Early Hominin Paleoecology

By Matt Sponheimer, Julia A. Lee-Thorp, Kaye E. Reed, Peter S. Ungar

University Press of Colorado

Copyright © 2013 University Press of Colorado
All rights reserved.
ISBN: 978-1-60732-224-5

Contents

Preface,
Acknowledgments,
Part 1: Paleoclimate and Paleoenvironment,
1. Faunal Approaches in Early Hominin Paleoecology Kaye E. Reed, Lillian M. Spencer, And Amy L. Rector,
2. Facies Analysis and Plio-Pleistocene Paleoecology Craig S. Feibel,
3. East African Hominin Paleoecology: Isotopic Evidence from Paleosols Jay Quade and Naomi E. Levin,
4. Tectonics, Orbital Forcing, Global Climate Change, and Human Evolution in Africa Mark A. Maslin, Beth Christensen, and Katy E. Wilson,
Part 2: Hominin Adaptations and Behavior,
5. Early Hominin Posture and Locomotion Carol V. Ward,
6. The Functional Morphology of Jaws and Teeth: Implications for Understanding Early Hominin Dietary Adaptations Peter S. Ungar and David J. Daegling,
7. Dental Microwear and Paleoecology Mark F. Teaford, Peter S. Ungar, and Frederick E. Grine,
8. Hominin Ecology from Hard Tissue Biogeochemistry Julia A. Lee-Thorp and Matt Sponheimer,
9. The Behavior of Plio-Pleistocene Hominins: Archaeological Perspectives David R. Braun,
Part 3: Analogies and Models,
10. Plants and Protopeople: Paleobotanical Reconstruction and Early Hominin Ecology Jeanne Sept,
11. Chimpanzee Models of Human Behavioral Evolution John C. Mitani,
12. Analogies and Models in the Study of the Early Hominins Clifford J. Jolly,
Contributors,
Index,

CHAPTER 1

Faunal Approaches in Early Hominin Paleoecology

Kaye E. Reed, Lillian M. Spencer, and Amy L. Rector

The paleoecology of early hominin species is more than simply reconstructing the habitats in which they existed. Ultimately we would like to know the ecological context before and after speciation and extinction events, and about the interactions of hominins with their environment, including other species. A first step toward this goal is to discover as much information as possible regarding the climate, geomorphology, vegetation physiognomy (habitat structure), and the faunal community. These factors build on one another such that climate, soil properties, and geomorphology are responsible for the vegetation, which, in turn, plays a fundamental role in controlling what other life forms can be supported. An understanding of extant African habitats is necessary to reconstruct ancient vegetation physiognomy for early Pliocene hominins. An appreciation of living mammals is also important in interpreting Pliocene environments when using faunal techniques. The most common data recovered with early hominins are other mammalian fossils, and these are targeted here for explaining how reconstructions of habitat and community ecology can be approached. Faunal analyses can be compared with other types of research such as palynology, fossil botanical studies, and isotopic analyses of soils and teeth to arrive at a better understanding of hominin paleoecology.

Fossil mammals found within the same deposits as early hominins can be used to answer a variety of questions relating to evolutionary paleoecology. First, fossil mammals have been used as indicators of habitats since early paleontological studies (e.g., Ewer 1958; Brain 1967; Leakey and Harris 1987). More recent work on this topic has emphasized the importance of determining taphonomic histories before reconstructions are attempted (e.g., Behrensmeyer and Hill 1980; Brain 1981; Behrensmeyer 1991; Soligo and Andrews 2005; Andrews 2006), but this caveat is still only rarely addressed. The majority of African hominin paleoecological work falls into the category of using faunal analyses for reconstructing ancient habitats, and forms the bulk of the work reviewed here. Second, studies of contemporaneous fauna are critical for investigating aspects of community ecology, such as guild structure. This avenue of research can be also used to determine possible differences between ancient Plio-Pleistocene and extant communities (e.g., Janis et al. 2004). Third, faunal studies can give insights into how hominins might have interacted with specific members of their shared community. For example, study of the members of the carnivoran guild (Marean 1989; Lewis 1997) can lead to hypotheses about how hominins might have avoided predation or competed with predators for access to meat. Finally, faunal studies can be used to answer questions of patterns and processes in the evolution of both hominins and other mammalian lineages (e.g., Vrba 1988, 1995; Behrensmeyer et al. 1997; Potts 1998; Bonnefille et al. 2004).

Faunal approaches in hominin paleoecology can be assigned to two types of studies. The first is analyzing individual fossil species of mammals and other fauna found at particular localities. This information can be used to reconstruct habitats and to look at species interactions with hominins. It is also a critical precursor for community studies. The second type of study examines communities as a whole, which is necessary for studies of community ecology and also for investigating evolutionary patterns in hominin lineages.

