One rule to grow them all: Using 'baby' teeth to predict hominin tooth size


February 24, 2016

What regulates the size of an organism’s teeth? The reduced size of our back teeth, or molars, is one of the defining attributes separating modern humans from hominins (our extinct relatives). The earliest hominins, known collectively as australopiths (best exemplified by “Lucy,” a member of the species Australopithecus afarensis), had larger teeth overall and their largest molar was positioned closer to the rear of the mouth. Fossil species within our own genus, Homo, not only had smaller teeth, but their biggest molar was situated closer to the middle of their jaw.

This trend of reduction in molar size continues in modern humans, where some of us may never develop our rearmost molar, or wisdom tooth. While these differences have been traditionally linked to variations in function and diet, constraints during dental development play an important role in how and why teeth grow to be a certain size. Download Full Image

A new study by an international team including ASU researchers published in the journal Nature, combined tools from embryology, comparative anatomy and computational biology to reveal that a single embryonic rule has regulated hominin tooth size. In addition, it is a subtle alteration in the expression of that rule that accounts for the varying patterns in tooth size encountered throughout the entire human evolutionary story.

“One of the more exciting findings of this new study is that the size of teeth, including the molars, in modern humans and all fossil hominins, conforms to the predictions of just one simple, elegant developmental rule called the ‘inhibitory cascade’,” said Gary Schwartz, a coauthor and paleoanthropologist with the Institute of Human Origins and School of Human Evolution and Social Change. 

“Under this simple rule, discovered in earlier research in mice, the size of one molar regulates the development of its neighbors, constraining the sizes of subsequently developing teeth,” added Susanne Daly, a coauthor and ASU doctoral candidate in evolutionary anthropology.

While paleoanthropological interest in this dental development rule focuses mostly on adult teeth, the researchers had a hunch that this was only part of the story.

“Most mammals, including all living humans and all extinct human ancestors, have two sets of teeth: a milk set, or ‘baby’ teeth, and an adult set,” said Kierstin Catlett, a coauthor and ASU doctoral candidate in evolutionary anthropology. “Milk molars and adult molars are both critical elements for breaking down food to fuel an individual’s growing body and brain.”

 

Milk teeth and adult molars

 

“As milk teeth and adult teeth develop side by side within growing faces, it is not surprising to think that one set has a powerful influence on how the other set develops,” added Schwartz.

The researchers found strong evidence that the inhibitory cascade pattern for adult molars was a direct outcome of how big their milk molars are.

“Amazingly, the size of milk molars, which start developing prior to birth, have a powerful cascading effect on the size of later-forming adult molars,” said Catlett.

This result encouraged the team to examine fossil hominin teeth from across the last 5 million years, focusing now on milk molars and adult molars together, as a developmentally linked set. They found that australopiths tended to follow one pattern, while sometime after 2.8 million years ago, a new pattern emerged in members of our own genus Homo. This suggests that selective pressures leading to this shift in pattern were a key adaptation in the lineage that eventually led to modern humans.

“One important implication of this work is the predictive power of the model, which now allows paleoanthropologists to predict the size of missing fossil teeth,” said Kathleen Paul, a coauthor and doctoral candidate who works with the School of Human Evolution and Social Change's Center for Bioarchaeological Research.

There appears to be a coordinated developmental dance between milk and adult teeth, such that slight deviations can produce profound downstream changes.

“What is really exciting is that our results fit an emerging picture that only a very small amount of tinkering during development, not a wholesale reorganization, is enough to generate a whole range of different anatomies, including the vast diversity in tooth sizes of our ancestors,” said Schwartz.

 

The Institute of Human Origins is a research center of the College of Liberal Arts and Sciences. The School of Human Evolution and Social Change and its Center for Bioarchaeological Research are both part of the College of Liberal Arts and Sciences.

Julie Russ

Assistant director, Institute of Human Origins

480-727-6571

ASU's Origins Project announces recipient of $10K Postdoctoral Lectureship Award


February 24, 2016

The Origins Project is pleased to announce Aomawa Shields of the University of California-Los Angeles is the recipient of the 2016 Origins Project Postdoctoral Lectureship Award.

This $10,000 award, the largest of its kind in the world, is offered to promising young scholar-scientists on the basis of their scholarly achievement and potential, as well as their skills in science communication. Part of the award is a week-long residency at Arizona State University where Shields will offer a series of departmental colloquia and a large public lecture in early April.  Aomawa Shields of the University of California-Los Angeles is the recipient of the 2016 Origins Project Postdoctoral Lectureship Award. Download Full Image

Shields’ research focuses on exploring the climate and potential habitability of extrasolar planets orbiting low-mass stars. Currently a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellow, Shields is also a University of California President's Postdoctoral Fellow in the UCLA Department of Physics and Astronomy and the Harvard-Smithsonian Center for Astrophysics.

She is also a TED Fellow whose TED talk “How We’ll Find Life on Other Planets” has nearly 1 million views. In addition to her many scientific accomplishments, Shields is a classically trained actor with an Master of Fine Arts degree, a skill she uses to communicate her love of science. 

“Dr. Shields rose to the top of a very impressive list of nominees,” said Origins Project director Lawrence Krauss. “Her cutting edge research in a topic of great current interest, combined with her clear communication skills, make her the ideal Origins Postdoctoral Award winner.”

“We are extremely happy she has accepted the award and are looking forward with anticipation to her visit, as should members of the ASU community and the public at large. Her presentations will be thrilling.”

For more information on the public lecture, please see: https://origins.asu.edu/events.

The Origins Project Postdoctoral Lectureship Award is one of many awards and scholarships that The Origins Project has available to students, researchers and scholars. For more information visit https://origins.asu.edu/awards-scholarships.

Written by Michelle Iwen, Origins Project