Dean Falk - US grants

We propose to test a series of hypotheses indicating that there is a genetic basis for variability in various cortical features (including asymmetries) by using advanced computer technology to analyze endocranial casts (endocasts) prepared from 600 skulls of rhesus monkeys (Macaca mulatta) from Cayo Santiago. An endocast, or replica of the interior braincase surface, reproduces accurate details of external brain morphology including sutures, venous sinuses, shape, and cortical sulci. The Cayo Santiago skeletal collection provides a unique research opportunity because it is the only primate osteological collection in the world that is associated with known maternal genealogies, ages at death, 30 years of behavioral/demographic records, and numerous publications on the heritabilities of external aspects of the skull. In this study, we will undertake a computer analysis of all endocast and cranial data in order to estimate heritabilities and genetic correlations of cortical features, and to compare the genetic bases for variation in osteological versus neurological features. Prior studies of endocasts have relied upon simplistic two dimensional data acquisition techniques which have sometimes led to erroneous conclusions. However, to understand the morphology of the brain's surface, accurate measurements of the location, orientation, length and other parameters for anatomically significant features must be available in three dimensions. An approach for three dimensional analysis of endocasts is already available in specialized diagnostic medical imaging centers and in discrete manufacturing computer aided design facilities. Specifically, medical computed tomography (CT) scanners have been applied to the study of endocasts, and surface measurements have been made manually with a specialized 3-dimensional digitizer unit. The proposed research is based on a collaboration between uniquely qualified researchers at three Institutions who propose to use this advanced imaging and computer aided design technology which is being made available to the project without cost.

The manner and mechanisms by which the human brain has developed its enormous relative size and intricate mode of patterning is one of the most fundamental questions in physical anthropology. Traditionally, the brain was thought to have first enlarged in response to, and concomitantly with, earliest human tool fabrication. It is now known, however, that human tools show up in the fossil record long before brain enlargement, and other theories have subsequently been offered to explain this uniquely human characteristic. This investigator explains brain enlargement as an indirect result of changes in cerebral blood flow and seeks to test two related hypotheses about the pattern of blood flow in hominoids. These hypotheses can be tested by examination of the interior of skulls, because the pattern of meningeal blood vessels provides an indelible imprint on the interior of the skull. The first hypothesis is that the branching pattern of middle meningeal blood vessels distinguishes between the two types of australopithecine fossil. The second is that a "strong" branching pattern correlates with larger cranial capacity. The latter hypothesis will be tested on a large number of human and ape skulls to determine the nature of the correlation between cranial capacity and blood vessel branching pattern in higher primates including humans. The research will provide further evidence regarding the "radiator" theory of brain enlargement, whereby bigger brains are claimed to be characterized by a more complicated network of vessels than smaller brains.

Casts of the interiors of skulls (endocasts) of our early ancestors indicate the volume and shape of their brains, and reproduce features of the cerebral cortex that were imprinted on the walls of their braincases during life. These features include convolutions, sulcal patterns, and blood vessels, which are sometimes correlated with known functions such as speech and right-handedness in living people. Endocasts are therefore useful for addressing questions about the origins of certain behaviors. Skulls of important Pleistocene fossil hominids are currently being CAT-scanned by an international team, and the resulting three dimensional (3D) CT data are available for generating and measuring images of endocranial surfaces from a computer screen. In order to validate use of 3D-CT to investigate brain surface morphology, a set of measurements will be obtained and compared from 3D-CT images and the corresponding traditional endocasts of several key hominid skulls. Additionally, the application of 3D geometrical methods for analyzing endocasts will be explored in an ape and human skull series. Once validated, these methods can be applied to new specimens, leading to further insights about brain evolution.