A second dichotomy exists between the taxonomic and ecological/functional approaches to faunal research. In taxonomic analyses, phylogeny plays an important role. Taxonomic methodologies are used occasionally to reconstruct environments (e.g., Vrba 1980), but the usual focus using these methods is to examine biogeographic and species-turnover patterns (Behrensmeyer et al. 1997; Bobe and Eck 2001). The second approach is often referred to as taxon free because species diversity, ecological diversity, or the results of functional studies are ecological representations of each species. Damuth (1992) has argued that results derived from these types of taxon-free data transform species-specific fauna, and by extension assemblages or communities, into parameters to be incorporated into ecological patterns that can then be compared with any other faunal community in space and time since the parameters used are not taxon specific. For example, it might be difficult to compare an Australian Macropoda (kangaroo) to an African Damaliscus (topi) on a phylogenetic level, but to compare them as terrestrial grazers of similar body size is possible. Both phylogenetic and taxon-free approaches have been important in understanding hominin paleoecology as well as in developing evolutionary scenarios (Figure 1.1).

As mammal species are most often recovered in the greatest numbers from hominin fossil localities, much of our discussion is devoted to analyses of mammalian fauna. However, the results of any research using mammals should be compared with other types of analyses — such as the study of amphibian, bird, and reptile fossils; research on paleoclimate, pollen, and depositional environments; and isotopic analyses — depending on the ultimate goals of the research. In this chapter, we present a brief overview of existing African habitats and African mammal communities. We then discuss issues of taphonomy such as time-averaging, collection bias, and other factors that may bias faunal assemblages such that they obscure paleoecological reconstructions. Faunal analyses are only as good as the data derived from the fossil localities. Third, we provide an overview of the types of analyses mentioned above — those focusing on individual species and those focusing on the community from phylogenetic and taxon-free perspectives. We then survey research that has been used to investigate three areas in hominin paleoecology: reconstructing habitats, reconstructing community ecology, and investigating species interactions between hominins and other mammals.

AFRICAN HABITATS

Reconstructing past African habitats is usually based on comparisons to extant habitats. Today, ecologists often refer to existing habitats by the dominant plant species, such as miombo woodland. Usually the best that can be accomplished for ancient vegetation, however, is to reconstruct the habitat physiognomy (structure) in which fossil hominins have been recovered. Actual plant-species identification can only be done through palynological and paleobotanical studies when these types of remains are present. Habitat structure simply refers to the architecture of the floral species — for example, forest or bushland — rather than to the actual species. Within Africa the assumption is that the fundamental architecture of past and extant habitats is similar. Habitats from different continents may be inappropriate for comparison to fossil localities in Africa because vegetation structure can be labeled the same (e.g., forests) but exhibit significant differences (Archibold 1995). For this reason, habitats that are present in Africa today are probably the best analogs (but see Andrews et al. 1979; Andrews and Humphrey 1999; Mendoza et al. 2005). Mendoza et al. (2005) have shown that terrestrial ecosystems can be separated best if placed into three categories: arid habitats with no trees, humid evergreen forests, and wooded savannas. While this is undoubtedly true, almost all of the early hominin localities in Africa would likely fall into the wooded savanna category.

Extant African habitats range from primary rain forests to deserts. The amount of rainfall, temperature, sunlight, evapotranspiration, soil type, landscape, and weather patterns/seasonality are thus indicative of these habitats, and habitats can, in return, inform on these climatic conditions. In the tropical belt, however, the seasonal pattern and the amount of rainfall are the critical determining factors of the vegetation structure (Archibold 1995). Identifying ancient habitats will thus provide limited information on these aspects of the climate.

African forests consist of tall trees with multiple canopies (White 1983). Forests need mean annual rainfall of greater than 1500 mm and/or consistent groundwater, and long wet seasons if the moisture is derived from rain. These conditions create a closed architecture. Deserts usually have stunted trees, if any, and small, succulent plants and/or bushes. The mean annual rainfall is usually less than 200 mm. Deserts have extreme seasonality, that is, long periods without rainfall. The desert habitat is open.

Every other habitat in Africa today is savanna, covering 65 percent of the continent (Archibold 1995). This does not mean that all savanna habitats are the same or that they cannot be differentiated from one another: the nature of the physiognomy depends on the amount and seasonality of rainfall. Thus, when attempting to understand human (and broader mammalian) evolution, conceptually separating savannas from other terrestrial ecosystems is better for understanding the habitats in which hominins existed, and the community relationships within the environment. Many classifications of savannas have been made, but we follow White (1983) and utilize classifications that may be meaningful in reconstructing past habitats. Savannas characteristically have grasses as ground cover and other arid-adapted plants that can survive long dry seasons. Unlike on other continents, in Africa most of the woodland trees are deciduous (Archibold 1995). This means that leaf development will occur in the wet season and leaves will fall during the driest months of the year (Hopkins 1970). This contrasts with evergreen species in which leaf production occurs in the dry season. See Table 1.1 for subdivisions of savanna habitats.

