Introduction
Understanding the dietary habits of extinct hominins is a challenging but crucial aspect of paleoanthropology. By analyzing dental and mandibular morphology, researchers can infer the types of foods consumed and the physical properties of those foods that shaped the evolution of hominin dentition. This article explores the evolution of diet in the Pan-Homo and hominin clades, focusing on 12 key variables related to dental and mandibular morphology. It examines how the physical properties of food, such as hardness and toughness, have influenced dental adaptations in various hominin groups.
Methods for Reconstructing Hominin Diets
Reconstructing the diets of extinct hominins involves multiple lines of evidence, including:
- Dental and gnathic morphology: Analyzing the shape and size of teeth and jaws.
- Functional analysis: Assessing how teeth and jaws function during food processing.
- Dental microwear: Examining microscopic wear patterns on tooth surfaces.
- Stable isotopes: Analyzing the isotopic composition of tooth enamel and bone.
- Archeology: Studying tools and other artifacts associated with food processing.
- Referential and analogical extant models: Comparing hominin morphology to that of living primates with known diets.
Focus on Dental and Mandibular Variables
This article concentrates on 12 variables (nine dental and three mandibular) for which data are available about extinct hominins. These variables provide insights into the dietary adaptations of different hominin groups. The expression of each of those variables is in extant chimpanzees, the earliest hominins, archaic hominins, megadont archaic hominins, and an inclusive grouping made up of transitional hominins and pre-modern Homo. Hypotheses about what the states of these variables would be in the last common ancestor of the Pan-Homo clade and in the stem hominin are also presented.
Taxonomic Considerations
Given the ongoing revisions in hominin systematics and the recognition of numerous hominin species, this analysis avoids identifying dietary adaptations at the species level. Instead, it focuses on broader morphological clusters, including:
- Earliest hominins: Ranging from 7 to 4 Ma, including Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus kadabba, and Ar. ramidus.
- Archaic hominins: Ranging from 4 to 2.5 Ma, including taxa such as A. afarensis and A.
- Archaic megadont hominins: Ranging from about 2.5 to 1.1 Ma, including P. boisei and Paranthropus robustus.
- Pre-modern Homo: Ranging from about 2 Ma to approximately 18 ka.
Dental Characteristics of Different Hominin Groups
Last Common Ancestor (LCA) of Pan-Homo
- The dentition of the LCA of chimpanzees/bonobos and hominins likely resembled that of the modern common chimpanzee.
- Incisors were likely large and procumbent (projecting forwards) when first erupted.
- Canines were sexually dimorphic, with males having more projection beyond other teeth than females.
- Premolar crowns were relatively small, with the second molar (M2) being the largest.
- Enamel thickness over the postcanine teeth may have differed from modern chimpanzees, potentially being thicker.
Earliest Hominins
- Incisors were probably also procumbent on eruption.
- Canine teeth are said to have been relatively small.
- The second molars were generally the largest of the molar teeth.
- Hominins all tend to have low blunt cusped molars and premolars - the so-called bunodont type of dentition.
Archaic Hominins
- Incisors were probably also procumbent on eruption.
- The second molars were generally the largest of the molar teeth.
- It is generally agreed that the earliest hominins to have had thick enamel were the archaic hominins.
- Hominins all tend to have low blunt cusped molars and premolars - the so-called bunodont type of dentition.
Archaic Megadont Hominins
- Incisors were more vertically implanted and relatively smaller.
- There was a distinct tendency for M3 to be as large, if not the largest, of the molars particularly with respect to the mandibular dentition.
- Enamel became even thicker in the ‘hyper-thick’ archaic megadonts.
- Hominins all tend to have low blunt cusped molars and premolars - the so-called bunodont type of dentition.
Pre-Modern Homo
- Incisors were more vertically implanted and relatively smaller.
- The first molar is usually the largest of the molars, following extensive reduction in overall tooth size.
- Hominins all tend to have low blunt cusped molars and premolars - the so-called bunodont type of dentition.
- Even in modern humans, the enamel of the postcanine teeth is much thicker than in the equivalent teeth of the extant apes
The Role of Food's Physical Properties
The physical properties of ingested foods exert a strong influence on tooth form. These properties can be categorized into:
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- External physical characteristics: The form of the external surface of the food item.
- Internal mechanical properties: The food's resistance to fracture.
Mechanical Properties: Hardness and Toughness
When a food particle is loaded by a force, stresses are created within the particle, potentially causing deformation. The relationship between stress and strain (deformation normalized to size) is crucial in understanding how foods break down.
- Young's Modulus (E): The stress/strain ratio, defining the ease of deformation.
- (ER)0.5: An index that defines the resistance of foods to fracture when force is limiting, closely related to what biologists understand as "hardness."
- (R/E)0.5: An index that defines resistance to fracture when deformation is limiting, closely related to what biologists term "toughness."
These indices can aid in considering dietary scenarios, particularly those involving plant foods, which have been central to debates about primate diets.
The Deformation Transition
The behavior of solid foods changes with respect to their particle sizes at a threshold given by (ER)0.5/Y, where Y is the yield stress. This threshold, known as the "deformation transition," is of great potential importance in understanding the evolution of dental adaptations.
Anterior Tooth Morphology and Food Preparation
Aspects of anterior tooth morphology are critical for food preparation prior to ingestion. For example, the incisors are essential for peeling fruit. The size and shape of incisors can indicate the importance of this function in a hominin's diet.
Postcanine Dentition and Food Processing
Features of the postcanine dentition, such as the overall and relative size of the crowns, are related to the reduction in the particle size of food. Larger molars with thicker enamel are better suited for processing tougher foods.
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Enamel Macrostructure and Microstructure
Information about the macrostructure (e.g., enamel thickness) and microstructure (e.g., extent and location of enamel prism decussation) of the enamel cap can be used to make predictions about the types of foods consumed by extinct hominins. Thicker enamel is generally associated with diets that include harder, more abrasive foods.
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