Many biologists believe teeth are basically adapted scales - that is, shark/ray type placoid scales, which also have both a soft, living inner pulp and a hard, enamelized shell. Bony-fish scales have neither, so they can simply grow in size, like our fingernails; placoid scales, with their enamel coating, can't.

If we only had one oversize tooth on each surface, we'd have to wait until we were our full adult size before replacing it, so how would the two tiny, non-expanding teeth we first developed to fit our infant mouths cope with our growing and widening jaw? Secondly, how would we feed while the permanent ones grew in?

This genesis may also explain why we uniquely discard our first dental set for a larger, more powerful one, as our mouth and jaw expand, and as we need more dental surface and durability to deal with our increased longterm intake requirements. Out of all the many specialized body parts that must change to meet our adult needs, our adapted dental scales are the only ones that use this process to conform to our eventual proportions. Other body parts either simply add more cells, or expand the volume of existing cells.