When Fishman started studying the embryonic zebrafish heart, however, he wasn't sure "if the heart would be decipherable, because the genes might be used more than once in development" In other words, a gene involved in the formation of the heart might have had another job earlier in an embryo's development, when the embryo was just a ball of cells. If Fishman created a mutation that made such a gene inactive, the embryo would never become anything more than that ball, and he'd never discover the gene's normal role in building the heart.

"Fortunately, that turned out to be a false worry" says Fishman. His team and others have found more than a hundred genes involved in heart development. After tinman-like genes have finished creating a simple tube, these other genes switch on, transforming the tube into the complex organ we're familiar with. And some of the genes took Fishman by surprise. By knocking out a single one of them, his group could create a heart that was missing its ventricle but was otherwise normal; knocking out a different one produced a heart that was missing valves but nothing else. These genes seemed to be in charge of little modules of genes that worked together to build specific parts of the heart. Fishman ended up finding about a dozen heart-module genes. "It was more than we could have hoped for" says the geneticist. "It meant we could dissect organ development, because we had individual elements that could be removed."

Fishman also realized that the way these heart-module genes work in living fish might hold a clue to the evolution of vertebrate hearts. It's possible that the complex, chambered heart didn't change gradually, with many genes evolving minor mutations that changed their functions. Instead, each of the heart modules may have existed in earlier vertebrates, where they performed other, still unknown jobs. Merely by tinkering with the master gene that controlled a module, evolution could have quickly invented a new structure for the heart. To picture the difference between these two kinds of evolution, imagine building a concrete bridge. If you build it by adding sand grains one at a time, it will take a lot longer than if you assemble it from large prefabricated blocks.

With powerful chambers, valves, and all the other parts of the vertebrate heart in place, the blood of an early fish could be pumped at higher pressures and therefore travel farther from the heart. And this souped-up circulatory system meant that fish could grow large enough to hunt down smaller prey. Thanks to a genetic revolution, Fishman suggests, vertebrates changed from lowly filter feeders into the ocean's top predators.

There's a built-in problem with the fish heart, though. It pumps blood through the gills, where the blood loads up with fresh oxygen before traveling through the rest of the body, nourishing muscles and organs as it goes. By the time it returns to the heart, it has used up a lot of the oxygen. The heart is like a waiter who is forced to eat only the scraps left at the end of a meal.

This design can cause trouble when a fish tries to swim fast: the harder it swims, the more oxygen is devoured by its swimming muscles, leaving even less to nourish the heart. But there are a couple of ways to get around this constraint. One is to divert blood back from the gills to the heart while it is still rich in oxygen. That's what some fish have done, evolving coronary arteries that move blood through the heart tissues. Tony Farrell, a physiologist at Simon Fraser University in British Columbia, has found that the flow of blood through the coronary arteries of a trout triples during exercise, enabling the fish's heart to keep pumping hard.

Another way to become a stronger swimmer may be to evolve lungs. Today lungs are found not only in land vertebrates but also in a few obscure fish lineages, such as gar, bichir, and lungfish. Many other species of fish have swim bladders, which they use to control their buoyancy. For a long time, paleontologists thought that lungs had evolved from swim bladders, helping fish survive in stagnant waters, where oxygen often ran low. But the fish themselves tell a different story. About 420 million years ago, evolution split jawed fish into two great branches: cartilaginous (such as sharks and rays) and bony (everything else, from lungfish and trout to sea horses). Sharks have neither swim bladders nor lungs, suggesting that both these useful organs must have evolved in bony fish after the split. Which came first? The most primitive branches of bony fish all have lungs, while swim bladders are found only in the teleost branch. The simplest explanation of the evidence is that the common ancestor of today's bony fish had lungs and that lungs turned into swim bladders in the lineage that led to teleosts.

The oldest fossils of bony fish all come from marine waters, where fish presumably didn't have to worry too much about running low on oxygen. So why did fish evolve lungs, when they had gills that seemed perfectly well adapted for getting oxygen from water? Colleen Farmer, a physiologist at the University of Utah, thinks that the evolution of lungs made fish better swimmers. In air-breathing fish, some of the blood that flows through the gills gets diverted to the heart, and when these fish swim hard, they tend to breathe more air. Farmer suggests that they're trying to keep their hearts supplied with enough oxygen. And that, she proposes, may have been the initial pressure driving the evolution of the first lungs some 400 million years ago. "These lunged fish," says Farmer, "were active predators cruising around the open ocean."

The question then becomes why (and when) so many fishes lost their lungs. Farmer speculates that the change started when rising to the surface to breathe became risky. About 220 million years ago, the sky began to fill with predators--scaly-winged pterosaurs and, eventually, birds--that snatched up the fish they saw while flying over the water. Perhaps fish species that lost their lungs flourished, while those with lungs became rare.

Fortunately for humans, one lineage of lunged fish hauled ashore about 360 million years ago, eventually giving rise to land-dwelling amphibians, reptiles, and mammals. In the millions of years that followed, these vertebrates have retooled their hearts in multiple ways. Hummingbirds evolved rapid-fire hearts that can beat twenty-two times per second; frogs and squirrels evolved the ability to slow down their hearts during hibernation. Whales returned to the sea and evolved huge hearts--in the case of the blue whale, a heart capable of pumping sixty gallons per minute. And crocodiles, when underwater, can redirect the flow of blood to bypass their lungs entirely. What makes these transformations all the more amazing is that the basic genetic recipe for the vertebrate heart hasn't changed much at all. When it comes to the heart, we all swim with the fishes.

"I've always been fascinated by how evolution produced complicated things like the heart and the brain," says freelance journalist Carl Zimmer ("The Hidden Unity of Hearts"), "and now scientists are coming up with good evidence for how it happened." Zimmer put his interest in evolution to good use in his last book, At the Water's Edge (Touchstone, 1999), which describes recent research on two of the most significant transitions in the history of life: from fish to four-legged land vertebrate and then, back to the water, from land mammal to whale. This September, Simon & Schuster will publish Zimmer's next book, Parasite Rex, a close look at tapeworms, flukes, and other long-underestimated parasitic organisms that just may turn out to be "the dominant force in the evolution of life." Zimmer also writes a monthly column on biomechanics for Natural History.

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