INTRODUCTION

In this part the contributors explore a variety of attributes of the paleobiology of fossil hominoids that may contribute to an understanding of the evolution of great ape intelligence. Our request to these experts, in each topic that is the focus of their contributions, was to focus attention on the way in which a single aspect of paleobiology may inform this issue. This is a twoedged sword. Many authors, most of whom had thought relatively little about the broader question of great ape intelligence (myself included), found new insights from the data they have long been contemplating from other perspectives. On the other hand, since we asked contributors to restrict themselves to a focused topic, many readers may feel that each contributor thinks their area of interest is the one most likely to “explain” great ape cognitive evolution. Nothing could be farther from the truth. All the contributors to this part of the volume recognize that their topic, whether it is environment and ecology, diet, locomotion, size, or life history, is one piece of the puzzle. When authors in this part of the book reach conclusions on the relevance of a specific aspect of fossil ape paleobiology to understanding great ape cognition, it is because the editors pushed them to do it, and all know that each attribute is but a facet of a very complex problem.

INTRODUCTION

Great apes and humans are the largest-brained primates. Aside from a few extinct subfossil lemurs, they are also the largest in body mass. Body size is a key aspect of a species' biology, a large organism having different energetic, ecological, and physical constraints than a small one. Brain size, in so far as it determines abilities to acquire, process, and act on information, is also a key aspect of a species' biology and is linked to body size. Large animals have different informational problems to solve than do small ones, hence their respective sensory organs and nervous systems are sized and organized differently.

Mammalian body and brain size scale consistently with each other (Figure 18.1). This relation is generally described by allometric exponents that vary between 2/3 and 3/4 (e.g., Bauchot & Stephan 1966, 1969; Hofman 1982; Jerison 1973; Lande 1979; Martin 1981; Martin & Harvey 1985; Stephan 1972). From a paleontological perspective, the body–brain size relation offers an appealing way to evaluate the cognitive abilities of fossil taxa, a problem of particular importance for understanding hominid evolution.

A convincing theoretical basis for this general allometric statistical pattern, however, remains elusive (Deacon 1997; Harvey & Krebs 1990). Body mass is not a strict determinant of brain size, as species of similar size can have different brain sizes and cognitive abilities (Pagel & Harvey 1989).

INTRODUCTION

Fossil endocasts, natural or artificial casts of the inside of a cranial vault, provide the most direct evidence of the evolution of the brain. Among fossil hominoids, the vast majority of endocasts come from Plio-Pleistocene hominids, and these have been described in detail, (Conroy, Vannier & Tobias 1990; Dart 1925; Falk 1980a, b, 1983a, b, 1987, 1990; Falk & Conroy 1983; Holloway 1974a, 1982, 1983a, 1984, 1995; Holloway & De la Coste-Lareymondie 1982; Martin 1983, 1990; Martin & Harvey 1985; Schepers 1946, 1950; Tobias 1967, 1971a, b, 1975, 1978, 1983, 1991, 1995). Fossil great ape endocasts are extremely rare and are thus far undescribed. Therefore, beyond extrapolation from an outgroup, little is known of the primitive condition from which modern great ape and human brains could have evolved.

Six specimens of the primitive Oligocene catarrhine Aegyptopithecus zeuxis from about 33–33.5 Ma are described (Radinsky 1973, 1974, 1977; Rasmussen 2002; Simons 1993). Among hominoids, only four specimens are sufficiently complete to estimate brain size: one for Proconsul nyanzae, an early Miocene (c. 18 Ma) primitive or stem hominoid that predates the emergence of the great ape and human clade, and three for the great apes Dryopithecus brancoi and Oreopithecus bambolii from between about 10 to 6 Ma (Begun 2002; Falk 1983a; Harrison 1989; Kordos 1990; Kordos & Begun 1997, 1998, 2001a; Walker et al. 1983).

The only fossil hominoid for which the endocast has yet been described is Proconsul.

Among the great apes that once ranged the forests of the Old World, only four species survive. Their evolutionary history reveals a huge range of morphological and behavioral diversity, all of which must be considered successful adaptations in their own time. Some of these attributes (large brains, sclerocarp and hard-object feeding, frugivory, folivory, gigantism, terrestriality, and suspensory positional behavior) survive in modern great apes. Our questions are: what combination of behaviors and attributes characterized the ancestor of living great apes? what was the significance of this suite of features for cognition? and how did it arise in evolution? To that end, we offer our model of a distinct great ape cognition along with its biological underpinnings and environmental challenges, then attempt to trace the evolutionary origins of this ensemble of features.

