Dental microwear and stable isotopes inform the paleoecology of extinct hominins

Abstract

Determining the diet of an extinct species is paramount in any attempt to reconstruct its paleoecology. Because the distribution and mechanical properties of food items may impact postcranial, cranial, mandibular, and dental morphologies related to their procurement, ingestion, and mastication, these anatomical attributes have been studied intensively. However, while mechanical environments influence skeletal and dental features, it is not clear to what extent they dictate particular morphologies. Although biomechanical explanations have been widely applied to extinct hominins in attempts to retrodict dietary proclivities, morphology may say as much about what they were capable of eating, and perhaps more about phylogenetic history, than about the nature of the diet. Anatomical attributes may establish boundary limits, but direct evidence left by the foods that were actually (rather than hypothetically) consumed is required to reconstruct diet. Dental microwear and the stable light isotope chemistry of tooth enamel provide such evidence, and are especially powerful when used in tandem. We review the foundations for microwear and biogeochemistry in diet reconstruction, and discuss this evidence for six early hominin species (Ardipithecus ramidus, Australopithecus anamensis, Au. afarensis, Au. africanus, Paranthropus robustus, and P. boisei). The dietary signals derived from microwear and isotope chemistry are sometimes at odds with inferences from biomechanical approaches, a potentially disquieting conundrum that is particularly evident for several species.

Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia

“…Thus, the remarkably wide range of C 3 to C 4 diets of by Au. afarensis across a temporally fluctuating environmental mosaic supports the inference of a generalist hominoid primate that exploited a broad range of habitats for food resources (1,22).…”

“…Therefore, the expansion of hominin diets to consume substantial amounts of C 4 /CAM foods signals a major ecological and adaptive divergence from the last common ancestor (LCA) that we shared with African great apes, which mostly occupy closed wooded habitats. C 4 /CAM food consumption is part of a general argument, based on several lines of evidence, that hominin diets diverged from the diets of the LCA during the shift to drier and more open environments in Africa during the Pliocene (1)(2)(3)(4). This dietary transition occurred subsequent to the known fossil record of Ardipithecus ramidus from Ethiopia at 4.4 Ma (5), which shows little evidence of C 4 /CAM food consumption.…”

Stable isotope-based diet reconstructions of Turkana Basin hominins

“…Some of the methods focus on the functional morphology of the masticatory system, others focus on tooth wear (both macroscopic and microscopic), and yet others focus on the physicochemical signatures that an animal's diet leaves within its hard tissues (1,2). Chemical methods include the use of strontium/calcium and barium/calcium ratios (3,4), but this study focuses on the analysis of stable isotopes of carbon (5)(6)(7)(8)(9).…”

Stable isotopes serving as a checkpoint

“…Later isotopic work in eastern Africa (8) showed a very high proportion of C 4 plants in the diet of Paranthropus boisei and began to suggest to some that the hypermasticatory apparatus characterizing this taxon was due to its adaptation to processing large quantities of low-quality vegetation rather than hard objects, as had been asserted for decades. Further, microwear studies cast doubt on assertions that P. robustus and P. boisei had similar diets, supporting the notion that phrenetic features may not necessarily reflect the presumed type of diet consumed (9).…”