The broad-based structural definitions of habitats are often the overall biomes of particular biogeographic regions, although there may be other habitats within them (White 1983), such as the Southern Savanna Grassland of South Africa (Rautenbach 1978). Various habitat structures often occur together in mosaic patterns within regions because of changes in soil types, subterranean water, and so on. River courses and lacustrine environments cause much of the mosaicism as they provide subterranean water that alters the general habitat close to the water. Thus it is possible to have riverine forests abutting almost desertlike habitats, such as along the Awash River that travels through extremely arid environs in Ethiopia. As most regions in Africa today possess a mosaic of habitats, it is reasonable to assume that ancient habitats were likely distributed in a similar manner. In eastern Africa, where the majority of hominin fossil localities occur within lacustrine and fluvial depositional environments, the vertical facies associations observed in the stratigraphy reflect ancient horizontal landscape associations (Miall 2000). That is, types of habitats across the landscape move horizontally through time due to common channel migration, change in fluvial regime (e.g., from meandering to braided), lake transgressions and regressions, and tectonic events.

ISSUES THAT CONFOUND THE RECONSTRUCTION OF HABITATS FOR FOSSIL LOCALITIES

The success of faunal analysis is related to the accurate estimation of taphonomic processes that contributed to the resultant fossil assemblages. If these factors are not considered, the paleoecological reconstruction and subsequent evolutionary analyses derived from these comparisons may be inaccurate.

Table 1.1. Modern savanna habitat descriptions. While all of these are considered to be savannas as they have grass as a ground cover, finer descriptions better describe individual habitat structures.

Taphonomy, strictly speaking, refers to the laws of burial (Efremov 1940). The term now usually refers to any alteration that may have occurred to a fossil at any time between the death of the animal and the fossil's placement in a museum. The taphonomic processes that have affected a fossil or a fossil assemblage may dictate that particular methods of analysis are inappropriate (Gifford 1981; Behrensmeyer 1991; Behrensmeyer et al. 2000). Therefore, it is important to discover the taphonomic information derived from both fossil fauna and other sources, such as the depositional environment, so that any incongruities resulting from these biases can be identified. Taphonomic processes that have influenced fossil localities can then be considered in the selection of methods to analyze the assemblage further (Behrensmeyer 1991). Taphonomy can be the focus of research and can therefore answer questions regarding modes of accumulation and pre- and postdepositional processes. However, we are more concerned in this chapter with briefly describing various confounding factors that may affect faunal analyses. When we explain methods of faunal analysis, we note which of these confounding aspects can be overcome as there are analytical methods that can minimize taphonomic overprint in faunal assemblages.

Time-averaging refers to the fact that most fossil deposits have accumulated over hundreds, if not thousands, of years. It is difficult to reconstruct a slice of time environment when the faunal accumulation used to predict the habitat is the result of some 10 kyr (thousand years) of deposition. We can never avoid this problem altogether, but analyses of particular species and their differences, if any, within deposits and also through time will help in this regard. That is, if species at the bottom of a section vary in dental dimensions from the same species at the top of the deposit, for example, then it is possible that what we have deemed as a single deposit is in fact more than one.

It is also necessary to determine if the fossil assemblage has been transported. An autochthonous assemblage is one in which no transport of specimens has occurred. Allochthonous assemblages refer to fossil deposits that have come from different habitats and yet appear to be a unified accumulation. These deposits would most likely be the result of fast-moving fluvial systems that wash many animals downstream during high-energy situations. These assemblages have a particular signature that can be interpreted before any ecological reconstruction is attempted. Lyman (1994) outlined taphonomic criteria with which to judge assemblages for identifying these biases, such as degree of abrasion and skeletal-part representation.

The accumulating agent refers to what or who was responsible for the fossil deposit — that is, animals dragging carcasses into caves, tar pits trapping animals, fluvial systems gathering up carcasses during flooding, hominin hunting or butchery practices, and so on. If one is interested in whether hominins were the hunters or the hunted (Brain 1975), then discerning the accumulating agent is the most important endeavor. Collection bias sometimes occurs when researchers are recovering fossils. It may be that there is no room in a museum for very large mammals, such as elephants, so they are left behind, or that the mode of collection (e.g., a walking survey) does not allow for the recovery of micromammals. Bone modification is the analysis of various alterations on the bone surface and can determine if the bones have been modified by carnivores, rodents, and/or hominins or if they have lain on the ground and weathered, or if they have been rolled or transported in fluvial settings (Behrensmeyer and Hill 1980; Behrensmeyer 1991; Lyman 1994).

Once the biases have been identified, researchers can select methods for reconstructing environments, identifying community structure, and examining species-turnover patterns that will minimize the effects of these biases. For example, if a fossil locality is depauperate in micromammals, it is likely that a comparison with extant faunal communities would be made only with macromammals. If hyenas have collected material of a certain size in the fossil record, then an extant database of animals should be created that takes this into consideration for subsequent comparisons with the fossil faunal assemblage. In other words, if one compares fossil communities with living communities to determine habitat, animals are being compared — that is, those of the same body size, such as micro-, mid-range, or macromammals, or those of the same mammalian orders, such as Artiodactyla or Primates (again of similar body sizes). Comparisons of recently deposited material — for example, assemblages acquired from hyena dens (Brain 1980) or fluvial flood remains, in which the originating habitat is known — will also minimize problems with reconstructing habitats when accumulating agents have modified the selection of fauna.

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