In hominid evolution, the shape of the inner frontal bone in the median sagittal plane has, in contrast to the outer vault, not changed since at least the Plio-Pleistocene (Bookstein et al., 1999). Nonetheless, inner vault size increased significantly (by ∼ 11%) while the size of the outer frontal profile did not. Thus, two of the more interesting questions to pose are: ‘At which other positions of the skull have major shape and size changes taken place?’ and ‘Could it be that the exocranium is involved in shape changes while the endocranium is involved in size changes?’. We approach these questions by analyzing general shape and size of both the exo- and endocranium in the median sagittal plane. Importantly, because the inner surface of the braincase provides information concerning brain evolution (Jerison, 1973) and because of the synevolution of cerebellar and frontal lobes (Seidler et al., 1997), our investigation also includes the occipital bone.

Geometric relations in the median-sagittal plane

Our sample includes 21 crania of modern humans of both sexes (10 females, 11 males): 15 from Central Europe, two San and two Bantu, and two Papuans. To these we added the stereolithographs of three mid- Pleistocene fossil hominid crania (Seidler et al., 1997): Kabwe (Broken Hill 1; Woodward, 1921), Petralona (Kokkoros & Kanellis, 1960), both of uncertain age – but probably in excess of 200 000 years old; and Atapuerca SH5 cranium (Arsuaga et al., 1993), about 300 000 years old.

Despite the emphasis on brain size in the classic paleoneurological literature, it has long been recognized that species-specific adaptations have neurological substrates that depend on more than just overall brain size. This concept is embodied in Harry Jerison's principle of proper mass (1973, p. 8): ‘The mass of neural tissue controlling a particular function is appropriate to the amount of information processing involved in performing the function.’ Thus brains do not merely enlarge globally as they evolve, their cortical and internal organization also changes in a process known as reorganization. The chapters in Part II explore the neurological underpinnings of some of the senses, adaptations, and cognitive abilities that are important for primates.

It is fitting that this section opens with a chapter on cerebral diversity by Todd Preuss that provides a clear synopsis of mammalian cortical anatomy, thus laying the groundwork for the rest of the volume. Preuss details the tension between classical studies that emphasize the microanatomical similarities between brains of different mammals and therefore highlight brain size as a main focus of natural selection, and more recent investigations that reveal a remarkable diversity in mammalian cortical organization and therefore emphasize neurological reorganization as a driving force during brain evolution (Preuss, 1993, 1995). There is no doubt where Preuss comes down in this debate, as is clear from his observation that ‘to focus exclusively on the ancestral features of cortical organization that mammals share, however, is to ignore those features of cortical organization that distinguish one group of mammals from another and provide the basis for their particular behavioral and cognitive abilities.’

Beginning with the 1973 publication of his classic monograph, Evolution of the Brain and Intelligence, Harry Jerison's ongoing research has had a profound impact on the questions, methods, and theoretical framework that continue to shape the field of brain evolution. On April 2,1998, researchers from Europe, Africa, and the United States gathered in Salt Lake City at the sixty-seventh annual meeting of the American Association of Physical Anthropologists to take part in a symposium that recognized and celebrated Harry Jerison's intellectual influence on the development of our discipline. The session, entitled ‘Current findings on mammalian, primate, and human brain evolution: A symposium in honor of Harry J. Jerison’, was the impetus for the present volume. In addition to contributions from participants at that symposium, several prominent investigators who could not be present have also contributed chapters. Although the fourteen chapters that comprise the bulk of this volume were intended to be ‘state-of-the-art’ reviews of the different sub-areas of brain evolution, many authors have gone much further by contributing new results of original research that would normally appear in peer-reviewed journals, and by providing glimpses of where they think the field is headed as we enter the twenty-first century. That they have chosen to do so is a reflection of their admiration, respect, and fondness for Harry Jerison.