Rodent Teeth

    Rodents include mice, rats, squirrels, chipmunks, beavers, porcupines, and many other species that live worldwide.  Rodents, such as mice, squirrels and beavers, have similarly shaped teeth that reveals their common ancestry and consume hard foods that require sharp front teeth and strong jaws to break down that food. 
     Rodents long ago ate hard foods because that type of food was available to them.  And, over the course of the life histories of these furry mammals, they developed teeth ever better adapted to such food.  Necessity IS the mother of invention.  And form does follow function.  Mice consume seeds, squirrels chew open nut shells to get the meat inside and beavers ingest twigs and bark from the trees they chew down with their large, orange front teeth. 
     But teeth wear down chewing such hard food, eventually becoming useless and allowing their owners to die of starvation or predation after the animals weakened.  Those animals reproduced little, if at all.  But by chance in the distant past, some rodents' teeth became genetically structured to grow throughout the lives of their owners.  That eliminated the problem of teeth wearing down, but if the teeth grew faster than they could be chewed down, the lower teeth could grow into the skull and the upper teeth could grow into the lower jaw, perhaps locking the mouth shut which again would lead to starvation or predation.  So, over many generations of rodents, a balance between the growth of teeth and their being worn down developed, to the benefit of surviving rodents to this day.
     All rodents today have similarly shaped teeth that are successfully balanced between their growth and wearing down from chewing hard food.  That similarity to me indicates that today's rodents are related to each other.  They are the surviving descendants of one species that was a member of a larger family of rodents in the long ago past.  Species of rodents that could not strike a balance between the growth and the wearing of their teeth became extinct, providing open niches for surviving rodents.
     The balance of the growth of rodent teeth and their wearing down is just one of innumerable changes in nature over eons that led to today's successful species.  And some of that current life won't be successful in the future.  Life is forever facing changes, including human-made ones, that eliminate some species, but make room for new, perhaps better adapted ones.  Life is never static.