A study of skull growth and tooth emergence reveals that timing is everything

October 6, 2021
Editor’s note: This story is featured in the 2021 year in review.

Six, 12 and 18. These are the ages that most people get their three adult molars, or large chewing teeth towards the back of the mouth. These teeth come in at a much later age than they do in our closest living relative, the chimpanzee, who get those same adult molars at around 3, 6 and 12 years old.

Paleoanthropologists have wondered for a long time how and why humans evolved molars that emerge into the mouth at these specific ages, and why those ages are so delayed compared to living apes. Scientists at Arizona State University and the University of Arizona unveil a study in Science Advances this week that they think has finally cracked the case. circular evolutionary tree skull growth A circular evolutionary tree displays skull growth and associated changes in the chewing apparatus in skulls of juvenile (inner ring) and adult (outer ring) primates. The study includes species of apes and humans (blue arrows), Central and South American monkeys (pink arrows), Asian and African monkeys (green arrows) and lemurs and lorises (yellow arrows). Image courtesy of H. Glowacka and G.T. Schwartz

Humans are unusual primates. We are highly intelligent, extremely social, remarkably resourceful, able learners and skilled teachers and, as a result, a remarkable evolutionary success story. A key aspect of our biology allowing these components of the human experience to evolve is our unique “life history,” or the overall pace of life, including how fast we grow, how long we are dependent on mothers for nutritional support, how long it takes us to reach sexual maturity and how long we live. Amazingly, clues to most of these components of our human biology are connected with our teeth.

The one dental feature intimately associated with the pace of growth and life history is the ages at which our adult molars cut through the gumline. For many decades, evolutionary anthropologists have leveraged the very tight relationship — which exists across all primates — between the pace at which these adult molars emerge into the mouth with the overall pace of life. Modern humans, for instance, grow up incredibly slowly, have a very long and protracted life history and emerge their adult molars very late in life, later than any other living or extinct primate.

“One of the mysteries of human biological development is how the precise synchrony between molar emergence and life history came about and how it is regulated,” said Halszka Glowacka, lead author and assistant professor at the University of Arizona, College of Medicine-Phoenix. Glowacka is also a doctoral graduate of the evolutionary anthropology program at ASU.

Glowacka and paleoanthropologist Gary Schwartz, a researcher with the Institute of Human Origins and professor in the School of Human Evolution and Social Change, published their study this week that provides the first clear answer — it is the coordination between facial growth and the mechanics of the chewing muscles that determines not just where but when adult molars emerge. This delicate dance results in molars coming in only when enough of a “mechanically safe” space is created. Molars that emerge “ahead of schedule” would do so in a space that, when chewed on, would disrupt the fine-tuned function of the entire chewing apparatus by causing damage to the jaw joint. 

For the study, Glowacka and Schwartz created 3D biomechanical models of skulls, including the attachment positions of each major chewing muscle, throughout the growth period in nearly two dozen different species of primates ranging from small lemurs to gorillas. When combined with details about the rates of jaw growth in these species, their integrative models revealed the precise spatial relationship and temporal synchrony of each emerging molar within the context of the growing and shifting masticatory system.

The authors note that this research establishes two things — it convincingly demonstrates that it is the precise biomechanical relationship between growing faces and growing chewing muscles that results in the tight and predictive relationship between dental development and life history, and it reveals that our species’ delayed molar emergence schedules are a result of the evolution of overall slow growth coupled with short jaws and retracted faces — faces situated directly beneath our braincase.

Their study revealed that the combination of how fast jaws grow with how long or protruding jaws will ultimately become in adults determines the timing of when molars will emerge. Modern humans are special among primates given our prolonged growth profiles and our retracted faces with short dental arcades.

“It turns out that our jaws grow very slowly, likely due to our overall slow life histories and, in combination with our short faces, delays when a mechanically safe space — or a ‘sweet spot,’ if you will — is available, resulting in our very late ages at molar emergence,” said Schwartz.

“This study provides a powerful new lens through which the long-known linkages among dental development, skull growth and maturational profiles can be viewed,” said Glowacka.

The researchers plan to apply their model to fossil human skulls to answer questions about when slowed jaw growth and delayed molar emergence first appeared in our fossil ancestors.

They also realize that the approach taken in this study could have implications for clinical dentistry. Because molars do not emerge until a point when enough facial growth has occurred and the sweet spot appears, “the finer details of the model could be explored in more samples to help understand the phenomenon of impacted wisdom teeth in humans,” noted Glowacka.

Julie Russ

Assistant director, Institute of Human Origins


The secret life of teeth: Evo-devo models of tooth development

ASU research explains variability in molar crown configuration

April 11, 2018

Across the world of mammals, teeth come in all sorts of shapes and sizes. Their particular size and shape are the process of millions of years of evolutionary fine-tuning to produce teeth that can effectively break down the foods in an animal’s diet. As a result, mammals that are closely related and have a similar menu tend to have teeth that look fairly similar. New Arizona State University research suggests, however, that these similarities may only be “skin deep.”

