A group coordinated by Dr. Vera Weisbecker examined whether the evolution of mammalian brain partitions follows conserved developmental constraints, causing the brain to evolve as an integrated unit in which the partitions scale with brain size. According to this ‘late equals large’ hypothesis, the timing of neurogenesis predicts the size of the partition such that later and more extended neurogenesis produces larger partitions due to the production of more neural precursors. In order to investigate the impact of neurogenesis on patterns of brain partition growth, the volumes of the whole brain and major partitions were reconstructed from soft-tissue diceCT scans of three marsupial species, including individuals with ages ranging from 1 day to adulthood. They tested three hypotheses consistent with a conserved brain partition growth: H1 postulates that partition scaling during development reflects the evolutionary partition scaling, and thus growth patterns should be uniform between species; H2 assumes that a neurogenesis-driven pattern of partition scaling is predictable from adult brain size, i.e. brain partitions scale with brain size; and H3 states that growth with age might differ between species according to brain size and/or neurogenetic events. Regressions of log partition volume against log rest-of-the-brain volume (whole-brain volume minus partition volume) showed significant interspecific differences in slopes and intercepts of most brain partitions, indicating diverse scaling patterns between species, which could not be predicted by adult brain size, as the smallest-brained species had intermediate slope to the other two. Growth curves of log partition volume against age were similar in all partitions within-species, but differed between species, particularly in growth rates, with the species with intermediate brain size having slower rates than the other two. Differences in growth patterns do not seem to be related to neurogenetic schedule as largest partitions are not especially late in their development and important maturation processes, like eye opening, occur closer to the end of the growth phase. Thus, none of the hypotheses are supported by these results, challenging the conserved neurogenetic schedules behind the evolution of mammal brain partitions. Moreover, the authors found high phylogenetic signal in brain partition scaling, revealing that a large part of the scaling relationship between brain and partition volumes is explained by phylogeny, which is more in agreement with a mosaic evolution of brain partition sizes, stressing its biological meaning and the level of mammalian brain plasticity. However, the intraspecific regular partition growth curves led the authors to contemplate the existence of an early brain partition pattern regulated by regional gene expression, and propose that further studies of brain partition evolution should integrate developmental neuromere expression models, neuron density, and patterns of neuron migration.
In their last review Jean-Jacques Hublin, Simon Neubauer and Philipp Gunz address the effects of hominins’ life histories in brain evolution. Encephalization in humans involved energetic costs that were sustained through changes in social structure and metabolic adaptations, including changes in the diet quality, as explained by the Expensive Tissue Hypothesis, and the ability to store energy in fat tissue, the primary source of energy for neonate brains. Because of the constraints of birth-giving associated with bipedalism, human brains develop mainly after birth. Also because of this prolonged development, the brain is exposed to a rich environment during its wiring process, with the child furthermore protected by the social community.
The study of fossil hominins’ brain development is only possible through the analysis of their endocranial casts, using cross-sectional samples. To establish their life histories it is necessary to attribute an age at death, generally according to the eruption timing of the teeth. Beyond variation in brain size, signs of brain reorganization can be investigated in fossil species to get insights about their cognitive capacities. Regarding Australopithecus, it is not yet clear how their brain development and life histories were more like that of humans or that of chimps. Homo erectus had body proportions and social structures closer to that of modern humans, but different brain organization and smaller brain capacity than that of latter Homo, pointing to a faster life history. Homo sapiens and H. neanderthalensis separately evolved similar brain capacities, although with different morphologies, and had different life histories, faster in the latter. Brain organization differs soon after birth, as modern humans undergo the so-called globularization phase, which does not exist in chimps or Neanderthals. This globular shape of the brain is characterized by the bulging of the parietal areas which may be linked to reorganization of the internal regions, like the precuneus.
There is still a debate about whether or not life histories in fossils can be investigated in terms of brain growth and development. The different brain morphology of Australopithecus when compared with African apes suggests that brain reorganization could have pre-dated encephalization. At last, our patterns of socio-cultural evolution might have been fundamentally responsible for the adaptive changes essential for the evolution of our big brains.
The middle cranial fossa houses the anterior and lateral portions of the temporal lobes. The evolutionary changes of this area in the human genus have been largely investigated by different teams coordinated by Markus Bastir and Antonio Rosas, at the Museum of Natural History, Madrid, Spain. They suggested that inter-specific differences in its morphology (namely a forward displacement of its anterior tip) can be associated with the relative enlargement of the temporal lobes described in modern humans, when compared with apes. They also provided morphological evidence of general differences in the endocranial base between modern humans and Neandertals. In their last article they include also considerations on the sulcal pattern, as visible on the endocranial surface. We have to keep in mind that the endocranial base is influenced by many different factors, and many of them are not associated with actual brain changes. The central position in the cranial base makes the middle cranial fossa sensitive to the development and evolution of the many surrounding structures. The same authors have shown before that the morphology of the middle cranial fossa is significantly correlated in terms of spatial organization with the morphology of the mandibular ramus due to direct physical interaction, being integrated as a modular unit. This integration can be associated with interactions between basicranium, brain and masticatory system during evolution and development. The middle cranial fossa also correlates significantly with the face, constituting a “bridge” for the interaction between the face and the neurocranium. In sum, the morphology of this area can be influenced by traits and processes associated with the face, with the many factors involved in the morphogenesis of the cranial base, as well as with the endocranial soft tissues (brain, meninges, vessels). Despite the neuroanatomical evidence of relatively larger temporal areas in our species, the exact correspondence and match between middle cranial fossa and temporal lobes may be more complex than a simple equivalence between a structure and its negative mould.
Ana Sofia Pedro
Edited by Emiliano Bruner
Springer Series in Bio-/Neuroinformatics
1. Paleoneurology resurgent! (Ralph Holloway, Columbia University); 2. Neuroscience and human brain evolution (Laura D. Reyes and Chet C. Sherwood, The George Washington University); 3. Computed tools for paleoneurology (Philipp Gunz, Max Planck Institute for Evolutionary Anthropology); 4. Functional craniology and brain evolution (Emiliano Bruner, Centro Nacional de Investigación sobre la Evolución Humana); 5. Human brain evolution: ontogeny and phylogeny (Simon Neubauer, Max Planck Institute for Evolutionary Anthropology); 6. Paleoneurology and behaviour (Natalie Uomini, University of Liverpool); 7. Neuroarchaeology (Dietrich Stout and Erin Hecht, Emory University); 8. Cognitive archaeology and the cognitive sciences (Frederick Coolidge, Thomas Wynn, Karenleigh Overmann, and James Hicks, University of Colorado, Colorado Springs); 9. Techniques for studying brain structure and function (Erin Hecht and Dietrich Stout, Emory University); 10. A digital collection of hominoid endocasts (José Manuel de la Cuétara, Universidad Autónoma de Madrid).
Functional craniology and brain evolution:
from paleontology to biomedicine
Frontiers in Neuroanatomy 8,19 (2014)