Dean Falk

Cerebral cortices of East african early hominids.

An endocast of the frontal lobe of a reconstructed skull, which is approximately 2 million years old, from the Koobi Fora region of Kenya appears to represent the oldest human-like cortical sulcal pattern in the fossil record, while the endocast from another skull from the same region produces an endocast that appears apelike in its frontal lobe and similar to endocasts from earlier South African australopithecines. New analysis of paleoanatomical evidence thus indicates that at least two taxa of early hominids coexisted in East Africa.

Brain evolution in Homo: The “radiator” theory

The “radiator” theory of brain evolution is proposed to account for “mosaic evolution” whereby brain size began to increase rapidly in the genus Homo well over a million years after bipedalism had been selected for in early hominids. Because hydrostatic pressures differ across columns of fluid depending on orientation (posture), vascular systems of early bipeds became reoriented so that cranial blood flowed preferentially to the vertebral plexus instead of the internal jugular vein in response to gravity. The Hadar early hominids and robust australopithecines partly achieved this reorientation with a dramatically enlarged occipital/marginal sinus system. On the other hand, hominids in the gracile australopithecine through Homo lineage delivered blood to the vertebral plexus via a widespread network of veins that became more elaborate through time. Mastoid and parietal emissary veins are representatives of this network, and increases in their frequencies during hominid evolution are indicative of its development. Brain size increased with increased frequencies of mastoid and parietal emissary veins in the lineage leading to and including Homo , but remained conservative in the robust australopithecine lineage that lacked the network of veins. The brain is an extremely heatsensitive organ and emissary veins in humans have been shown to cool the brain under conditions of hyperthermia. Thus, the network of veins in the lineage leading to Homo acted as a radiator that released a thermal constraint on brain size. The radiator theory is in keeping with the belief that basal gracile and basal robust australopithecines occupied distinct niches, with the former living in savanna mosaic habitats that were subject to hot temperatures and intense solar radiation during the day.

The Archaeology of Perception: Traces of Depiction and Language [and Comments and Reply]

Depiction, particularly the making of images to resemble things, can only have emerged prehistorically incommunities with shared systems of meanings. We argue, on the basis of an articulation of Gibsons ecological theory of perception, Meads distinction between communication and language, and a portmanteau theory of language and mind relying on the insights of, among others, Ryle, Vygotsky, and Olson, that depiction transforms communication into language. The rapid change in numerous practices observable at the end of the Upper Pleistocene becomes understandable when communication is seen to be tuming into language as here defined. It is for this reason that the period in question represents the point of evolution of modem human beings.

Advanced Computer Graphics Technology Reveals Cortical Asymmetry in Endocasts of Rhesus Monkeys

Lengths of cortical sulci were measured on ten endocranial casts (endocasts) from skulls of rhesus monkeys, using advanced computer technology that permits analysis and imaging of surface morphology in three dimensions. Sulcal lengths were compared in left and right hemispheres and, contrary to earlier reports, the length of the left Sylvian fissure was found to be significantly longer than its right counterpart, as is the case for chimpanzees and humans. This asymmetry in humans is thought to be associated with asymmetrical representation of language functions in the left hemisphere and, although this report is the first to demonstrate a significantly longer left Sylvian fissure in rhesus monkeys, our results are in keeping with psychophysical evidence that suggests that Macaca is left hemisphere dominant for perception of meaningful vocalizations. We attribute the difference between our findings and previous reports to the sensitivity of the new computer technology used to collect data from endocasts.

LB1's virtual endocast, microcephaly, and hominin brain evolution.

Earlier observations of the virtual endocast of LB1, the type specimen for Homo floresiensis, are reviewed, extended, and interpreted. Seven derived features of LB1s cerebral cortex are detailed: a caudally-positioned occipital lobe, lack of a rostrally-located lunate sulcus, a caudally-expanded temporal lobe, advanced morphology of the lateral prefrontal cortex, shape of the rostral prefrontal cortex, enlarged gyri in the frontopolar region, and an expanded orbitofrontal cortex. These features indicate that LB1s brain was globally reorganized despite its ape-sized cranial capacity (417cm(3)). Neurological reorganization may thus form the basis for the cognitive abilities attributed to H. floresiensis. Because of its tiny cranial capacity, some workers think that LB1 represents a Homo sapiens individual that was afflicted with microcephaly, or some other pathology, rather than a new species of hominin. We respond to concerns about our earlier study of microcephalics compared with normal individuals, and reaffirm that LB1 did not suffer from this pathology. The intense controversy about LB1 reflects an older continuing dispute about the relative evolutionary importance of brain size versus neurological reorganization. LB1 may help resolve this debate and illuminate constraints that governed hominin brain evolution.

