My science fiction is based on two species, the R’il’nai and Humans, and their crossbreds, the Ril’noids, living together. One of the major differences between the two parent species is in life span. The Humans have what we would consider a normal life span. The R’il’nai, while not immortal, do not age beyond maturity. A number of my characters have been alive for millennia. Crossbreds can show either pattern.

This leads to all kinds of interesting situations in the society. How do the two species interact, for instance? How many Humans would want to marry someone who would never grow old? How does a R’il’nian act toward someone he or she knows will grow old and die while the R’il’nian is still young? This is in the background of all of my plots.

Here, however, I am addressing a different problem.

Most of the cells in our bodies are constantly turning over. I can imagine a creature that looks and acts human with a near-infinite life span, except for one thing. Teeth.

Tooth enamel wears, and unlike skin, it is not constantly replaced from within. Modern dentistry can do a lot to repair wear, but I’m having to have enamel repairs already. Young mammals are born with two sets of tooth buds, one that grows into teeth suited for the small jaw of a juvenile; the second set adult sized, and that’s it. People who lived thousands of years would wear out their teeth. How to handle the problem?

The R’il’nai would have to have an essentially infinite number of replacement teeth. When a tooth was worn out, it would be shed much as a child sheds its milk teeth, and replaced by a new tooth. How? They must have some tooth stem cells in their jaws, just as we have blood stem cells in our bone marrow. Assuming that a tooth would last for 50 or 60 years, this would mean that the R’il’nai and non-aging R’il’noids are teething roughly every two or three years. I don’t think I’ve actually mentioned that, but if a R’il’noid seems to be in a particularly bad humor, he or she may be teething.