When a baby greets the world with an open mouthed cry, its pink gums already contain teeth.
The rest of the primary teeth, also known as baby or milk teeth, begin forming right after birth and usually appear in the mouth when a child reaches 6-14 months old. The order and time that primary teeth appear varies from child to child, some teething as late as 25-33 months old, with girls tending to teethe slightly before boys. The first teeth to appear are the incisors. When all four of the front incisors have erupted, the lateral incisors, the small teeth on either side of the front center teeth, will show up, probably two at a time. Then come the canines and first and secondary molars for a grand total of 20 primary teeth.
Primary teeth remain reliably in place until around the age of six when they start to loosen on their own to make room for permanent teeth. This process may last from age six to 12, but by age 13 only permanent teeth remain, with girls generally losing their baby teeth earlier than boys. In a time frame of seven years, 28 permanent teeth replace 20 baby or primary teeth. By the age of 21, four wisdom teeth will appear, rounding out the total number of 32 permanent teeth.
The third set of molars, known as the wisdom teeth, can cause painful glitches in the tooth life cycle. At one point in tooth evolution they provided valuable chewing aid for people, but as modern people ate more refined foods, the jaw shrank even though the tooth patterns stayed the same. Some people are born with a jawbone that can't hold another set of molars. When this happens the wisdom teeth become impacted, or wedged in sideways between the jawbone and the molars that arrive at age 12. They can grow, get stuck, and cause an infection. Five million Americans have about 10,000,000 wisdom teeth extracted every year.
Age and the daily chewing grind affect the life cycle of a tooth. As teeth and the people who have them age, they become more prone to cavities in tooth dentin and since older people often produce less saliva, the bacteria that cause tooth decay often flourish. Older teeth are also vulnerable to periodontal disease, which means that even if the tooth itself is healthy, the bone and gum that anchor the tooth can become seriously infected and the tooth can fall out.
Dentists say that there is not an evolutionary dead end for the elderly tooth. They point out that people are more aware of the importance of dental health and faithfully brush after meals and floss every day. Some dentists predict that dentures will become as obsolete as wooden false teeth in the tooth life story.