The Evolution of Early Hominin Diet PDF

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GJJD2 ARCLG 179 B2 The Evolution of Early Hominin Diet Introduction Diet is an essential part of the relationship between an organism and its environment (Ungar 2012). For living primates, diet influences several facets of their lives, including geographic range, body size, breeding strategy, and lo...

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The Evolution of Early Hominin Diet Introduction Diet is an essential part of the relationship between an organism and its environment (Ungar 2012). For living primates, diet influences several facets of their lives, including geographic range, body size, breeding strategy, and locomotion (Clement and Hillson 2013; Ungar and Sponheimer 2011). Likewise, it was undoubtedly equally as important and influential in the lives of our extinct hominin ancestors. Environmental changes, which often alter available food resources and introduce new challenges, “have surely driven changes in early hominin diets and with them the evolution of our genus” (Ungar 2012). In fact, major changes in diet are often considered “key milestones” in hominin evolution (Ungar and Sponheimer 2011). The change from a primarily plant-based diet to one with meat has often been cited as a key motivation behind the transition to early Homo (Bunn 2006), although others suggest the inclusion of underground storage organs was more important (O’Connell et al. 2002; O’Connell et al. 1999; Ungar 2012). Still others suggest that the additional preparation of food with tools and cooking was critical (Ungar 2012; Wrangham and Conklin-Brittain 2003; Wrangham et al. 1999). Understanding how diet shifted between Australopithecus, Paranthropus, and early Homo can illuminate some of the major driving forces of human evolution. Modeling major changes in the evolution of hominin diet can be done through the examination of several lines of evidence, using both direct and indirect methods. However, which methods are usable varies depending on the time period being studied, as well as the archaeological and fossil evidence available. Archaeological remains, such as stone tools and animal bones, can be used to estimate the diet of more recent hominins, however the earliest

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hominins are non-existent archaeologically. Their limited representation in the fossil record is an additional problem. Fortunately, the parts of the skeleton most relevant to the study of diet, teeth and jaws, are also the most likely tissue to fossilize (Lee–Thorp 2002). Dental tissues are considerably more resistant than normal bone to chemical and physical degradation (Clement and Hillson 2013; Lee–Thorp 2002). The size, shape, structure, and microwear patterns of teeth are commonly examined to infer aspects of diet (Ungar 2012). Further information can be gathered through stable isotope analyses and trace elements (Lee‐Thorp et al. 2003; Sponheimer et al. 2013). Reconstructing paleoenvironments can also suggest what types of food were available during the time periods and regions in which our hominin ancestors lived (Alemseged and Bobe 2009). In this essay I will discuss the various lines of evidence used to study and evaluate the diet of early hominins and how it evolved over time. I will also discuss the dietary importance of plants and animal foods and how the development of cooking influenced human evolution.

Lines of Evidence Adaptive Evidence Teeth are adapted to provide preliminary processing of food and for most mammals, the morphology of their teeth is correlated with diet (Andrews et al. 1991). The abundance of teeth in the hominin fossil record has allowed researchers to investigate the evolution of hominin diet using several aspects of dental morphology including tooth size, tooth shape, and enamel structure. Incisor Size For primates, there is a correlation between incisor size (relative to body or first molar (M1) size) and type of diet. Frugivorous primates tend to have relatively large incisors, likely

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adapted for consuming large, husked fruits, while folivores tend to have relatively small incisors, as they eat smaller objects that do not require large front teeth (Andrews et al. 1991; Groves and Napier 1968; Hylander 1975; Teaford and Ungar 2000; Ungar 2012). In the hominin lineage, Australopithecus taxa plot on or near the regression line for incisor allometry in extant catarrhines (Figure 1), demonstrating a moderate incisor size, while Paranthropus robustus plots below with a relatively small incisor. Relative incisor size appears to then increase above the line with Homo habilis and Homo rudolfensis, followed by a decrease back to the line in Homo erectus and then below it again with Homo sapiens. The differences in incisor size suggests that there were notable shifts in diet related to incisor use, however, it is important to note that sample sizes for each hominin species are extremely small (n =1-2) and the body weight estimates are rough and uncertain (Teaford et al. 2002; Ungar 2012). Molar Size Relative molar size has also been used as an indicator of diet, although the same limitations in sample size and weight estimates still apply. In living primates, folivores have longer molars than frugivores for most primate groups, however this is not the case in cercopithecoids (Kay 1977; Vinyard and Hanna 2005) and in many primate species, there is a significant difference in relative cheek teeth size between males and females (Harvey et al. 1978). Because the relationship between molar size and diet is inconsistent among living primates, any conclusions drawn should be done cautiously. Several studies have shown an increase in molar size (in both absolute surface area and megadontia quotient) throughout time in the Australopithecus and Paranthropus taxa, followed by a reduction in Homo (Figure 2) (McHenry and Coffing 2000; Teaford and Ungar 2000; Ungar 2012). The significance of this change is unclear, although the enlarged cheek teeth and robust jaws of the australopithecines are typically explained as an adaptation to processing large amounts of

