The evolution of neurocranial morphology in Homo sapiens is characterized by bulging of the parietal region, a feature unique to our species. In modern humans, expansion of the parietal surface occurs during the first year of life, in a morphogenetic stage which is absent in chimpanzees and Neandertals. A similar variation in brain shape among living adult humans is associated with expansion of the precuneus. Using MRI-derived structural brain templates, we compare medial brain morphology between humans and chimpanzees through shape analysis and geometrical modeling. We find that the main spatial difference is a prominent expansion of the precuneus in our species, providing further evidence of evolutionary changes associated with this area. The precuneus is a major hub of brain organization, a central node of the default-mode network, and plays an essential role in visuospatial integration. Together, the comparative neuroanatomical and paleontological evidence suggest that precuneus expansion is a neurological specialization of H. sapiens that evolved in the last 150,000 years that may be associated with recent human cognitive specializations.

Evidence for expansion of the precuneus in human evolution.

Paleoneurology deals with the study of brain anatomy in fossil species, as inferred from the morphology of their endocranial features. When compared with other living and extinct hominids, Homo sapiens is characterized by larger parietal bones and, according to the paleoneurological evidence, also by larger parietal lobes. The dorsal elements of the posterior parietal cortex (superior parietal lobules, precuneus, and intraparietal sulcus) may be involved in these morphological changes. This parietal expansion was also associated with an increase in the corresponding vascular networks, and possibly with increased heat loads. Only H. sapiens has a specific early ontogenetic stage in which brain form achieves such globular appearance. In adult modern humans, the precuneus displays remarkable variation, being largely responsible for the longitudinal parietal size. The precuneus is also much more expanded in modern humans than in chimpanzees. Parietal expansion is not influenced by brain size in fossil hominids or living primates. Therefore, our larger parietal cortex must be interpreted as a derived feature. Spatial models suggest that the dorsal and anterior areas of the precuneus might be involved in these derived morphological variations. These areas are crucial for visuospatial integration, and are sensitive to both genetic and environmental influences. This article reviews almost 20 years of my collaborations on human parietal lobe evolution, integrating functional craniology, paleoneurology, and evolutionary neuroanatomy.

https://cir.cenieh.es/bitstream/20.500.12136/652/1/Human%20paleoneurology%20and%20the%20evolution%20of%20the%20parietal%20cortex_Bruner_2018.pdf

Human Paleoneurology and the Evolution of the Parietal Cortex.

Folding of the primate brain cortex allows for improved neural processing power by increasing cortical surface area for the allocation of neurons. The arrangement of folds (sulci) and ridges (gyri) across the cerebral cortex is thought to reflect the underlying neural network. Gyrification, an adaptive trait with a unique evolutionary history, is affected by genetic factors different from those affecting brain volume. Using a large pedigreed population of ∼1000 Papio baboons, we address critical questions about the genetic architecture of primate brain folding, the interplay between genetics, brain anatomy, development, patterns of cortical–cortical connectivity, and gyrification’s potential for future evolution. Through Mantel testing and cluster analyses, we find that the baboon cortex is quite evolvable, with high integration between the genotype and phenotype. We further find significantly similar partitioning of variation between cortical development, anatomy, and connectivity, supporting the predictions of tension-based models for sulcal development. We identify a significant, moderate degree of genetic control over variation in sulcal length, with gyrus-shape features being more susceptible to environmental effects. Finally, through QTL mapping, we identify novel chromosomal regions affecting variation in brain folding. The most significant QTL contain compelling candidate genes, including gene clusters associated with Williams and Down syndromes. The QTL distribution suggests a complex genetic architecture for gyrification with both polygeny and pleiotropy. Our results provide a solid preliminary characterization of the genetic basis of primate brain folding, a unique and biomedically relevant phenotype with significant implications in primate brain evolution.

https://www.genetics.org/content/genetics/200/2/651.full.pdf

Cortical Folding of the Primate Brain: An Interdisciplinary Examination of the Genetic Architecture, Modularity, and Evolvability of a Significant Neurological Trait in Pedigreed Baboons (Genus Papio)

The spatial relationships between brain and braincase represent a major topic in surgery and evolutionary neuroanatomy In paleoneurology, neurocranial landmarks are often used as references for brain areas In this study, we analyze the variation and covariation of midsagittal brain and skull coordinates in a sample of adult modern humans in order to demonstrate spatial associations between hard and soft tissues The correlation between parietal lobe size and parietal bone size is very low, and there is a marked individual variation The distances between lobes and bones are partially influenced by the dimensions of the parietal lobes The main pattern of morphological variability among individuals, associated with the size of the precuneus, apparently does not influence the position of the neurocranial sutures Therefore, variations in precuneal size modify the distance between the paracentral lobule and bregma, and between the parietal lobe and lambda Hence, the relative position of the cranial and cerebral landmarks can change as a function of the parietal dimensions The slight correlation and covariation among these elements suggests a limited degree of spatial integration between soft and hard tissues Therefore, although the brain influences the cranial size and shape during morphogenesis, the specific position of the cerebral components is sensitive to multiple effects and local factors, without a strict correspondence with the bone landmarks This absence of correspondent change between brain and skull boundaries suggests caution when making inferences about the brain areas from the position of the cranial sutures The fact that spatial relationships between cranial and brain areas may vary according to brain proportions must be considered in paleoneurology, when brain anatomy is inferred from cranial evidence

