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, who became an HHMI investigator at the University of Wisconsin–Madison in 1990, is an internationally recognized evolutionary biologist. In April 2010, Carroll was named vice president for science education at HHMI, where he is responsible for directing the Institute’s portfolio of science education activities. HHMI’s science education programs are helping to inspire and train a new generation of students and scientists and to advance the public understanding of science.
A noted chronicler of Charles Darwin’s scientific legacy, Carroll has spent his career studying the genetic underpinnings of evolution and development, identifying the molecular mechanisms that lead to new traits and species. His “Dialogues of Discovery” lecture will detail the search for the origins of species and describe some of the great adventures that have occurred in that quest over the past 200 years. Carroll will chronicle the exploits of a group of explorers who walked where no one had walked, saw what no one had seen, and thought what no one else had thought. “Their achievements sparked a revolution that changed, profoundly and forever, our perception of the living world and our place within it,” Carroll says.
Carroll’s research focuses on identifying the ways new animal forms have evolved, and his studies of a wide variety of animal species have revealed how changes in the genes that control animal development shape the evolution of body parts and body patterns. Using the tools of genetics and 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 explains. “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—contained the set of “master” genes needed to grow appendages, such as legs, arms, claws, fins, and antennae. Moreover, Carroll says, 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 notes. “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 studying the common fruit fly, Drosophila melanogaster, to reveal how genes control the development and evolution of animal form. His innovative approach to studying evolution has led scientists to a more detailed understanding of how animal patterns and diversity evolve. For example, 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 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 provides critical evidence of how 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 explains. “These accidents are then preserved as small changes in the DNA.”
In addition to his scientific achievements, Carroll is recognized as an exemplary educator. In 2009, he received the Viktor Hamburger Outstanding Educator Prize from the Society for Developmental Biology. He is also a recipient of the Distinguished Service Award from the National Association of Biology Teachers. Along with David Kingsley, a fellow HHMI investigator, Carroll delivered the Institute’s 2005 Holiday Lectures on Science, “Evolution: Constant Change and Common Threads.”
He is also the author of several books, including Remarkable Creatures: Epic Adventures in the Search for the Origins of Species, a finalist for the 2009 National Book Award in nonfiction. He writes a monthly column (also called “Remarkable Creatures”) for the science section of The New York Times and has served as a consulting producer for the public television program NOVA. In March 2010, Carroll received the Stephen Jay Gould Prize, in recognition of his efforts to advance public understanding of evolutionary science.