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low-quality, hard foods, such as nuts and hard-shelled fruits (Ungar and Sponheimer 2011), while the reduction in Homo could be the result of a relaxation in selection pressures due to the introduction of tools and cooking (Ungar and Sponheimer 2011). Tooth Morphology and Structure The shape of primate teeth, in particular molar teeth, also reflects the fracture properties of the food each species eats. For example, primates that eat tough leaves often have more occlusal relief than those who typically eat hard objects and these relationships are conserved with wear (Kay 1984; M'kirera and Ungar 2003; Meldrum and Kay 1997; Ungar and M'kirera 2003). Occlusal morphology differences between hominins suggest that Paranthropus consumed more hard-brittle food, while early Homo would have been better at shearing tough items and Australopithecus falls somewhere in the middle (Bailey and Wood 2007; Teaford et al. 2002; Ungar 2007; Wood and Strait 2004). Tooth enamel thickness is also argued to be an adaptation related to diet and food fracture properties. Thicker enamel protects better against breakage, but thinner enamel wears quicker and provides a jagged surface, beneficial for processing tough foods (Dumont 1995; Kay 1981; Ungar et al. 2006; Ungar and M'kirera 2003). Studies focusing on tooth size, shape, and structure offer important evidence about fracture properties and masticatory stresses in early hominin diets, however their results can be misleading (Ungar and Sponheimer 2011). They indicate the dietary adaptation and phylogenetic history of each species and what they are capable of eating, but that does not always match what specific individuals actually eat (Lee‐Thorp et al. 2003; Ungar and Sponheimer 2011). Even extant primate taxa regularly eat food that does not match the current morphology of their teeth (Lee‐Thorp et al. 2003). For more precise evidence of what each fossil specimen ate, we need to use other methods.

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Non-Adaptive Evidence Dental Microwear Dental microwear, the study of microscopic wear on the surface of teeth, is one of the best methods for reconstructing early hominin diets (Scott et al. 2005; Walker et al. 1978). Scratches and pits appear on the surface of a tooth as a direct result of use and each mark is representative of an actual chewing event (Ungar 2011; Ungar and Sponheimer 2011). Different types of food leave behind different wear patterns. Hard, brittle foods (nuts, bones) typically leave pit marks on the occlusal surface of teeth, while tough foods that require shearing (leaves, meat) leave long, parallel striations (Figure 3) (Ungar 2010; Ungar and Sponheimer 2011). Surface complexity corresponds to the hardness of food eaten (Figure 4) (high complexity = heavy pitting) and the directionality (anisotropy) of the wear corresponds to food toughness (high anisotropy = highly aligned scratches) (Ungar and Sponheimer 2011). Studies of early hominin microwear (Scott et al. 2005; Ungar et al. 2008; Ungar et al. 2012; Ungar et al. 2010) reveal somewhat surprising microwear patterns that do not always match what is expected based on morphology. Australopithecus individuals and Paranthropus boisei do not have the microwear pattern of high complexity and heavy pitting consistent with a hard-object feeder, unlike originally expected; they also have low to moderate anisotropy, indicating they did not shear tough leaves (Ungar and Sponheimer 2011). Paranthropus robustus, in contrast, had very high complexity and very low anisotropy, as well as the highest amount of variation of all early hominins (Scott et al. 2005). This distribution is similar to hard-object fallback feeders, which eat harder foods when their preferred softer foods are absent (Scott et al. 2005). Early Homo microwear patterns show evidence of a generalized diet (Ungar et al. 2012). Homo erectus, in particular, has much more variation in the levels of microwear complexity than Homo habilis, suggesting a very broad diet (Ungar and Sponheimer 2011).

