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.
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 at ASU's Institute of Human Origins and School of Human Evolution and Social Change (SHESC).
"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," adds 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," continues Catlett.
"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," adds 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 with SHESC'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.
"A simple rule governs the evolution and development of hominin tooth size," released in Nature on Feb. 24, 2016, Alistair R. Evans, E. Susanne Daly, Kierstin K. Catlett, Kathleen S. Paul, Stephen J. King, Matthew M. Skinner, Hans P. Nesse, Jean-Jacque Hublin, Grant C. Townsend, Gary T. Schwartz and Jukka Jernvall.