Symposium on ‘Human evolution: ancestors and relatives’

Abstract

Substantial effort and attention has been paid to determining the evolutionary history and relationships of the living great apes (including modern humans). The vast majority of the existing (but until recently largely unexploited) gross morphological evidence, and the more recently obtained molecular evidence, especially from DNA, supports the hypothesis that modern humans are more closely related to chimpanzees (and bonobos) than they are to any other living primate, and that the sister taxon of the chimpanzee and bonobo/human clade is the gorilla. Indeed, modern humans and chimpanzees are evidently so closely related that it is no longer appropriate to place these taxa in different families, the Hominidae and the Pongidae, respectively. Instead, they are placed in a separate subfamily, the Homininae (informal name ‘hominines’), and then differentiated at the level of the ‘tribe’, which is a less inclusive category between the family and the genus. Modern humans, and all the fossil taxa more closely-related to modern humans than to living chimpanzees or bonobos (i.e. all the members of the human clade) are usually now put in the tribe Hominini (informal name ‘hominins’). The equivalent tribe for the chimpanzee and bonobo clade is the Panini (informal name ‘panins’). Gorillas, although more closely related to chimpanzees/bonobos and modern humans than to the only Asian great ape, the orangutan, are usually allocated to their own subfamily, the Gorillinae (informal name ‘gorillines’). Various attempts to calibrate the evolutionary history of the African great ape clade suggest that the hypothetical last common ancestor (LCA) of chimpanzees/bonobos and modern humans existed somewhere in the interval between 8 and 6 Ma. However, we know relatively little about this elusive LCA. So when Bernard Wood was asked to organize an ASGBI symposium on human evolution, he and his co-organizer, Sarah Elton, extended invitations with very specific instructions. Potential contributors were asked to make, for their particular specialism, predictions about what the hypothetical LCAs of the hominin/panin and hominin clades would have been like. They were also asked to trace the subsequent evolution of that particular system or function within the hominin clade, and then to speculate about when in human evolution the modern human condition for that system or function appeared. These are, by any criterion, rigorous demands and the organizers are still amazed that the invitees signed-on to this vision of the symposium. The fact that many contributors were recruited from the membership of the ASGBI is a tribute to the scope and quality of palaeoanthropological and morphological research in the UK. The symposium was held at the Society's 2007 Winter Meeting at St Anne's College, Oxford, and all the papers in this issue of the Journal of Anatomy were prepared by speakers at the symposium, together with their research colleagues. A previous ASGBI symposium devoted to human evolution reviewed the hominin fossil record and focused on the methods used to extract as much information as possible from the fossil evidence for human evolution. The authors who contributed to that symposium were asked to address the subsequent review papers, published as a special issue of Journal of Anatomy in 2000, to a non-specialist/graduate student audience and their success in doing so is reflected in the high citation rates of many of the contributions. Sticking with this model, all the authors contributing to the current special issue were charged with preparing a text that would be accessible to non-specialists and to students, and the organizers are grateful to their colleagues for doing this; it is much easier to hide behind jargon that to write a paper in language that is accessible. We hope that the utility of the papers in this 2008 issue of the Journal of Anatomy will at least match, if not exceed, that of the papers in the 2000 issue. The 2007 symposium necessarily covered a wide range of topics, from the molecular evidence for human evolution to specific regions of the body. This breadth is reflected in the contributions to this special issue. Arguably the most important step towards understanding human evolution in the last two decades has come not from the discovery of fossil evidence, but from comparative and molecular biology. The fruits of such work have yielded estimates of the likely time period for the LCA of modern humans and chimpanzees/bonobos. Thus, although not strictly anatomical or morphological, the first paper in this volume, by Brenda Bradley, is an up-to-date, uniquely comprehensive and comprehensible review of the molecular evidence for the relationships within the ape part of the Tree of Life. This is a rapidly moving and complex field and we are especially grateful to Brenda for her clear explanations