Without jaws, most vertebrates-including us-would be stuck hanging around in the ocean or on the ground, unable to bite and scooping up or filtering food. We’d also be smaller. Instead, we’re fearsome predators and herbivores, with big brains and an infinite range of food sources. We have evolution to thank for our fortune-and Jon Mallatt to thank for helping us appreciate the fact.
“The evolution of jaws a half billion years ago was the single most important factor in the success of vertebrates,” says Mallatt, associate professor in the School of Biological Sciences and in basic medical sciences.
Mallatt began his study of the evolution of jaws as a graduate student, with an analysis of how fish and lampreys feed. In subsequent years, his anatomical comparisons of fish, lampreys, and other animals led him to propose the then novel idea that jaws evolved as a breathing apparatus that became adapted for feeding, rather than just as a feeding apparatus.
In the process he also analyzed the hagfish, a jawless vertebrate that has a mechanism of feeding similar to the lamprey’s. Many considered it the most primitive vertebrate, but Mallatt wasn’t sure.
“I wondered if hagfish were a good model of the primitive vertebrate that jaws came from or just a bizarre, slimy animal that doesn’t tell us much about what early vertebrates were like,” says Mallatt. “I always believed the second.”
Mallatt found that he couldn’t answer his questions about the hagfish and about the evolution of jaws just by studying the anatomical structures. “Anatomical characteristics are complex,” says Mallatt. “Many genes control each one.” It’s much easier to model how evolutionary change happens at the genetic level, by looking at the DNA.
Mallatt chose the gene sequence of the 28S component of ribosomal RNA as a means to determine phylogeny, or how organisms are related to each other. Ribosomal RNA is fundamental to life, involved in making proteins, so its structure and function can’t change much. If there is evolutionary change, it comes slowly, he says. Comparing the DNA sequence of this gene in distantly related animals can reveal how the animals are related to each other.
Mallatt, one of the first to use the 28S DNA sequences to study phylogeny in animals, confirmed what he had always felt about hagfish, that it is not a primitive vertebrate, but rather the second member of a natural group with the lamprey. Now he’s using the sequences to address an even bigger question: how all animals are related to each other.
“If you want to tell the story of how important things evolved, you must get the relationships right first,” says Mallatt.