WWU scientist finds surprising link between teeth and the evolution of pregnancy
by Mary Gallagher
One of the many characteristics that set humans apart from other primates is our fetal growth rate, particularly how much our brains grow in utero. Humans have the highest prenatal growth rate of all primates living today. But how and when this high prenatal growth rate evolved has been a mystery, until now.
A new study led by Western Washington University paleoanthropologist Tesla Monson found a key piece of the answer in an unexpected place: the relative sizes of fossilized molars. Monson was the lead author of a study of teeth, prenatal growth rates and the evolution of pregnancy that was just published in the Proceedings of the National Academy of Sciences in October.
“While I don’t think that our humanness comes down to only our teeth, I do think that a lot of our humanness is recorded in our teeth,” said Monson. “This opens up a huge window to understanding pregnancy and gestation. We can take dental material and learn about pregnancy in human ancestors and other fossil primates.”
Monson and her colleagues studied data collected from fossilized molars and fragments of skulls from the Late Miocene epoch into the Plio-Pleistocene, from about 6 million years ago to roughly 12,000 years ago. They found that the primates’ prenatal growth rate and endocranial volume increased over the course of the last 6 million years, arriving at a human-like gestational growth rate just under 1 million years ago.
Monson says these findings line up with research into another key factor in human evolution: the use of tools. Around the time prenatal growth rates and brain sizes were increasing, human ancestors were changing their social structures and using more tools to gather resources. More resources were needed to support the increase in prenatal growth rates — and bigger brains — so collaboration and tool use was likely increasingly important. And more efficient gathering and sharing of resources, of course, fuels more growth.
“What’s really interesting to me,” said Monson, “is that we reach a prenatal growth rate that separates us from all other living apes between half a million to a million years ago, well before the evolution of the human species around 200–300 thousand years ago.”
Scientists have long hypothesized that the evolution of pregnancy is central to human evolution. This is because the relatively large brain of humans, compared to overall body size, results in difficult labor and delivery in humans. But the human brain is also known to be a key factor in the evolutionary success of our species — particularly the size of the human brain compared to the size of the human body, a concept known as encephalization. “We know that humans are one of the most encephalized animals to have ever existed,” Monson said. The evolution of our large brain is associated with increased technology and tool use, art, language development, complex social structures, and the ability to survive in ecologically diverse landscapes, among other things. This balance between growing a bigger brain and the pelvic constraints brought on by our relatively small body size is further impacted by the metabolic cost exacted on the body of the gestating parent.
“Teeth are a proxy for what is happening in other parts of the body.”
Hypotheses about the evolution of pregnancy have been hard to test, though. Many biomedical and bioanthropological studies have explored modern human gestation but studying the evolution of gestation has been much more difficult. This is because fossilized bone fragments of primates that lived millions of years ago provide a lot of insight into the evolution of hard tissues, but far less direct information about behavior and life history traits like pregnancy.
An assistant professor of anthropology at WWU, Monson and her colleagues, including Andrew Weitz, a postdoctoral research fellow in Western’s Environmental Science Department, and scientists from the Berkeley Geochronology Center and the National Center for Research on Human Evolution in Spain, studied fossils from the group of primates that includes apes and African and Asian monkeys; they found prenatal growth rates are highly correlated with endocranial volume and, surprisingly, with variation in the proportions of the molar teeth. This demonstrates that teeth can provide a proxy for both prenatal growth rates and brain size, which is especially important for scientists hoping to study the gestational development of our human ancestors, because dental remains are some of the most plentiful skeletal parts in the fossil record.
“Teeth are a proxy for what is happening in other parts of the body,” Monson said. “We show that they can be used like a roadmap to navigate the maze of interweaving genetic and developmental effects and understand more about life history in the past.”
Monson, who completed her doctorate in integrative biology at the University of California, Berkeley, leads Western’s Primate Evolution Lab, studying primate evolution, life history, reproductive ecology and the growth and development of the skeletal system. One of the graduate students in the lab will be building off this research, and Monson hopes to include more graduate and undergraduate students in the work going forward.
Discovering this crucial link between molar proportions and prenatal growth rates has created many new questions for evolutionary researchers, Monson says, including understanding the genetic mechanisms behind the relationship. Another key question is whether this relationship extends to other mammals, she says.
Learn more about Western Washington University’s programs in Anthropology at chss.wwu.edu/anthropology
