Scarce as Hen's Teeth

Most everyone knows that birds come with a hard beak, empty of any teeth. Thus the old saying I often heard my mom say, when something couldn't be found: “It's as scarce as hen's teeth.” Little did Mom know that hens—or, rather their deep ancestral forebears—once did have teeth. In fact, those toothy critters were birds' very deep ancestors (actually, dinosaurs), who lived over 100 million years ago.

In that ancient time the ancestors of today's birds were small, toothed dinosaurs that cohabited the planet with the really big toothy dinosaurs, like the formidable T-Rex. A great many species of dinosaurs presided over all living creatures on Earth, until that fateful day when an asteroid crashed down near the Mexican Gulf coast, 65 million years ago. The resultant disruption to Earth's climate soon wiped out the great dinos, but the smaller, feathered ones managed to squeak through.

Well before that asteroid crash, however, those bird ancestors had already lost their teeth. How do we know? New genetic analyses show that the toothed dinos long ago first lost their teeth at the front of their jaw, and the tooth loss then gradually moved rearward on the jaw. As this loss progressed, the hard beak slowly developed (I guess to give them something hard to mash their food with).

Bird evolution continued to occur, as the proto-birds also evolved flight feathers and even began to sing. As a fascinating example of how genes are shared across species (good evidence that we all co-evolved from even more primitive critters), the 50 or so genes that permitted birds to begin to sing are shared by us humans. These genes give us the ability to speak—and yes, sing, but certainly not like a bird.

But back to hen's teeth: Was a toothless bird more suited for surviving the asteroid impact than the dentally-equipped large dinosaurs? It was more likely a case of their larger size and the over-specialization of the huge dinos that did them in. In any case, the surviving proto-birds found countless available niches, after their dino cousins disappeared. When many niches are available, a species can rapidly multiply into many new species. That's why Earth currently supports more than 10,000 species of birds.

So, the proliferation of birds happened because the big dinosaurs, some 65 million years ago, became scarcer than hen's teeth.