Dietary competition played a key role in the evolution of early primates


August 1, 2018

Since Darwin first laid out the basic principles of evolution by means of natural selection, the role of competition for food as a driving force in shaping and shifting a species’ biology to outcompete its adversaries has played center stage. So important is the notion of competition between species, that it is viewed as a key selective force resulting in the lineage leading to modern humans.

The earliest true primates, called “euprimates,” lived about 55 million years ago across what is now North America. Two major fossil euprimate groups existed at this time: the lemur-like adapids and the tarsier-like omomyids. Dietary competition between these similarly adapted mammals was presumably equally critical in the origin and diversification of these two groups. Though it has been hinted at, the exact role of dietary competition and overlapping food resources in early adapid and omomyid evolution has never been directly tested. Diagram competition vs noncompetition 2 Three models of niche competition between euprimates and non-euprimate mammals. Non-euprimates thrived across North America prior to euprimate arrival about 55 million years ago (large tree, left). After euprimate arrival (center column), these two groups could have: occupied separate niches with no competition (top row, right); occupied the same niche with one group ultimately displacing the other to reduce competition (middle row, right); or coexisted with minimal competition (bottom row, right). Download Full Image

New research published online Tuesday in the Proceedings of the Royal Society B led by Laura K. Stroik, an alumna of ASU’s School of Human Evolution and Social Change (SHESC) and currently assistant professor of biomedical sciences at Grand Valley State University, and Gary T. Schwartz, associate professor with SHESC and research scientist at ASU’s Institute of Human Origins, confirms the critical role that dietary adaptations played in the survival and diversification of North American euprimates.

“Understanding how complex food webs are structured and the intensity of competition over shared food resources is difficult enough to probe in living communities, let alone for communities that shared the same landscape nearly 55 million years ago,” Stroik said.

The researchers utilized the latest in digital imaging and micro CT scanning on more than 350 fossil mammal teeth from geological deposits in North America. They sought to quantify the 3D surface anatomy of molars belonging to extinct representatives of rodents, marsupials and insectivores — all of which were found within the same geological deposits as the euprimates and were thus likely real competitors.

The high-resolution scans allowed them to capture and quantify details of how sharp, cresty or pointy the teeth were. In particular, they looked at molars, or teeth at the back of the mouth, useful in pulverizing and crushing food or prey. The relative degree of molar sharpness is directly linked to the broad menu of dietary items consumed by each species.

Tooth ct scans

Examples of micro CT scans of molars and the types of measurements researchers were looking at.

Stroik and Schwartz used these aspects of molar anatomy to compute patterns of dietary overlap across some key fossil groups through time. These results were then weighed against predictions from three models of how species compete with one another drawn from the world of theoretical ecology. The signal was clear: Lineages belonging to the adapids largely survived and diversified without facing competition for food. The second major group, the omomyids, had to sustain periods of intensive competition with at least one contemporaneous mammal group. As omomyids persisted into more recent geological deposits, it is clear that they evolved adaptive solutions that allowed them to compete and were usually victorious.

"The results showed adapids and omomyids faced different competitive scenarios when they originated in North America," Stroik said. 

“Part of what makes our story unique is that for the first time we compared these fossil euprimates to a range of potential competitors from across a diverse group of mammals living right alongside adapids and omomyids, not just to other euprimates,” Schwartz said. “Doing so allowed us to reconstruct a far greater swath of the ecological landscape for these important early primate relatives than has ever been attempted previously.”

The key advance of this new research is the demonstration that diet did in fact play a fundamental role in the establishment and continued success of euprimates within the North American mammalian paleocommunity. An exciting outcome is the development of a new quantitative tool kit to diagnose patterns of dietary competition in past communities. This will now allow them to explore the role that diet and competition played in how some of these fossil euprimates continued to evolve and diversify to give rise to living lemurs and all higher primates.

Julie Russ

Assistant director, Institute of Human Origins

480-727-6571

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

480-727-6571