COGNITION

All living great apes express a distinctive grade of cognition intermediate between other nonhuman primates and humans. Their cognition normally reaches rudimentary symbolic levels, where symbolic means using internal signs like mental images to stand for referents or solving problems mentally. It supports rudimentary cognitive hierarchization or metarepresentation to levels of complexity in the range of human 2 to 3.5 year olds, but not beyond (in this volume, see Blake, Chapter 5, Byrne, Chapter 3, Parker, Chapter 4, Russon, Chapter 6, Yamakoshi, Chapter 9).

Great apes' high–level cognitive achievements are generalized in that they manifest system wide and relatively evenly across cognitive domains (Russon, Chapter 6, this volume).

INTRODUCTION

Relations among living great apes and humans have been worked out in recent years to the satisfaction of most researchers, from both the molecular and the morphological/fossil approaches (Begun 1999; Begun, Ward & Rose 1997; Page & Goodman 2001; Satta, Klein & Takahata 2000; Shoshani et al. 1996). It is now widely recognized that humans and Pan (chimpanzees and bonobos) are members of a clade (evolutionary lineage) to the exclusion of other living primates, and that among living apes gorillas are next most closely related, orangutans after that, and hylobatids (gibbons and siamangs) after that. The living great apes, humans, and their ancestral lineages then form a natural evolutionary group, the hominids (Table 2.1 and Figure 2.1). Where fossil hominids (Miocene to Pleistocene great apes and humans) fall within this framework is the subject of intense debate, but this question is not critical to the theme of this chapter.

Phylogenetic parsimony suggests that characteristics shared among all members of the hominid group probably evolved once in their common ancestor rather than repeatedly in the separate lineages of the Hominidae. In some cases the symplesiomorphic (shared primitive) nature of characters shared among living hominids is supported by fossil evidence, such as brain size, body mass and rate of maturation (Begun & Kordos, Chapter 14, Gebo, Chapter 17, Kelley, Chapter 15, this volume). Among the shared characteristics that cannot be directly confirmed in the fossil record is great apes' distinctive intelligence.

This book arose from three realizations. First, there is an important need for good models of great ape cognitive evolution. Studies of comparative primate cognition over the last two decades increasingly show that all great apes share a grade of cognition distinct from that of other nonhuman primates. Their cognition appears to be intermediate in complexity between that of other nonhuman primates and humans, so it offers the best available model of the cognitive platform from which human cognition evolved. Understanding the position of the great apes is then essential to understanding cognitive evolution within the primate order and ultimately, in humans. Second, existing reconstructions of the evolutionary origins of great ape cognition are all in need of revision because of advances in research on great ape cognition itself, on modern great ape adaptation, and on fossil hominoids. Third, developing an accurate picture of the evolutionary origins of great ape intelligence requires bringing together expertise from a highly diverse range of fields beyond modern great ape cognition. Essential are current understandings of the brain, life histories, social and ecological challenges, and the interactions among them in both living and ancestral hominids.

We therefore assembled a team of contributors with expertise spanning the topics currently recognized as relevant to cognitive evolution in the great ape lineage, with the aim of piecing together the most comprehensive picture possible today. We asked all our contributors to explore the implications of their realm of expertise for cognition and cognitive evolution.

INTRODUCTION

Recent fossil discoveries, methodological advances, and ongoing analyses of hominoid comparative molecular biology and morphology have led to dramatic changes in our current understanding of relations among the great apes and humans. The late nineteenth-century view of these relations held that humans and the African apes were most closely related, and that the Asian great ape, the orangutan, was a more distant relative (Darwin, 1871; Huxley, 1959). For most of the twentieth century however, the great apes have been placed not only in their own family (the Pongidae), but also in their own separate evolving lineage, to the exclusion of humans. In the past thirty years, a few molecular biologists have been questioning the evolutionary reality of the pongid lineage, suggesting a return to the Darwin–Huxley view, placing African apes closer to humans, but based on the results of comparative molecular biology. During that time, the techniques of molecular systematics have been greatly improved and refined, to the point where researchers are today actually comparing nucleotide sequences, the fundamental, specific arrangement of DNA molecules, as opposed to overall DNA similarity (hybridization) or the products of DNA metabolism (proteins).

This work in molecular biology has been joined by recent developments in method and theory in palaeontology and comparative anatomy. Foremost among these is the widespread adoption of cladistic methodology, but other developments include more comprehensive analysis of hominoid comparative anatomy, and a more complete hominoid fossil record.