About 420 million years ago, fish evolved jaws. 99% of vertebrates still have them — including humans — but they may not be a very useful adaptation.
New research published this week shows that the arrival of jaws didn't give any particular advantage over other fish, and that the design remains fundamentally unchanged since the mutation's early arrival. Millions of years ago, there were creatures with a diversity of jaw types, but the design settled down some 415 million years ago, and has remained more or less the same ever since.
At that point, the vast majority of fish were covered in armored plates, and only a tiny fraction of the vertebrates had jaws. As much as we like to think that jaws are an extremely useful mutation, the jawed organisms didn't out-compete their jawless brethren. Jawless organisms went into decline some 30 million years after the evolution of jaws. And when jawless creatures went extinct, the jawed didn't take over their niches, or become more diverse. Our jaw designs simply didn't help us against our floppy-chinned competitors.
The researchers aren't sure why the basic design of the jaws has been static for so long, but it's tempting to believe because it's good enough that further mutation didn't bring any additional benefits.