For decades, scientists studying evolution have relied on fossil records and animal morphology to painstakingly piece together the puzzle of how animals evolved. Today, growing numbers of scientists are using DNA evidence collected from modern animals to look back hundreds of millions of years to a time when animals first began to evolve. One of those leading the charge is molecular biologist Sean Carroll.
Carroll's research focuses on the way new animal forms have evolved, and his studies of a wide variety of animal species have dramatically changed the face of evolutionary biology. Using genetics and the tools of molecular biology, he is looking back to the dawn of animal life some 600 to 700 million years ago. It is so long ago that there are virtually no fossils or other physical clues to indicate what Earth's earliest animals were like.
"Evolution encompasses all of biology—it is our big picture," Carroll said. "When I was a student, we had a grand picture of animal evolution from the fossil record, but no knowledge whatsoever of how new animal forms arose. That is the mystery that I want to tackle."
Carroll's studies have uncovered evidence that an ancient common ancestor—a worm-like animal from which most of the world's animals evolved—had a set of "master" genes to grow appendages, such as legs, arms, claws, fins, and antennas. Moreover, Carroll noted, these genes were operational at least 600 million years ago and are similar in all animals, from humans to vertebrates, insects, and fish. What is different, however, is the way these genes are expressed, leading some animals to develop wings, and others to grow claws or feet.
"We found the same mechanism in all the divisions of the animal kingdom," Carroll noted. "The architecture varies tremendously, but the genetic instructions are the same and have been preserved for a very long period of time."
Carroll is also probing the common fruit fly, Drosophila melanogaster, to elucidate how genes control the development and evolution of animal morphology, or form. This innovative approach to studying evolution has led scientists to a more detailed understanding of how animal patterns and diversity evolve.
By analyzing the genetic origin of the decorative spots on a fruit fly wing, Carroll has discovered a molecular mechanism that helps to explain how new patterns emerge. The key appears to lie in specific segments of DNA, rather than genes themselves, that dictate when during development and where on an insect's body proteins are produced to create spots or other patterns.
The same molecular mechanism is likely at work in other animals, including humans, and helps to explain the pattern of stripes on a zebra or the technicolor tail of the peacock. Carroll and his colleagues chose to study the evolution of the wing spot on fruit flies because it is a simple trait with a well-understood evolutionary history. While ancient fruit fly species lack spots, some species have evolved spots under the pressure of sexual selection. The wing spots offer a survival advantage to males, who depend on the decorations to "impress" females to choose them in the mating process.
The discovery is important because it provides critical evidence of the way that animals evolve new features to improve their chances of reproductive success and survival. "We now have convincing proof that evolution occurs when accidental mutations create features such as spots or stripes that impart an advantage for attracting mates, hiding from or confusing predators, or gaining access to food," Carroll explained. "These accidents are then preserved as small changes in the DNA."