Final answer:
The human brain is not completely developed at birth due to space constraints within the skull and the necessity for subsequent development. Extensive folding of the cerebral cortex allows for more gray matter to fit inside the limited cranial cavity. Additionally, significant changes occur during adolescence, including synaptic pruning and increased myelination, contributing to brain maturation.
Step-by-step explanation:
Understanding the complexities in the development of the human brain, it's evident that the brain is not completely developed at birth for several reasons. The limitation of the head size by the birth canal and the need for the brain to fit inside the cranial cavity necessitates the extensive folding seen in the cerebral cortex. This folding allows a greater amount of gray matter, essential for brain functions, to be packed within the limited space.
Furthermore, embryonic development establishes the basic structure of the nervous system, which undergoes significant changes and maturation into adolescence. Areas like the forebrain, midbrain, and hindbrain develop from primitive structures into more sophisticated and larger regions, as seen in the comparison between an early embryonic state and the adult brain. Finally, changes during adolescence, such as synaptic pruning and increased myelination, contribute to the brain's efficiency and functionality, leading to cognitive development.