Two different papers have been published this month on the evolution of the supraorbital anatomy in humans. The first article is on Neanderthal facial morphology, and it was coordinated by Stephen Wroe, of the FEAR lab. Here a comment on the Daily Mail. The second article, by Ricardo Miguel Godinho and coauthors, links supraorbital morphology and social dynamics, and it was commented in a News and Views by Markus Bastir.
Primates are unique among mammals for having a brain much larger than expected for body size. An important aim in paleoneurology is understanding how cerebral structures reorganized to accomodate primate cerebral expansion. The brain comprises only soft-tissue and does not fossilize so paleoneurologists rely on endocasts, either physical or digital molds of the cranial cavity, to estimate the macro-anatomy of the brain. Continuing computational advances and powerful imaging techniques have allowed the generation of increasingly higher-resolution digital endocasts. Gonzales et al. (2015) generated a high-resolution endocast of the 15 Myr-old fossil cercopithecine Victoriapithecus macinnesi using micro-CT scans. By using computational methods, taphonomic distortion was corrected and a new endocranial volume (ECV) of 35.6 cm3 reported for Victoriapithecus which is much smaller than the previous value 54 cm3. This new, smaller ECV places Victoriapithecus within the range of extant strepsirrhines but outside the range expected of extant and fossil cercopithecoids including the 32 Myr-old fossil species Aegyptopithecus zeuxis which had an ECV within the expected range for fossil cercopithecoids.
Despite Victoriapithecus exhibiting a very small ECV and falling below the range for extant cercopithecoids, the fossil does exhibit the ‘frog-shaped’ sulcal pattern shared only among cercopithecines. This sulcal pattern suggests Victoriapithecus is a cercopithecine, the ‘frog-shaped’ sulcal pattern is such a diagnostic trait that it is not shared by the leaf-eating colobines but only present in cercopithecines. The olfactory bulbs in Victoriapithecus are unusually large relative to the small ECV. Large olfactory bulbs are present in extant strepsirrhines and the fossil catarrhine Aegyptopithecus zeuxis but reduced in all extant and fossil cercopithecoids and hominoids. The presence of small olfactory bulbs in the 18 Myr-old hominoid Proconsul versus the large bulbs in Victoriapithecus suggested olfactory bulb reduction may have evolved independently in both cercopithecoids and hominoids.
Harrington et al. (2016) compared digital endocasts generated from micro-CT of three adapiform fossil primates including the 48 Myr-old Notharctus tenebrosus, 47 Myr-old Smilodectes gracilis and 45 Myr-old Adapis parisiensis. Results of endocranial volume (ECV) were consistent with other studies revealing an ECV of 7.6 cm3 for Notharctus, an ECV of 8.3 cm3 for Smilodectes while Adapis had an ECV of 8.8 cm3. The sulcal morphology of these adapiforms was also consistent with previous studies showing the defining feature of the primate brain, the Sylvian sulcus, is species-specific in these adapiforms. The Sylvian sulcus is well-defined in Adapis, occurs only as a shallow depression in Notharctus but is entirely lacking in Smilodectes. The absence of the Sylvian sulcus in Smilodectes is not understood but as it is absent in other mammals, this may represent a retained ancestral trait from before the divergence of primates from other mammals.
The cerebral organization of Notharctus and Smilodectes showed both possessed larger temporal and occipital lobes relative to brain size with smaller olfactory bulbs and frontal lobes. This trend might indicate cerebral reorganization favoring larger visual-auditory structures located in the temporal-occipital regions of the brain versus smaller visual-olfactory structures in the frontal region. The olfactory bulbs of these adapiforms were small and blunt relative to endocranial volume and predicted body mass but uniquely, Adapis parisiensis had the largest olfactory bulbs, placing it within the range of extant strepsirrhines. These studies reveal how little is understood about primate paleoneurology and the evolutionary trends of different primate lineages with implications for the human fossil record.
Intracranial infections represent serious brain diseases that occur in various forms and often may be hard to recognize in their earlier stages. A fast diagnosis is crucial for an effective treatment. Various technique of CT and MRI imaging have been developed to distinguish the symptoms in the brain and its associated tissues (see for example Hsu 2010). Radiologists recognize several categories of infections according to the origin (e.g. congenital and neonatal), location (intra-axial, extra-axial), or characters. In general, infections of the brain parenchyma, meninges and ventricles can have bacterial, viral, fungal, or parasitic origins. Bacterial infections usually develop from early cerebritis (inflammation of the cerebrum) to formation of abscesses (accumulation of infectious material and microorganisms) within the cranium. Some bacteria have more specific effects. Streptococcus pneumonia and Neisseria meningitides are common cause of bacterial meningitis (inflammation of the meninges and the cerebrospinal fluid). Tuberculomas, abscesses and tuberculous meningitis indicate presence of Mycobacterium tuberculosis (TB). Frequent viral infections are Herpes Simplex Encephalopathy involving Herpes simplex virus 1 (HSV-1), or infections induced by Human Immunodeficiency Virus (HIV) leading to cerebral atrophy and white matter disease. Fungal infections usually generate abscesses filled with fungi. Cryptococcosis and fungal meningitis are frequent fungal infections in some specific geographical regions. Examples of parasites causing intracranial infections are Toxoplasma gondii, or Taenia solium causing cysticercosis, which also leads to acquired epilepsy (see Vaccha et al. 2016). Consequences of intracranial infections could be in some cases lethal, in other cases they can cause a severe damage. For instance, infections of meninges and cerebrospinal fluid leading to meningitis can further evolve into subdural (between the dura mater and the underlying arachnoid layers) and epidural (between the meninges and the bone) abscesses, hydrocephalus (excessive accumulation of fluid in the brain), ventriculitis (inflammation of the ventricles containing and circulating cerebrospinal fluid throughout the brain), and venous thrombosis. Brain infection can be extended into the bone tissue and cause cranial pathologies like osteomyelitis or mastoiditis. Sometimes the infections can even lead to bone fractures. The origin of brain infection is often associated with traumas, when the microorganism spread from the wound into the soft tissues of the endocranium.
