Volume 6, Issue 42 March 2009
Biting into Evolution
Paleontologist Leslea Hlusko spends three to six months a year prospecting for early hominid fossils in East Africa.
Bury a body, and time will take its toll. Flesh rots and even bone returns to the dust from whence it came. Teeth, however, are tough enough to withstand the processes of decay for much longer, making them among the best-preserved and most abundant mammalian fossils.
So it's no wonder that paleontologist Leslea Hlusko studies teeth as a means to observe the processes of evolution. A professor of integrative biology at UC Berkeley, Hlusko studies variation and genetic change in the dentition of modern primates. By obtaining a better understanding of how genes control tooth anatomy in living primates, she hopes to gain insights into the fossil record and human evolution.
To do this, Hlusko has amassed a collection of more than 600 dental castings from a colony of baboons at the Southwest Foundation for Biomedical Research in Texas. Her laboratory has painstakingly measured characteristics such as length and width, the presence of extra cusps, and enamel thickness for each dentition.
Because Hlusko has pedigrees for every baboon going back several generations, she can use statistical methods to identify whether or not these dental phenotypes, or expressed traits, are inherited. She can also tell whether these traits change independently or in lockstep with other dental characteristics.
Graduate student Sarah Amugongo examines a baboon skull in the UC Museum of Vertebrate Zoology. After studying the intricacies of primate teeth, many of Hlusko's undergraduate students go on to dental or medical school. Photo credit: Leslea Hlusko
This approach is known as quantitative genetics. "It's Mendel's pea plants-looking at how variation is inherited in a family," Hlusko says. "If genetic effects underlie the variation in a trait, then we know that the variation will respond to selection," Hlusko says.
Hlusko has already applied these findings to help interpret the fossil record. For years, anthropologists have used tooth enamel thickness as a means to help sort early primate fossils into either chimpanzee or hominid lineages. Their reasoning was that modern chimpanzees have a fairly thin coating of enamel on their molars, while human teeth have a somewhat thicker coating.
Hlusko's work shows that in baboons, enamel thickness changes independently from any other tooth trait measured. Independent traits can shift rapidly from generation to generation, because no other characteristics are impacted by the change.
"We argued that paleontologists might want to be careful about using enamel thickness as such an important trait in determining whether fossils are early hominids or early chimps," Hlusko says, "because our findings suggest this trait can vary from thick to thin quite quickly over evolutionary time, perhaps just one hundred thousand years or so."
Hlusko is now determining if the inheritance patterns she has found in baboons and has gone on to model genetically in mice, hold true for other primates. She and her students are now in the midst of a project measuring the teeth of other baboons and Old World monkey specimens in museum collections across the United States.
Hlusko and Dr. Jackson Njau of the National Natural History Museum in Arusha are using satellite technology to survey Tanzania for promising new fossil sites. Tanzania's Mount Hanang can be seen in the background. Photo credit: courtesy Leslea Hlusko
"I've seen evolutionary quantitative genetics hold up in other systems such as body and wing dimensions in flies, beak and body measurements in finches, and head size in New World monkeys. But nobody's yet looked at teeth or brought in fossils before," Hlusko says. If she sees the same patterns in such distantly related living species, the principles are likely to be universal enough to use to interpret primate fossils.
Hlusko is now reaching out to other colleagues who are working on the genetics and evolution of the skeleton, around the Bay Area. To provide a framework for these meetings, Hlusko has organized a consortium of biologists called the Genetics and Evolution of the Skeleton Research Initiative. Participants so far include medical researchers, evolutionary biologists, anthropologists, and paleontologists like Hlusko. The group will hold its first symposium on March 26, and feature scientists from UC Berkeley, UC San Francisco and Lawrence Livermore National Laboratory.
"By bringing everyone together we create an opportunity to share our various research methodologies and perspectives. Such a multidisciplinary approach to skeletal biology will likely help us better design our research questions and end up with more insightful answers," Hlusko says. She has high hopes that the initiative and the symposium will yield fruitful research partnerships and interdisciplinary study opportunities for students in the future.
A program schedule for the March 26 symposium is available as a PDF: Program Schedule