A Theory Evolves
How evolution really works, and why it matters more than ever
But new research also shows that evolution works in ways Darwin did not imagine. Many creatures still appear quite suddenly in the fossil record, and the growing suspicion is that evolution sometimes leaps, rather than crawls. For example, the first complex animals, including worms, mollusks, and shrimplike arthropods, show up some 545 million years ago; paleontologists have searched far and wide for fossil evidence of gradual progress toward these advanced creatures but have come up empty. "Paleontologists have the best eyes in the world," says Whitey Hagadorn of Amherst College, who has scoured the rocks of the Southwest and California for signs of the earliest animal life. "If we can't find the fossils, sometimes you have to think that they just weren't there."
A new understanding of Earth's history helps explain why. Scientists have learned that our planet has been rocked periodically by catastrophes: enormous volcanic eruptions that belched carbon dioxide, creating a super greenhouse effect; severe cold spells that left much of the planet enveloped in ice; collisions with asteroids. These convulsions killed off much of life's diversity. Once conditions improved, says Harvard paleontologist Andy Knoll, the survivors found a world of new opportunities. They were freed to fill new roles, "experimenting" with new body plans and evolving too rapidly to leave a record in the fossils.
We may owe our own dominance to the asteroid impact that killed the dinosaurs 65 million years ago. As mammals, we like to think that we're pretty darned superior. The sad truth: "Mammals coexisted with dinosaurs for 150 million years but were never able to get beyond little ratlike things," says Knoll. "It was only when the dinosaurs were removed that mammals had the ecological freedom to evolve new features."
Whether evolution worked fast or slow, theorists labored to explain how it could produce dramatic changes in body structure through incremental steps. Half an eye would be worse than none at all, creationists were fond of arguing. But "partial" eyes turn out to be common in nature, and biologists can trace eye evolution from the lensless flatworm eyespot to the complex geometry of vertebrate eyes. Now "evo-devo" biologists, who study how fertilized egg cells develop into adults, are discovering powerful new ways evolution can transform organisms. They are finding that changes in a handful of key genes that control development can be enough to drastically reshape an animal.
Master switches. The central discovery of evo-devo is that the development and ultimate shape of animal bodies are orchestrated by a small set of genes called homeotic genes. These regulatory genes make proteins that act as master switches. By binding to DNA, they turn on or shut down other genes that actually make tissues. All but the simplest animals are built in segments (most obvious in creatures like centipedes, but also apparent in human vertebrae), and the Hox family of homeotic genes interacts to determine what each segment will look like. By simple genetic tinkering, evo-devo biologists can tweak the controls, making flies with legs where their antennae should be, or eyeballs on their knees.
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