The teeth at the back of our mouths — the molars — have a series of bumps, ridges and grooves across the chewing surface. This complex dental landscape is the product of the spatial arrangement of cusps, which are conical surface projections that crush food before swallowing. How many cusps there are, how they are positioned and what size and shape they take together determine a molar's overall form or configuration. teeth A simple, straightforward developmental rule — the “patterning cascade” — is powerful enough to explain the massive variability in molar crown configuration over the past 15 million years of ape and human evolution. Photo courtesy Pixabay.com

Over the course of hominin (modern humans and their fossil ancestors) evolution, molars have changed markedly in their configuration, with some groups developing larger cusps and others evolving molars with a battery of smaller extra cusps.

Charting these changes has yielded powerful insights into our understanding of modern human population history. It has even allowed us to identify new fossil hominin species, sometimes from just fragmentary tooth remains, and to reconstruct which species is more closely related to whom. Exactly how some populations of modern humans, and some fossil hominin species, evolved complex molars with many cusps of varying sizes, while others evolved more simplified molar configurations, however, is unknown. 

In a study published this week in Science Advances, an international team of researchers led by ASU’s Institute of Human Origins and School of Human Evolution and Social Change found that a simple, straightforward developmental rule — the “patterning cascade” — is powerful enough to explain the massive variability in molar crown configuration over the past 15 million years of ape and human evolution.

“Instead of invoking large, complicated scenarios to explain the major shifts in molar evolution during the course of hominin origins, we found that simple adjustments and alterations to this one developmental rule can account for most of those changes,” said Alejandra Ortiz, a postdoctoral researcher with the Institute of Human Origins (IHO) and lead author of the study.

Model of molar cusps

CT-rendered chimpanzee cranium (left) with enlarged image of a virtually extracted molar (middle). The outer layer, called enamel, is rendered transparent revealing the 3-D landscape of a molar’s underlying dentine core. The location of embryonic signaling cells that will determine future cusp position is indicated by yellow spheres (middle). The distribution of these signaling centers across the dentine landscape is measured as a series of intercusp distances (red arrows in right, top), which determines the number of cusps that will ultimately develop across a molar crown, as well as the amount of terrain mapped out by each cusp (dashed lines in right, bottom). Image credit: Alejandra Ortiz and Gary Schwartz

In the past decade, researchers’ understanding of molar cusp development has increased a hundredfold. They now know that the formation of these cusps is governed by a molecular process that starts at an early embryonic stage. Based on experimental work on mice, the patterning cascade model predicts that molar configuration is primarily determined by the spatial and temporal distribution of a set of signaling cells.

Clumps of signaling cells (and their resultant cusps) that develop earlier strongly influence the expression of cusps that develop later. This cascading effect can result in either favoring an increase in the size and number of additional cusps or constraining their development to produce smaller, fewer cusps. Whether this sort of simple developmental ratchet phenomenon could explain the vast array of molar configurations present across ape and human ancestry was unknown.

Using state-of-the-art microcomputed tomography and digital imaging technology applied to hundreds of fossil and recent molars, Ortiz and her colleagues created virtual maps of the dental landscape of developing teeth to chart the precise location of embryonic signaling cells from which molar cusps develop. To the research team’s great surprise, the predictions of the model held up, not just for modern humans, but for over 17 ape and hominin species spread out across millions of years of higher primate evolution and diversification.

“Not only does the model work for explaining differences in basic molar design, but it is also powerful enough to accurately predict the range of variants in size, shape and additional cusp presence, from the most subtle to the most extreme, for most apes, fossil hominins and modern humans,” Ortiz said.

These results fit with a growing body of work within evolutionary developmental biology that says very simple, straightforward developmental rules are responsible for the generation of the myriad complexity of dental features found within mammalian teeth.

“The most exciting result was how well our results fit with an emerging view that evolution of complex anatomy proceeds by small, subtle tweaks to the underlying developmental toolkit rather than by major leaps,” said Gary Schwartz, a study coauthor, paleoanthropologist with IHO and associate professor with the School of Human Evolution and Social Change.

This new study is in line with the view that simple, subtle alterations in the ways genes code for complex features can result in the vast array of different dental configurations that we see across hominins and our ape cousins. It is part of a shift in our understanding of how natural selection can readily and rapidly generate novel anatomy suited to a particular function.

“That all of this precise, detailed information is contained deep within teeth,” continued Schwartz, “even teeth from our long-extinct fossil relatives, is simply remarkable.”

“Our research, demonstrating that a single developmental rule can explain the countless variation we observe across mammals, also means we must be careful about inferring relationships of extinct species based on shared form,” said Shara Bailey, a coauthor and paleoanthropologist at New York University. “It is becoming clearer that similarities in tooth form may not necessarily indicate recent shared ancestry,” added Bailey, who, in 2002, was the first doctoral graduate to be affiliated with IHO.

Julie Russ

Assistant director, Institute of Human Origins