Introduction
Paranthropus boisei, an extinct hominin species, has fascinated paleoanthropologists for decades. Known for its robust craniodental features, particularly its large jaws and teeth, P. boisei was initially hypothesized to have consumed a diet of hard foods. However, isotopic and microwear analyses have challenged this view, suggesting a diet of compliant/tough foods like grasses and sedges. This article delves into the complexities surrounding the dietary habits of Paranthropus boisei, examining the morphological adaptations, biomechanical analyses, and isotopic evidence that contribute to our understanding of what this hominin species ate.
Craniodental Morphology and Feeding Adaptations
Paranthropus boisei exhibits a suite of craniodental features that have traditionally been interpreted as adaptations for feeding on hard foods. These include:
- Large and anteriorly placed attachments for the muscles of mastication.
- Huge, blunt premolars and molars with thick enamel.
- A massive mandible with a tall ramus.
- Visor-like zygomatics rising above the premolars.
Based on comparative and mechanical grounds, these traits have been hypothesized to be adaptations for efficiently producing high bite forces and strengthening the face against feeding stresses (Rak, 1983; Tobias, 1967). The anteriorly placed zygomatic root, in particular, is thought to play a key role in absorbing stresses associated with loads applied to the massively enlarged premolars (Rak, 1983). The inflated zygomatic arch and straight zygomaticoalveolar crest should reinforce the zygomatic arch and the entire midface against the pull of a hypertrophied masseter muscle (Rak, 1983).
Biomechanical Analyses and Bite Force
While the robust morphology of P. boisei suggests an adaptation for generating high bite forces, recent mechanical analyses have challenged this assumption. Finite element analysis (FEA), an engineering method used to examine how objects of complex geometry and material properties respond to complex loads, has been employed to assess the structural strength and bite force production efficiency of the P. boisei cranium.
One study used FEA to compare a well-preserved P. boisei cranium (OH 5) to crania of Australopithecus africanus and chimpanzees (Pan troglodytes) (Smith et al., 2015). The results showed that the facial skeleton of P. boisei is structurally strong, exhibits a strain pattern different from that in chimpanzees and A. africanus, and efficiently produces high bite force.
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However, the mammalian jaw functions as a Class III lever system, where the muscle force is applied between the biting tooth and the right and left temporomandibular joints (TMJs). For the system to be stable, the resultant of the masticatory muscle force vectors must fall within the triangle of support defined by the biting tooth and the two TMJs. P. boisei's combination of distally positioned molar teeth with a masseter muscle that originates far forward on the face could make it especially at risk of experiencing distractive reaction forces that would put the working-side TMJ into tension during unilateral molar biting (Greaves, 1978; Spencer, 1995, 1998, 1999).
According to the "constrained lever model," mammals adapted to generate high bite forces on the mesial teeth (incisors, canines, premolars) should combine anteriorly placed adductor muscles with either an anteriorly shifted tooth row, or a tooth row exhibiting reduced or missing distal teeth (molars). In contrast, mammals adapted to generate high bite forces on the distal teeth should exhibit widely separated TMJs but narrow dental arcades. P. boisei does not neatly fit either of these configurations, raising questions about the efficiency of its bite force production on the molars.
Walker (1981) argued that high bite forces were needed not necessarily to generate high stresses within hard foods, but rather to maintain occlusal pressures across a tooth row with an expanded occlusal surface. In this scenario, australopiths had a “high volume” or “bulk feeding” diet of food tissues of varying quality and material properties. In other words, larger teeth allow more food to be processed with each chew (e.g., Lucas, 2004).
Isotopic Evidence and Dietary Ecology
Isotopic analyses of P. boisei teeth have provided crucial insights into its dietary ecology. Stable carbon isotope analysis, in particular, has revealed the proportion of C3 and C4 plants in the diet. C3 plants, such as trees and shrubs, have lower δ13C values than C4 plants, such as grasses and sedges.
A study using stable isotopes showed that P. boisei had a diet dominated by C4 biomass, such as grasses or sedges (Cerling et al., 2011). Its diet included more C4 biomass than any other hominin studied to date, including its congener Paranthropus robustus from South Africa (Cerling et al., 2011). This finding suggests that P. boisei was well-adapted to utilizing open habitats in the Plio-Pleistocene.
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Dental Microwear Analysis
Dental microwear analysis involves examining the microscopic wear patterns on tooth surfaces to infer the types of foods consumed. Hard foods tend to produce more complex and abrasive microwear patterns, while soft foods produce smoother patterns.
Microwear analyses of P. boisei teeth have yielded mixed results. Some studies have suggested a diet of compliant/tough foods (Ungar et al., 2008; Van der Merwe et al., 2008; Cerling et al., 2011a; Ungar and Sponheimer, 2011), while others have suggested an adaptation to consume hard foods, perhaps including grass and sedge seeds (Jolly, 1970; Lucas et al., 1985; Peters, 1987; Strait et al., 2009).
The Debate: Hard Foods vs. Compliant/Tough Foods
The dietary habits of Paranthropus boisei remain a subject of ongoing debate. While its craniodental morphology suggests an adaptation for hard food consumption, isotopic and some microwear analyses point to a diet of compliant/tough foods like grasses and sedges.
One possible explanation for this apparent contradiction is that P. boisei may have consumed a mixed diet, utilizing its powerful jaws and teeth to process a variety of foods, including both hard and tough items. Another possibility is that the robust morphology of P. boisei was not primarily an adaptation for hard food consumption, but rather for processing large quantities of lower-quality foods, as suggested by the "bulk feeding" hypothesis (Walker, 1981).
It has been suggested that P. boisei consumed a diet of compliant/tough foods like grass blades and sedge pith. However, the blunt occlusal topography of this and other species suggests that australopiths are adapted to consume hard foods, perhaps including grass and sedge seeds.
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Postcranial Evidence and Tool Use
The recent discovery of hand and foot bones unambiguously associated with P. boisei has shed new light on its locomotor repertoire and potential tool-making abilities (Mongle et al.). The analysis of these bones shows that although P. boisei was capable of making and using tools, its hand shared similarities with members of our own genus Homo while evolving its own capabilities (Orr et al.).
The ability to make and use tools would have allowed P. boisei to access a wider range of food resources, potentially including underground storage organs (USOs) and other difficult-to-obtain items.
Implications for Understanding Early Hominin Evolution
The study of Paranthropus boisei diet has important implications for understanding the ecological roles of different early hominin species. The fact that P. boisei consumed a diet dominated by C4 biomass suggests that it occupied a different ecological niche than other hominins, such as Australopithecus africanus, which had a more varied diet.
The new discovery also sharpens debate over the ecological roles of different early hominin species.
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