Squirrels belong to the order of mammals, called Rodentia (so much better sounding than "rodents"), which are characterized by continuously growing incisors. Like many of us working in today’s business world, dental isn’t covered for Squirrels either, so they are responsible for keeping their own teeth from overgrowing by gnawing.
Squirrels belong to the family Sciuridae and being such adaptable little critters, are found all over the world, from rainforests to deserts to city parks -- with the exception of Australia and Antarctica. There are nearly 280 species of squirrels and the oldest squirrel fossil dates back about 35 - 40 million years ago. Over those millions of years, squirrels have developed a strategic ability to get seed out of even the most ingenious squirrel-proof bird feeders. And they don’t easily give up. We like that.