and presentation of all the lines of evidence. In a similar vein, Bernard Wood and Nicholas Lonergan summarize the fossil evidence for human evolution, in order to provide a general context for the papers that follow. However, the bulk of their paper is devoted to explaining the theory and practice of the methods researchers use to generate taxonomic and systematic hypotheses (i.e. hypotheses about how many species and genera should be recognized within the hominin clade, and hypotheses about the internal structure of the hominin clade). As human evolution did not take place in a vacuum, Sarah Elton's review makes predictions about what the environments would have been like at the time of the LCA and during the history of the hominin clade. Part of what makes species distinct is the rate at which the individuals belonging to those species proceed through the various stages of immaturity to adulthood. This is the domain of life history and Shannen Robson and Bernard Wood review the evidence researchers use to look at ontogeny in extinct taxa, and the ways the timing of ontogenetic events differ within the great ape (i.e. modern humans, chimpanzees and bonobos, gorillas and orangutans) clade. They also speculate about life-history evolution within the hominin clade. If you asked the proverbial occupant of the top deck of the Clapham omnibus what are the most obvious differences between modern humans and chimpanzees, they are likely to point to the differences in brain size, and to the many ways the capacities and behaviour of the two organisms differ. Chet Sherwood, Francys Subiaul and Tad Zawidzki, in a magisterial survey, make predictions not only about the differences between the macrostructure and microstructure of the brains of the members of the great ape clade, but also about the likely behaviours of the LCAs. Unusually for a palaeoanthropologically based review of the brain, they have attempted to link these two types of evidence. The remaining contributions address either anatomical regions (cranial base, face and hand) or functional systems (locomotion and posture, intra-oral feeding and mastication) and speculate about what the evolution of those regions and systems would have been like in the African ape clade, and particularly in the hominin clade. The cranial base is a region of the hominin cranium that has undergone substantial spatial rearrangement and Lisa Nevell and Bernard Wood trace these changes immediately prior to, and within, the hominin clade. Sam Cobb summarizes the evidence for the face and helpfully melds information about the fossil evidence with information about growth and development. Peter Lucas, Paul Constantino and Bernard Wood select the aspects of dental and gnathic morphology that are best represented in the hominin fossil evidence, and then they make predictions about the states of these various variables and characters in the various LCAs. The remit given to Robin Crompton, Evie Vereecke and Susannah Thorpe was in many ways the most difficult. The postcranium is the poor relative of the skull when it comes to fossil evidence, and even when there is postcranial fossil evidence, unless researchers are lucky enough to find associated skeletons they do not always know which heads go with which limbs. Thorpe, Crompton and colleagues have recently suggested that the upright posture and bipedal gaits of fossil hominins and modern humans may have their origin in the type of bipedalism that is occasionally seen in orangutans. This would mean that hominins need not have evolved from an ancestor with a chimpanzee/bonobo-like gait. This and the prodigious amount of information assembled in their review suggests that it is likely to be both controversial and influential. The final contribution by Matt Tocheri, Caley Orr, Marc Jacofsky and Mary Marzke harks back to the main research interest of John Napier, a distinguished member of the ASGBI and one of the founders of primatology. Among the findings Matt Tocheri and his colleagues focus on in their review is the suggestion that a major reorganization of the hand took place after rather than before, or at the same time, as the origin of Homo (as traditionally defined). We owe particular thanks to the co-authors who had all the ‘grit’ of preparing a paper, and none of the ‘gravy’ in the form of being a guest of the Society in Oxford. We are especially grateful to those authors who are not members of the Society; they honour the ASGBI by their contributions. We would also like to express our thanks to the reviewers, and the Council of the ASGBI for the financial support that made the symposium possible. We also acknowledge the support and fortitude of the Editor of the Journal of Anatomy, Gillian Morriss-Kay, and the Managing Editor, Edward Fenton. Their editorial skills and patience are equally remarkable and we thank them very much for their help and advice over the past months.