Modern human brain growth and development. Contribution to brain evolution in hominids

Abstract

Human phylogenetic history is directly related to brain evolution. But many biologic processes related to the appearance of this complex organ are unknown, mainly due to the fact that it is an organ composed of soft tissue, which is not sensitive to the fossilization processes. Hence, to infer human brain evolution it is essential to study the indirect evidences it leaves in the cranial bones, such as the endocranial size (cranial capacity) and shape. In this sense, the hominid fossil record has an important cranial representation in relation to other bones. However, in order to interpret the information the cranium provides about the brain it shelter and infer evolutive theories, it is vital to understand the relationship between the brain and the endocranial vault. In this PhD, modern human endocranium and brain growth and development will be characterized from a morphometric point of view, with the aim of defining how these two structures interact and correlate throughout maturation from birth to adulthood. This body of knowledge will be applied to enlighten our interpretations of the different indirect evidences we have about the human brain evolution. In this way, the present thesis research will not only contribute to our understanding of brain evolution in the human lineage, but it will also assist future medical research that investigate human brain and cranial growth and development trajectories. In order to answer these questions two data bases were created: one of them consisting of computed tomographic (CT) images to study bone structure maturation, and the other one consisting of magnetic resonance (MR) images to quantify ontogenetic changes in the soft brain tissue. These data bases contain individuals in a range from birth to the age of 31. The data was analysed by means of geometric morphometric techniques, which allow the statistic separation of size and shape changes throughout ontogeny, in this particular case. The results showed that the brain and endocranium present a close ontogenetic relationship from birth to the first adolescence (approximately to the age of 10 in females and 12.5 in males). From this time onwards the brain starts loosing volume (mainly gray matter due to neuronal rearrangements), and therefore, the close relationship between brain cortex and endocranial vault gradually diminishes, at the same time that the brain modifies its shape. For this reason, brain shape changes from adolescence onwards are not rejcted in endocranial regions. An important contribution was the construction of accurate and precise brain / endocranial volume (BV/ECV) ratio formulas dependent of sex, age and endocranial size, which may serve to extract better information from cranial data. A third main subject of this PhD was the study of asymmetric patterns in both brain and endocranium. In this sense, it was shown that the brain macroscopic asymmetries and the endocranial petal pattern are not the same for the different periods analysed, and they even change their trajectories through ontogeny. Hence, the adult asymmetric patterns are not the same than in the sub-adults. Finally, sexual dimorphism was investigated in both structures, and the characterization of growth and development divergences between females and males could be done through heterochonic processes. Growth and development of the brain and its surrounding bony endocranial tissue could be characterized in the human species, with the aid of 3D medical images and new geometric morphometric techniques specially developed for this study. New information about the ontogenetic relationship between these two structures was discovered, constituting an important tool that will enlighten human studies about brain evolution.