Introduction

Teeth are one of the most distinctive body parts for vertebrates and have played a central role in their evolution. The molecular pathways and morphogenetic processes included in tooth development have been the focus of vigorous investigation over the past few decades, and the tooth is an important model system for many areas of research.¹ Along with that, the preservation of teeth in the fossil record makes these organs vital for the work of paleontologists, anthropologists, and evolutionary biologists.²

From the archaean Eon to today

The first finding of tooth-like structures is believed to be in the posterior pharynx of jawless fishes more than 500 million years ago. With the evolution of jawed vertebrates, teeth developed on oral jaws and helped to create the dominance of gnathostomes on land and in water. Two theories have been put forward regarding the evolution of teeth, these are: The “Outside-in” Theory suggests that teeth evolved from ectoderm-derived, skin denticles that folded and assimilated into the mouth. The “Inside-out” theory suggests that teeth evolved from endoderm, with the formation of pharyngeal teeth in jawless vertebrates and moved anteriorly to the oral cavity with the evolution of jaws. However, recent research suggests that neither of these are majorly correct.¹

Fate-mapping techniques using transgenic axolotls showed that teeth formed generally regardless of whether the oral epithelium was derived from ectoderm or endoderm. Taking existing species into consideration, such as certain cichlids, they possessed both oral and pharyngeal teeth. Pharyngeal teeth develop on pharyngeal jaws whereas oral teeth develop in hox-negative, ectoderm-derived regions. Pharyngeal teeth of jawless vertebrates appear to utilize a predated gene network that connects to the origin of oral jaws, oral teeth, and ectodermal appendages. Despite the differences between oral and pharyngeal teeth, they are both oral and also show prominent similarities in their gene regulatory networks. From these multiple studies, it is evident that teeth have evolved both inside and out and that the gene network for mesenchyme has remained a constant throughout this evolution.¹

Change in number and morphology of teeth

Both humans and rodents have been considered to evolve from a common mammalian ancestor that is thought to have had a full set of teeth comprising three incisors, one canine, four premolars, and three molars in each dental quadrant that replaced its teeth a single time. In addition to modifications in the number of teeth, the morphology of mammalian teeth is enormously diverse. These modifications involve variations in cusp shape and crest organization. A fascinating case of evolution in mammals especially humans is the case of multi-cusped teeth.³

The question that arises when the case of different types of teeth are discussed is why did mammals need more complex teeth in the first place? How was the selection pressure so strong that it caused such a diversity in tooth morphology? A multi-cusp dentition allows for more proficient mastication of food is just one of the features that mammals have evolved in order to acquire extra energy; enhanced particle size reduction by the teeth can improve digestion by making more surface area available for contact with digestive enzymes. In some mammals, one cusp may become dominant articulating with the basin of the opposing tooth to create a mortar-and-pestle configuration for better crushing of food.

However, it is still arguable whether dentition evolved solely because of the need to manipulate and process a wider range of food items and maintain integrity of the structure of a tooth.³

Dollo’s Law and the peculiar case of frogs

Dollo’s law of irreversibility states that structures that are lost over evolutionary time cannot be regained in the same form. However, an exception is the case Gastrotheca guentheri which is known as the only frog with teeth on the lower jaw. Mandibular teeth were lost in the ancestor of frogs more than 200 million years ago and subsequently regained in G. guentheri.

Recent work has acknowledged that teeth have been completely lost more than 20 times in frogs, which is a higher occurrence of evolution than in any other major vertebrate ancestory.³ G. guentheri may have re- evolved mandibular teeth by using the same developmental mechanism used in the formation of upper jaw teeth. The gene regulatory network that controls tooth initiation, development, and differentiation is likely modular and this organization may be responsible for shaping this pattern of heterogeneous tooth loss across jaw regions. Investigating the developmental genetics of tooth formation in the jaws of frogs may provide insights into whether a transient tooth signaling program is present in the lower jaw, providing the possible mechanism underlying the re-evolution of lost mandibular teeth in G. guentheri.

Does the environment play a role in evolution?

Ecological reactions of mammals to climate change are renowned in the fossil record by adaptive significance of morphological variations. To understand the role of dietary behaviour on functional changes of dental morphology in rodent evolution, research carried out to examine evolutionary change of tooth shape in late Miocene Siwalik murine rodents showed an inclination toward C4 diets during late Miocene ecological change indicated by carbon isotopic evidence. Tooth shapes of the two species examined (Progonomys clade and Karnimata clade) are similar at their sympatric origin but get distant from each other with decreasing overlap through time.

Shape change in the molars and premolars of Karnimata clade is related with greater efficiency of chewing for tough diets than in the Progonomys clade. Larger body mass in Karnimata may be correlated to exploitation of lower-quality food items, such as grasses, than in smaller bodied Progonomys. The functional and Eco physiological aspects of Karnimata exploiting C4 grasses are linear with their isotopic dietary needs relative to Progonomys.⁴