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Microwear analyses provide important and direct details of hominin diets, but they still have limitations. Microwear can really only indicate the consistency of food eaten and studies must omit foods that do not leave impressions on the surfaces of teeth, such as insects and flesh (Lee‐Thorp et al. 2003). The “last supper effect” should also be taken into account; microwear features are constantly worn over and only reflect diet in the last few days or weeks of life (Grine 1986). Stable Carbon Isotopes Stable carbon isotope analyses can be used to determine the relative proportion of C3 (trees, bushes, shrubs, forbs) and C4 (grasses, sedges) plants in an extinct hominin individual’s diet (Ungar and Sponheimer 2011). The stable isotopes of plants eaten by an individual (or for faunivores, the plants eaten by its prey) are incorporated into the teeth and bones of that individual and the isotopic composition of these tissues becomes reflective of its diet (Cerling et al. 1999; Koch et al. 1998; Lee-Thorp et al. 1989; Ungar and Sponheimer 2011). In the case of human evolution, stable isotope analyses can be used to determine if any of the early hominin species had diets similar to those of extant apes and evaluate the percentage of C3 and C4 plants in their diets based on δ13C values (Figure 5). Results indicate that the early hominins that have been analyzed using stable isotopes can be roughly separated into three groups: those with relatively low δ13C values (Ardipithecus ramidus and Australopithecus anamensis) similar to the C3 dominated diets of savanna chimpanzees (Schoeninger et al. 1999; Sponheimer et al. 2013; Sponheimer et al. 2006a), those with intermediate δ13C values (Australopithecus africanus, Australopithecus afarensis, Paranthropus robustus, and early Homo) (Lee-Thorp et al. 2000; Sponheimer et al. 2006b; van der Merwe et al. 2008; van der Merwe et al. 2003) indicating a mixed C3/C4 diet, and those with high δ13C values and strongly a C4 diet (Paranthropus boisei) (van der Merwe et al. 2008).

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While stable carbon isotope analyses reveal useful and interesting information, there is some difficulty in explaining the results, as there can be several possible explanations for what is observed. In particular, we cannot discriminate between folivorous, frugivorous, or carnivorous diets as all three are based on C3 plants, nor can we tell which type of C3 or C4 foods were eaten (Clement and Hillson 2013; Sponheimer et al. 2013). Trace Elements (Sr/Ca) It is possible, however, to distinguish between folivorous, carnivorous, or underground storage organ based diets through the use of trace elements (Lee‐Thorp et al. 2003). The ratio of strontium (Sr) to calcium (Ca) in an individual’s teeth or bones is reflective of the foods eaten and their trophic level. Herbivores have lower Sr/Ca ratios than the plants they eat, and carnivores have lower Sr/Ca ratios relative to their prey (Elias et al. 1982; Ungar and Sponheimer 2013). Additionally, leaf-eating herbivores would have lower Sr/Ca ratios compared to animals that eat stems or underground storage organs (Sillen et al. 1995). It is important to note however, that a carnivore would only have a reduced Sr/Ca ratio compared to the particular prey it eats, and as a result, it may overlap with herbivores (Lee‐ Thorp et al. 2003). There is also a high level of variability within species, further complicating any analyses (Burton et al. 1999; Sillen 1992). When Sr/Ca ratios are examined in South African early hominins, Australopithecus africanus had the highest ratios, early Homo the lowest, and Paranthropus robustus was intermediary (Figure 6) (Balter et al. 2012). When compared to other fauna, early Homo fit within the Sr/Ca range for carnivores, Paranthropus robustus for browsers, and Australopithecus africanus was indistinguishable from both grazers and browser (Balter et al. 2012). It is possible that early Homo and Paranthropus robustus both had relatively typical browser/carnivore diets, while Australopithecus africanus had a more complex diet.