The parietal lobe has a unique central location in the brain, and it is involved in higher cognitive functions. Investigating its functions and connectivity is essential to understand its role in uniquely human abilities. Two recent works have put emphasis on the importance of the parietal lobe for tool use.
Catani and colleagues investigated the intralobar parietal connectivity in human and monkey brains, using diffusion imaging tractography. In general, the patterns of white matter connectivity are similar in both species, although with some differences for areas that are distinct in humans. The larger tract connects the superior parietal lobule (SPL) to the angular and supramarginal gyri of the inferior parietal lobule, within the IPS. The authors suggest it might act to mediate the interaction between the two lobules during object manipulation, and to coordinate both dorsal and ventral visuospatial networks. The second and third larger tracts link the postcentral gyrus to the inferior parietal lobule and to the SPL, respectively. These might transmit tactile and proprioceptive information on the body orientation relatively to an object for guiding motor actions and grasp. The connection between the postcentral and the angular gyri was only observed in humans, leading the authors to highlight its role in specific cognitive functions. Particularly, its connections to SPL are key for tool use, mathematical thinking, and language and communication.
Kastner and coworkers reviewed the organization and function of the dorsal pathway of the visual system of monkey and human brains, focusing on the areas of the posterior parietal cortex within and adjacent to the intraparietal sulcus (IPS). Monkeys and humans have diverged in the functional contributions of the IPS since their last common ancestor, as some functionally-defined areas that are located within the IPS in monkeys have been partially relocated outside this sulcus in humans. The authors suggest this relocation might be due to expansion for accommodation of human-specific abilities, such as tool use. They hypothesize that humans might have developed a derived and advanced tool network from the modification of the macaque circuit for object manipulation. First, the human dorsal vision pathway must provide object shape information regardless of size and viewpoint, facilitating object recognition and mental manipulation. Second, object information is integrated with cognitive information such as working memory, allowing maintaining the information over a period of time. Finally, humans have areas that respond specifically to tool use, some of which also integrate frontal and temporal networks involved in action recognition and semantic knowledge related to tools and actions, respectively.
Both studies point at the parietal lobes and visuospatial integration as key elements for human cognitive capacity, as suggested by evidence in paleoneurology, evolutionary neuroanatomy, and cognitive archaeology.
Bruner E. & Ogihara N. 2018. Surfin’ endocasts: the good and the bad on brain form. Palaentologia Electronica 21.1.1A: 1-10.
Following bachelor’s work in applied physics at Caltech and a first career as a research engineer at NASA/JPL, Brian Metscher completed his PhD in the then-new interdiscipline of evo-devo at the University of California, Irvine. He did postdoctoral research on the development and evolution of appendages and teeth at The Natural History Museum (London) and Penn State University, and then served five years as an Assistant Professor in southern Indiana. During the summers he carried out research at Yale University and came to the University of Vienna in 2006, to set up the imaging lab in the Department of Theoretical Biology, where he is now Senior Scientist. He helped to establish X-ray microtomography as an essential method for imaging ex vivo biological samples, especially embryos and invertebrates. His lab is developing new and refined sample preparation and imaging methods, with applications including molecular imaging and imaging of specific cells types. He coordinates a MicroCT Methods Forum. Here a brief interview …
What are the basic principles of these methods mixing histology and digital anatomy?
MicroCT provides 3D images of intact samples at resolutions that overlap with what is achieved by light microscopy of sectioned material. Contrast-enhanced X-ray images give only histomorphological information, so microCT images are a powerful complement to traditional histology, which takes advantage of a vast array of stains with different tissue specificities. MicroCT gives a 3D overview and context for more detailed section-based images from histology (and also electron microscope).
So, you stain specimens before microtomographic scan … what about these staining techniques?
The familiar X-ray images of bones or teeth inside the body are possible because the dense calcium-rich materials absorb a lot more X-ray energy than the soft tissues around them – skin, muscle, and internal organs, which are made up mostly of proteins and water. To make soft tissues clearly visible in X-ray images, it helps to add a contrast agent: this can be a suspension of an iodine- or barium-containing liquid injected or swallowed, as is common in clinical radiology examinations. In the case of non-living samples (ex vivo imaging, most of what I do), the sample can be stained with a substance that actually binds to the tissues and has a higher X-ray absorption. The contrast stains used most often are inorganic iodine, phosphotungstic acid, and (less frequently) osmium tetroxide. None of these is specific to any one tissue type, but they do allow the different tissues and structures to be distinguished clearly in the X-ray images.
What kind of expertise, career, and tools are necessary to work in this field?
As with any kind of biological imaging, it is necessary to have a good understanding both of the biological systems under study and of how the imaging systems actually work. So a strong background in microscopy, histology, and image acquisition and analysis is important. And one must always complement one’s own expertise with good working collaborations with partners in other fields.
What is, at present, the most intriguing current challenge?
We would really like to make microCT imaging more tissue- and molecule-specific. Thus I have collaborative projects to test new staining methods and calibrate their functions in microCT images with histological baselines. And my lab is working on refining the antibody imaging method we published a few years ago to make this a more robust and routine method for 3D imaging of gene expression and other molecular patterns in developmental, comparative, and medical-related research.