The brain and the braincase: a spatial analysis on the midsagittal profile in adult humans.

https://hal.inria.fr/hal-01440065/document

Morphoarchitectural variation in South African fossil cercopithecoid endocasts

The development of formalin-free fixatives is an urgent issue in gross anatomy because of the health hazard and the tissue-hardening actions of formalin. We recently identified the fixative, antimicrobial, and preservative effects of N-vinyl-2-pyrrolidone (NVP), a precursor of the water-soluble macromolecular polymer polyvinylpyrrolidone, in animal experiments. The aim of the present study is to investigate whether NVP solution can be used as an alternative to formalin in human cadaveric dissection. Twelve donated cadavers were infused with NVP via the femoral and common carotid arteries using a peristaltic pump. Experienced teaching staff members in our department dissected the cadavers and examined their macroanatomical properties. The NVP-embalmed corpses showed no sign of decomposition or fungal growth. The bodies remained soft and flexible. Notably, the shoulder, elbow, wrist, phalangeal, hip, knee, cervical spine, and temporomandibular joints were highly mobile, almost equivalent to those of living individuals. The range of motion of most joints was greater in the NVP-fixed than formalin-fixed cadavers. Under the dermis, the subcutaneous fat was markedly reduced and the connective tissues were transparent, so the ligaments, cutaneous nerves, and veins were easily discernible. The abdominal wall and the visceral organs remained pliable and elastic, resembling those of fresh cadavers. The lungs, liver, and gastrointestinal tract were moveable in the thoracic and abdominal cavities and were readily isolated. NVP can be used successfully as a fixative and preservative solution for human cadavers; furthermore, NVP-embalmed bodies could be valuable for learning clinical skills and for training, and for developing innovative medical devices. Clin. Anat. 31:90-98, 2018. © 2017 Wiley Periodicals, Inc.

A new substitute for formalin: Application to embalming cadavers.

In order to evaluate the extent of the subdivisions of Neanderthal brains, we explored methods to determine the extent of subdivisions of brains in extant primate species. In the present study, we analyzed skulls and brains of macaque monkeys (Macaca fascicularis). Under deep anesthesia, five aged monkeys were perfused transcardially with phosphate-buffered 10 % formalin. The heads were scanned using a Toshiba Asterion CT scanner, and the reconstructed skulls and endocasts were compared with the convolutional patterns of the brain. In contrast to adult humans, which barely exhibit impressions in the upper part of the calvaria, the endocasts of the monkey skulls showed marked impressions of the cerebral sulci and gyri through the entire surface. On the dorsolateral surface, we identified most of the major sulci including the principal, arcuate, central, intraparietal, lunate, lateral, and superior temporal sulci, as well as the gyri in-between. On the ventral surface, we identified the medial and lateral orbital sulci, and the anterior middle temporal sulcus. Some of the individual differences in sulcal patterns were also observed on the endocast surface. We can thus infer the extent of major subdivisions of the macaque cerebral cortex by creating endocasts.

Cerebral Sulci and Gyri Observed on Macaque Endocasts

C-terminals in the mouse branchiomotor nuclei originate from the magnocellular reticular formation.

In order to estimate the extent of cortical subdivisions in the now extinct Neanderthals, we explored the relationship between skull and brain morphology in extant primate species, including humans. In this study, we used macaque monkeys (Macaca fascicularis) to determine the relationship between the location of sutures and the location of major sulci of the brain. Among the sulci examined, the inferior limb of the arcuate sulcus showed a close spatial relationship with the ventral portion of the coronal suture. The inferior limb of the arcuate sulcus is homologous to the ventral part of the precentral sulcus in modern humans, and it defines the caudal border of the prefrontal association cortex. These findings prompted us to examine whether a similar relationship could be observed in Neanderthals and modern humans, in order to obtain a more accurate inference of the extent of prefrontal cortical development in Neanderthals.

Yoshinori Haizuka

Papers

A new substitute for formalin: Application to embalming cadavers.

Cerebral Sulci and Gyri Observed on Macaque Endocasts

C-terminals in the mouse branchiomotor nuclei originate from the magnocellular reticular formation.

The Coronal Suture as an Indicator of the Caudal Border of the Macaque Monkey Prefrontal Cortex