For more than a billion years, these simple multicellular organisms evolved internally without great changes in external form. But beginning about 545 million years ago, during a period known as the Cambrian, evolution headed down a different path. A wealth of new organisms suddenly appears in the fossil record; they have hard shells, segmented bodies, and wildly different kinds of legs, antennae, spines, and claws. All the major types of animal lines—including organisms that would evolve into the vertebrates—appear during the Cambrian period, along with many bizarre biological experiments that proved unsuccessful.

The sudden appearance of these different body plans has always puzzled biologists. How did all these animals evolve so quickly? Was it the result of abrupt and dramatic genetic changes?

A relatively small change in a regulatory region can have a dramatic effect on the body plan of an organism.

HHMI investigators Sean Carroll, David M. Kingsley at Stanford University School of Medicine, and Nipam H. Patel at the University of California, Berkeley, have been investigating these questions from a relatively new perspective. They are leaders in the new field of evolutionary developmental biology—evo devo, for short—which relates the development of organisms to the regulation of genes. According to these investigators, evolutionary changes in body plan do not necessarily require changes in the number of genes or in the protein products of genes. Instead, evolution can create new kinds of organisms simply by experimenting with how genes are turned on and off as an organism develops from a single fertilized cell to its mature form. "If you want to tinker with body patterns, you tinker with genetic switches," says Carroll.

Besides containing the genetic sequences that dictate the order of amino acids in proteins, DNA contains noncoding sequences that tell cells when and where specific proteins should be expressed during development. These regulatory regions undergo evolutionary changes just as the coding regions of DNA do. But a relatively small change in a regulatory region can have a dramatic effect on the body plan of an organism. It can change the number of segments of an organism, and it can alter the appendages—themselves often segmented—that emerge from a body segment.

Patel, for example, studies this process in crustaceans, the segmented organisms that first appeared in the Cambrian period. "The particular animal we work on [the crustacean Parhyale hawaiensis] is remarkable because each segment comes from an individual row of cells in the embryo, so it's very easy to keep track of what goes on," he says. Patel and his colleagues have identified a number of genes that play a role in the segmentation process, and they have begun modifying the regulation of these genes to gauge the effects on development. They also have been relating changes in expression of the genes to changes in the segmentation of fossilized crustaceans. "In different species, different appendages have become specialized to do different things, and we're trying to develop a molecular understanding of how that occurs."

Patel, Kingsley, and Carroll all emphasize the importance of understanding how ecological forces have shaped an organism's evolution. Kingsley, for example, studies the evolutionary genetics of the stickleback—a small bony fish that lives in lakes, oceans, and coastal habitats throughout the Northern Hemisphere. He chose the stickleback as a model organism, he says, because different forms can be crossed and because thousands of papers have been written about the fish and its adaptation to different environments. "We were able to leverage a rich history of biological work to develop a full picture of this organism, from its DNA to its ecology," he says.

After the Cambrian period, animals moved from oceans onto land and evolved into insects, amphibians, and reptiles. The dinosaurs that came to dominate suddenly went extinct, quite likely because a giant asteroid hit Earth. In the absence of dinosaurs, early mammals diversified and spread, eventually producing many of the creatures familiar to us today.

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