|
|
|
   
   
|
HARVARD GAZETTE ARCHIVES What Killed the Dinosaurs? It's more complex than a comet's impact, says Charles Marshall By Alvin Powell
When Charles Marshall was a child in Australia, his imagination was fired by the idea that an alien world in his backyard had vanished without a trace. A world dominated by volcanoes, ferns, and dinosaurs had been replaced by the lone sheep that placidly kept the lawn trimmed and supplied the family with wool. But unlike many other former dinosaur-crazy 5-year-olds, Marshall’s passion led him to a career in paleontology, a lifetime of trying to understand the interplay between inheritance, environment, and catastrophe in directing evolution’s path. "I was one of those 4- or 5-year-olds who got addicted to dinosaurs," said Marshall, who in July was named a professor in the departments of Earth and Planetary Sciences and Organismic and Evolutionary Biology. "What struck me was that they were not imaginary, but real; that they were utterly different from what we know today; and that we can only learn about them through the fossil record." Marshall’s work attracted media attention in 1996. He and University of Washington geologist Peter Ward concluded that there may have been other causes than just the comet or asteroid impact responsible for the extinctions at the end of the Cretaceous Period, when all living dinosaur species died out. Marshall used a statistical analysis of the fossil record to conclude that a major drop in sea level – preceding the impact by 150,000 to 30,000 years – may have led to as much as 25 percent of the huge number of extinctions that took place. Another 25 percent of the extinctions, the pair theorized, were so-called "background" extinctions and would have happened without the impact or the drop in sea level. Marshall’s theory lacks the simplicity of a comet crash, but reflects the complexity of his broader work to uncover the secrets of evolution. Working on a molecular scale, Marshall is also trying to understand how the genetic circuitry that controls gene expression evolves, by transplanting the control circuitry of a known beetle gene into the fly Drosophila and observing what happens. "Surprisingly, most of the genes that lead to structures such as legs or wings seem to be preserved [in both] beetles and flies, despite their divergence 300 million years ago," Marshall said. "It appears that morphological innovation stems from changes in the time and place of expression of particular genes. It’s not that a fly and a beetle look radically different because they have radically different genes. They actually have the same genes that are just switched on in different places and times." What happens with the beetle circuitry in the fly, Marshall said, is about the same as what happens with the fly’s own circuitry. That leads Marshall to conclude that not only are the genes of flies and beetles largely the same, but so also is at least some of the control circuitry. This suggests that the differences among species may be the result of relatively minor DNA changes. Biology Professor Daniel Hartl, chair of the Department of Organismic and Evolutionary Biology, described Marshall’s statistical analysis of the fossil record as "pathbreaking" and said his energy, creativity, and enthusiasm make him a good addition to the Department. "He is one of the few people who can deal on the one hand with the fossil record and on the other hand do wet lab work in molecular biology," Hartl said. "We are always looking for energetic and creative people who love ideas and who have a passion for science." Marshall’s work both with genes and with the fossil record aim toward a common goal: understanding the driving forces behind evolution. "Are geologic changes (such as a comet crash) largely the cause, or are the causes the internal unfolding of genetic [instructions], or is it a complicated interplay between geologic change and genetic changes?" Marshall asks. Fascinated by Systems Marshall said he was attracted to biology more out of a curiosity about the complexity of living systems than out of a love of animals. The family’s only pet was a sheep that they would shear for its wool on the dining room table, every family member taking a body part and snipping away with scissors. Marshall’s father conducted research at the Australian National University, a fact that had unforeseen repercussions for Marshall’s toys. "My soccer ball would get borrowed because he wanted to solve spherical geometry problems," Marshall said. "I’d get it back all covered with ink." Marshall attended Australian National University himself and received a bachelor’s degree in zoology, mathematics, and paleontology in 1984. From there, he traveled to the University of Chicago, where he received a master’s degree in 1986 and a doctorate in 1989, both in evolutionary biology. After postdoctoral work at Indiana University, Marshall became an assistant professor in the Department of Earth and Space Sciences at the University of California at Los Angeles in 1991. He was tenured in 1993 and made full professor in 1997. Marshall said he was attracted to Harvard by the quality of its faculty and of its student body. In teaching, Marshall said he takes a three-phase approach. For entry-level undergraduates, he said it’s critical to get them to fall in love with the subject. Phase two, for undergraduate concentrators and for beginning graduate students, is to teach them the key findings and techniques relevant to the subject. And for more advanced students, he said, he strives to help them develop their individuality in the subject as well as their own mastery. "At the Core level, the primary focus is to get students excited by the material," Marshall said. "By phase three, they are developing the individuality that is the pinnacle of the creative process."
Copyright 1999 President and Fellows of Harvard College |
