system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we have "baby" teeth?

What is the purpose of baby teeth? Why don't we just keep them and have additional teeth grow in?
gpt
Actually, you're asking the wrong question.  Let me explain.

We have two sets of teeth, like most mammals (some mammals, such as rodents, only have one set of teeth, and still others, such as baleen whales, have none at all).  But our distant ancestors, hundreds of millions of years ago, had the ability to grow endless sets of teeth.  Today, we see this ability retained in modern fish, amphibians, and reptiles.  

Now, replacement teeth seem to grow in a really stereotyped manner.  There's a layer of cells in the mouth that lay just medial (tonguewards) of the tooth, called the successional lamina.  In mammals, the successional lamina is  contained within the cavity that holds  the root (the alveolus) but in reptiles and amphibians, the successional lamina will often be part of the medial gumline.  The successional lamina contains stem cells as well as epithelial cells.  Periodically, the successional lamina will form a pocket, that will then fold up in various ways, then differentiate into an enamel organ and pulp cavity, which will then produce a tooth.  The remainder of the successional lamina will then proliferate to create more successional lamina, and will later repeat the whole cycle.

In mammals, this is disrupted.  The successional lamina forms a replacement tooth (your adult tooth) but it does not form a new successional lamina.  This is not the case in all people; some people are able to produce a third or even a fourth set of teeth, but this is the case in most people, and indeed in most mammals.

So now we know how humans came to have only two sets of teeth.  But why?  It's kind of ridiculous, considering that a tooth really doesn't last very long, even with modern dentistry.  However, our early mammal ancestors were small and likely had short lifespans, much like modern shrews, rodents, possums, and other small furry creatures.  

This was, of course, probably also the case for many of our early nearly-mammalian ancestors, and our early somewhat-mammalian ancestors, and before that our early almost-amniote (egg-laying) ancestors, our early almost-amphibian ancestors, and so on. But our early mammalian ancestors did something special; they fed their offspring milk.  The benefit of milk is that it doesn't require an adult dentition to consume, and it contains a lot of energy, minerals, and other things the body needs to grow.  So baby mammals grow up really, really fast.  Furthermore, as soon as a baby mammal reaches sexual maturity, it stops growing.  So, a baby reptile or amphibian or almost-mammal might take a few years to reach adulthood, and during that time, it's going to be using its teeth to eat plants, or insects, or other reptiles, amphibians, or almost mammals, and once it becomes an adult, it's going to *keep* growing for a while longer, although it will slow down considerably.  So, a reptile, amphibian, or almost-mammal is going to need to keep on replacing those teeth as they grow, because the teeth that were good for catching small insects when they're a few months old won't be all that good for catching small amphibians or reptiles when they're a year or two old, and so on, so they need to keep on making more teeth and larger teeth as they grow.  A mammal, however, doesn't need to do that.  That first generation of teeth (baby teeth) are good enough to get it through the growing stage of its life, and when the mammal reaches adulthood, it gets its adult teeth, at which point it's as large as it's going to get, and thus will never need to replace its teeth.  And because our early mammal ancestors probably only lived a couple of years at most, they didn't have to worry about cavities too much, because by the time their adult teeth were too damaged to use well, they had likely produced plenty of little baby early mammal ancestors, and, more importantly, had probably been devoured by a small dinosaur anyways.  So our early mammal ancestors did away with additional replacement teeth, because they simply didn't need them.  They still needed those baby teeth, and they still needed adult teeth, so that's why we have two generations of teeth.

**tl;dr: "baby" and "adult" teeth are holdovers from our reptile-like ancestors, who replaced their teeth continuously throughout their lifespan.  We lost the "continuously" but still kept the "replacement."**