Planet Earth

Death From the Pleistocene Sky

By Carl ZimmerMar 1, 1996 6:00 AM


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Jaws may be nature’s version of a plowshares-to-swords program: They seem to have originally evolved to help fish breathe better.

When our vertebrate ancestors debuted about 520 million years ago, they didn’t take the world by storm. Inches long and jawless, they grabbed helpless worms with their lips, much as a toothless, armless man might eat a hot dog. Only when fish developed jaws 460 million years ago did Earth see serious predation. As soon as jaws evolved, there was a revolution, says Jon Mallatt, a zoologist at Washington State University. You got all these giant-jawed vertebrates that were at the top of the food chain and eating really big things--just mean, nasty carnivores all of a sudden. And one of these groups gave rise to the land vertebrates. So jaws were a big event. Without them we wouldn’t be here.

Paleontologists have believed that jaws evolved precisely because they made fish better predators. But now Mallatt has called that view into question. The predatory prowess afforded by jaws, he argues, was an evolutionary bonus. Jaws evolved in the first place, he says, not for better biting but for better breathing.

The conventional view dates to a nineteenth-century German anatomist named Karl Gegenbaur. Gegenbaur noted a resemblance between the jaws of embryonic sharks--among the more primitive of jawed fishes--and skeletal arches, behind the mouth in all fish, which support the gills. Gegenbaur proposed that the frontmost gill arches had become enlarged in some jawless fishes, allowing the mouth to clamp down on a wriggling victim. This innovation gradually led to full-blown jaws.

Gegenbaur focused on bones. Mallatt, on the other hand, has looked at the soft parts of fish. To analyze the transition to jaws, he has compared sharks with lampreys, a surviving jawless fish. Like sharks, he has found, lampreys have big cheeks and lips, which house the muscles that control the mouth; the similarity extends to their cartilage, nerves, and arteries. And while fossils of extinct jawless fish provide few details, they too have traces of the same features. In other words, cheeks and lips seem to have originated before the jaw and remained intact as the jaw evolved.

That observation is hard to square with Gegenbaur’s theory. If the protojawed fish had large cheeks and lips, its gill arches must have been too far from the front of its mouth to have been useful for grabbing, even if they became enlarged. It would be as if the cartilage in our voice box suddenly started grabbing food, says Mallatt.

Yet Mallatt thinks Gegenbaur was right about the gill arches becoming jaws; it’s just their initial use he got wrong. The arches support gills, after all--and gills let a fish breathe. After a fish draws water into its mouth, it squeezes the arches to force the water over the gills at the back of the mouth. Blood vessels in the gills pull oxygen from the water and dump carbon dioxide back into it before it flows out the gill slits. The animal then relaxes its gill arches, drawing in more water.

Jawed fish have more powerful arch muscles than lampreys. They can suck water in quickly, get more oxygen, and swim faster. But to prevent the fast-flowing water from regurgitating out, they shut their mouth with each breath. Mallatt argues that the earliest protojaws evolved as a first step toward this breathing system. With enlarged front gill arches--still useless for eating--a fish could stop the backwash by clamping the arches together, sealing off the gills from the front of the mouth.

The enlarged arches and their more powerful muscles, though, also let fish suck in prey as well as water, and pin the wriggling prey with their protojaws. They could thus attack bigger, faster prey, and an evolutionary feedback began--culminating in full-blown jaws that extend to the front of the mouth and can bite. Mallatt sees signs of the origin of jaws even today in almost any modern fish. Most fish don’t really use their jaws for biting so much as they use them for clamping down on prey they’ve suction-fed in. The real evidence is that feeding in almost all jawed fishes is an exaggerated ventilatory act: suck ’em in and close the mouth.

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