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Contextual Evidence Paleoenvironmental Reconstruction As diet is a direct link between an individual and its environment, dynamic changes in environment likely influenced dietary and adaptive changes throughout human evolution (Alemseged and Bobe 2009; Ungar et al. 2006). Reconstructing the paleoenvironmental context of each hominin taxa can provide an additional angle from which to assess potential hominin diets. The type of environment an individual lives in can limit and influence their food choices (e.g a savanna animal is more likely to rely on grasses than tree fruits) (Ungar and Sponheimer 2013). Additionally, if any of the early hominins depended on a specific food for survival and those food sources disappeared with an environmental shift, it could have led to their extinction or been the driving force behind an adaptive morphological transition (Ungar et al. 2006). Environments can be reconstructed using many different methods. Often the most common taxa of a fossil assemblage are used to infer the most likely environment and confirmed using sedimentological and other related evidence (Alemseged and Bobe 2009). Fossilized plant remains, pollen, phytoliths, and soil isotopes are additional methods that are useful in interpreting paleoenvironments (Bamford 1999; Bonnefille et al. 2004; Cerling 1992; WoldeGabriel et al. 1994). Unfortunately, even if we are able to reconstruct the environments early hominins likely inhabited, this does not tell us much about what they actually ate. Rather, it shows what would have been available and provides context for other lines of evidence. Additionally, we need to know the distribution of edible foods in these landscapes in order to understand their eating habits (Peters 2007; Ungar and Sponheimer 2013). It can show, however, whether specific hominin taxa were specialized for particular environments or more generalized and lived in diverse habitats, as well any similarities or inconsistencies in hominin diet across environment type (Alemseged and Bobe 2009).

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Archaeological and Zooarchaeological Remains Archaeological evidence can be an important source of information for the diets of hominin taxa, however there are limitations on what survives in the archaeological record. For most early hominins we do not have any archaeological artifacts from the time period that they were alive. Of the early hominin archaeological material we do have, stone tools and butchery marks on animal bones reveal the most about diet. The earliest examples of stone tools appear around 2.6 million years ago (Semaw et al. 1997; Semaw et al. 2003) and the earliest cut marks on bone are from around 2.5 million years ago (De Heinzelin et al. 1999) (although there is controversial evidence of cut-marked bones from 3.4 million years ago (McPherron et al. 2010)). It is likely that hominins from earlier time periods also made and used tools, but out of perishable materials (Panger et al. 2002). The existence of early stone tools alone does not prove they were used in food acquisition, however the presence of butchery marks on animal bones provides support for this idea (Ungar et al. 2006). Furthermore, stone tools were probably very versatile implements. Microwear on some Oldowan stone tool artifacts suggests they were used to prepare vegetation, possibly for food or to construct tools from plant materials (Keeley and Toth 1981). The function of stone tool kits also probably varied between hominin taxa and groups, as they do for extant chimpanzee groups and modern human foragers (Ungar et al. 2006). At any rate, it is clear that the development of stone tools greatly expanded hominin dietary options. The presence of cut-marked animals bones suggests that the development of stone tools enabled a shift in the hominin diet from a primarily frugivorous diet that may have included small animals, to one that included medium to large sized animals (Blumenschine and Pobiner 2007). The combination of both cut marks and carnivore teeth marks on zooarchaeological remains demonstrates which other animals hominins interacted and competed with for food sources (Blumenschine and Pobiner 2007). Butchery marks from

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stone tools are clearly morphologically distinct from carnivourous teeth marks, as well as rodent gnawing and other taphonomic marks (Blumenschine and Pobiner 2007). The development of stone tools greatly expanded the dietary options of early hominins, and butchery marks on zooarchaeological remains provides concrete evidence that, beginning about 2.5 million years ago, these hominins were incorporating food from larger animals into their diet. What is less clear, however, is exactly which hominin taxa were making these stones tools and using them in food acquisition. Several hominin species were alive around 2.5 million years ago and multiple species are often found at the same site. This makes it nearly impossible to determine which species was the creator of the stone tool artifacts or cut marks on bones (Lee‐Thorp et al. 2003; Ungar et al. 2006).

Plant vs. Animal Food in Hominin Diets The increasing incorporation of animal meat and tissue into the hominin diet was one of the most dramatic and influential dietary changes that occurred during human evolution (Bunn 2006). It is thought that the spread of savanna grasslands and the decrease of forest resources pushed hominins to include more and more meat in their diets in order to maintain the same level of dietary quality (Milton 1999; Milton 2003; Ungar et al. 2006). The development of stone tools also improved and expanded their hunting abilities and strategies. A feedback loop was created, with the increase in protein and energy from the meat allowing for the growth of larger brains, which in turn led to greater intelligence and more complex cognition; resulting in more complex social systems, division of labor, and better hunting strategies (Isaac 1971; Isler and van Schaik 2009; Ungar et al. ...