Brain shape in human microcephalics and Homo floresiensis

Because the cranial capacity of LB1 (Homo floresiensis) is only 417 cm3, some workers propose that it represents a microcephalic Homo sapiens rather than a new species. This hypothesis is difficult to assess, however, without a clear understanding of how brain shape of microcephalics compares with that of normal humans. We compare three-dimensional computed tomographic reconstructions of the internal braincases (virtual endocasts that reproduce details of external brain morphology, including cranial capacities and shape) from a sample of 9 microcephalic humans and 10 normal humans. Discriminant and canonical analyses are used to identify two variables that classify normal and microcephalic humans with 100% success. The classification functions classify the virtual endocast from LB1 with normal humans rather than microcephalics. On the other hand, our classification functions classify a pathological H. sapiens specimen that, like LB1, represents an ≈3-foot-tall adult female and an adult Basuto microcephalic woman that is alleged to have an endocast similar to LB1s with the microcephalic humans. Although microcephaly is genetically and clinically variable, virtual endocasts from our highly heterogeneous sample share similarities in protruding and proportionately large cerebella and relatively narrow, flattened orbital surfaces compared with normal humans. These findings have relevance for hypotheses regarding the genetic substrates of hominin brain evolution and may have medical diagnostic value. Despite LB1s having brain shape features that sort it with normal humans rather than microcephalics, other shape features and its small brain size are consistent with its assignment to a separate species.

Cortical asymmetries in frontal lobes of rhesus monkeys (Macaca mulatta).

Cortical sulci were digitized and their lengths determined with 3-dimensional computer technology on 335 endocranial casts from rhesus monkeys with known maternal genealogies, ages at death, and sex. Non-metric data were also collected from 403 endocasts. Frontal lobes were directionally asymmetrical with lengths of the left central, right rectus (principal), and right lateral orbital sulci significantly longer. The positions of the medial and lateral ends of the central sulcus were significantly different in the two hemispheres, and there was significant protrusion of the frontal lobe (petalia) on the right side. Together, these data indicate elongation of the right orbital and dorsolateral frontal lobe. The asymmetries reported here probably involve short-term memory for visual information. This raises interesting questions about the extent to which macaques are right hemisphere dominant for processing visual information as compared to humans.

Evolution of the Primate Brain

The mammalian order of primates is known for a variety of species that are lively, curious, social, and intelligent. Nonhuman primates are of special interest to people, not only because they are appealing and entertaining to watch, but also because certain species (e.g., of macaques or baboons) are genetically close to humans, which makes them excellent animal models for medical research. As curious primates ourselves, we wonder about our evolutionary origins. One way to address this topic is to study and compare species from living primates that are thought to approximate broad stages (or grades) that occurred during some 65 million years of primate evolution. Thus, one may compare particular anatomical structures or behaviors across appropriate representatives from the series prosimian-> monkey-> ape -> human. When possible, such a comparative method should be supplemented with the direct method of studying fossil primates, which adds elements of specificity and time to the picture.

Human cortical asymmetries determined with 3D MR technology

A method is described for obtaining clear 3D magnetic resonance (MR) images of the cortical surface of the brain in living human subjects. By combining volume composite and depth encoded images, we have obtained surface coordinate data that resulted in highly repeatable measurements of sulcal lengths and cortical surface areas in eight normal adult volunteers. Sulcal lengths were determined for specific parts of the Sylvian fissure, central sulcus and frontal operculum. Additionally, angles were computed between the anterior and posterior limbs of the pars triangularis and the ascending and horizontal limbs of the posterior Sylvian fissure. The cortical surface areas enclosed by these limbs were also computed. Finally, thirteen non-metric cortical features (e.g., petalias) were scored from the 3D MR images. All measurements were compared in right and left hemispheres. In addition to corroborating cortical asymmetries reported in the literature, we observed previously unrecognized directional asymmetries in the length of the anterior limb of the pars triangularis, length of the ascending limb of the posterior Sylvian fissure, and position of the lateral end of the central sulcus. We attribute the finding of three new directional asymmetries for the human cortex, as well as the high repeatability of our measurements, to the sensitivity and accuracy of the 3D MR imaging technology that has recently become available.

The cerebral cortex of Albert Einstein: a description and preliminary analysis of unpublished photographs

Upon his death in 1955, Albert Einstein’s brain was removed, fixed and photographed from multiple angles. It was then sectioned into 240 blocks, and histological slides were prepared. At the time, a roadmap was drawn that illustrates the location within the brain of each block and its associated slides. Here we describe the external gross neuroanatomy of Einstein’s entire cerebral cortex from 14 recently discovered photographs, most of which were taken from unconventional angles. Two of the photographs reveal sulcal patterns of the medial surfaces of the hemispheres, and another shows the neuroanatomy of the right (exposed) insula. Most of Einstein’s sulci are identified, and sulcal patterns in various parts of the brain are compared with those of 85 human brains that have been described in the literature. To the extent currently possible, unusual features of Einstein’s brain are tentatively interpreted in light of what is known about the evolution of higher cognitive processes in humans. As an aid to future investigators, these (and other) features are correlated with blocks on the roadmap (and therefore histological slides). Einstein’s brain has an extraordinary prefrontal cortex, which may have contributed to the neurological substrates for some of his remarkable cognitive abilities. The primary somatosensory and motor cortices near the regions that typically represent face and tongue are greatly expanded in the left hemisphere. Einstein’s parietal lobes are also unusual and may have provided some of the neurological underpinnings for his visuospatial and mathematical skills, as others have hypothesized. Einstein’s brain has typical frontal and occipital shape asymmetries (petalias) and grossly asymmetrical inferior and superior parietal lobules. Contrary to the literature, Einstein’s brain is not spherical, does not lack parietal opercula and has non-confluent Sylvian and inferior postcentral sulci.