Brain Development and Evolution
Brain Size and Intelligence
Correlation of Brain Size and Intelligence
It is significant to consider body size alongside brain size when evaluating intelligence.
Brain Size and Mammals
Encephalization Factor
This factor accounts for the deviation of brain weight to body weight ratio across different species and classes.
Brain Regions Among Primates
Cortex Size
As brain size increases, the size of the cortex increases correspondingly, unlike other regions of the brain.
Figure 6.15: Displays these changes in apportionment of brain regions among primates.
Evolution of Brain Regions
Expansion of Cortex
Evolution allows for the growth of later-developing brain regions, reflecting increasing neuron complexity and cortex expansion.
Figure 6.16: Illustrates this evolution and its implications.
Development of Brain Regions
General Development Trends
Regions of the brain that develop later in gestation or developmental stages tend to become larger.
Suggests that minor genetic changes during late brain development can lead to significant changes in brain structure.
Historical Perspective on Bipedal Species and Cranium Size
Evolutionary Data Chart
Ardipithecus, Australopithecus, Homo habilis, Homo erectus, and Homo sapiens:
Reported cerebral volumes:
Ardipithecus: 350-400 cm³ (similar to a chimp)
Australopithecus: Volume data not specified
Homo habilis: 600-700 cm³
Homo erectus: 700 to 1400 cm³ (expanded over 1.5 million years)
Homo sapiens: 1400 cm³ approximately 150,000 years ago
Significant cultural developments correlated with increases in cerebral volume include:
Tool-making, agriculture, use of fire, and urbanization.
Human Brain Evolution
Cognitive and Social Developments
Tool-making skill likely reduced selective pressures for large jaws and teeth in archaic humans and related to social tolerance.
Selection Pressures and Brain Size
Rapid Brain Size Expansion
Indicates potential survival advantages; however, the specific selection pressures behind this change remain uncertain.
Social Brain Hypothesis:
Proposes that a larger cortex facilitates the cognitive complexity associated with managing social relationships (Dunbar, 1998).
Sexual Selection Hypothesis:
Suggests natural selection for traits that help attract and stimulate potential mates, providing evolutionary context for human creativity, humor, and language.
Other pressures may include behavioral innovations and social learning through observation.
Costs of a Large Brain
Challenges of Large Brain Size:
Long gestation periods, childbirth difficulties, and prolonged dependency on parental care.
While the brain comprises only 2% of body weight, it consumes a significant portion of metabolic energy at rest, complicating its development due to the intricate genetic factors involved.
Potential Disorders:
Larger brains are associated with higher risks of developmental disorders such as schizophrenia and autism due to the complexity of brain development.
Differences Between Humans and Chimpanzees
DNA Similarity:
Humans and chimpanzees share approximately 95% of their DNA; key differences account for the varied cognitive abilities.
Specific genes such as ASPM gene show significant variation between species; its deficiency in humans results in severe disabilities and reduced brain size.
Brain Development Factors
Intrinsic and Extrinsic Factors:
Development involves both genetic and environmental influences:
Genotype: Genetic traits within an individual.
Phenotype: Observable physical characteristics stemming from genotype-environment interactions.
Early Development of the Nervous System
Zygote and Embryo Formation:
The fertilized egg (zygote) forms three layers:
Ectoderm: becomes the nervous system.
Mesoderm: develops into muscles, the heart, red blood cells.
Endoderm: forms lungs, endocrine glands, pancreas.
Transcription Factors (TFs):
These proteins bind to DNA and broadly activate gene expression crucial to early development and segmentation in vertebrates.
Stages of Neural Development
Six Stages:
Neurogenesis: Formation of neurons and glia.
Cell migration: Movement of cells to their final locations.
Differentiation: Cells adopt specific phenotypes based on brain regions.
Synaptogenesis: Formation of synaptic connections.
Neuronal Cell Death: A controlled process for fixing neuron counts (apoptosis).
Synaptic Rearrangement: Elimination and addition of synapses after they are initially formed.
Detailed Stages in Neural Development
1. Neurogenesis
Neurons are formed from the ventricular zone (VZ) of the neural tube, regulated by genetic instructions and extracellular signals like TFs.
Most adult neurons maintain their numbers post-division; however, some brain regions can continue neurogenesis into adulthood (e.g., olfactory bulb, dentate gyrus).
2. Cell Migration
Types of migration:
Radial Migration: Neurons travel from VZ to the outer surface on scaffolding cells (e.g., radial glia).
Tangential Migration: Cells move along the rostral-caudal axis of the CNS, completing their migration by birth for primates.
3. Differentiation
Neurons adopt specific characteristics required for their respective brain regions.
Neural Cell Adhesion Molecules (NCAMs): Chemicals guiding neuron migration and synaptic targeting.
4. Synaptogenesis
Ongoing throughout life as connections form and are pruned; slows in later years.
5. Neuronal Cell Death
Occurs via programmed cell death (apoptosis).
Neurotrophins like NGF and BDNF are crucial for neuron survival and target matching.
6. Synaptic Rearrangement
Refers to the dynamic process of synapse formation and elimination.
Impact of Glia in Brain Development
Gliogenesis: Continues throughout prenatal and postnatal development; increases lifetime production of gliocytes.
Myelination: Principally occurs post-birth; enhances action potential conduction and cognitive functioning.
Frontal Lobe Development
Myelination can be studied with diffusion MRI; shows that myelination in the frontal lobe is incomplete into late adolescence (12-16 years) and years into adulthood (up to 30).
Influencing Factors in Brain Development
Hypoxia and other incidents during pregnancy can lead to neurodevelopmental disorders (e.g., cerebral palsy).
Behavioral Teratology: Examines maternal environment impacts on fetal development, emphasizing drug exposures during pregnancy (e.g., alcohol, thalidomide).
Gene Studies in Development
Genetic Models: Use of organisms like Drosophila, C. elegans, and mice to investigate developmental genetics.
Specific mutations, such as weaver and reeler mutants, exhibit notable developmental impairments.
Conditional and Transgenic Studies
Introduction of specific genes or inactivation in mice to model human conditions; however, must contend with limitations on developmental timing and expression.
Understanding Environment and Gene Expression
Epigenetics: Examines heritable changes in gene expression without DNA alteration; influences phenotype based on an individual's experiences.
Common Mechanisms:
DNA Methylation: Inhibits gene expression at specific locations.
Histone Modification: Alters histone proteins to facilitate DNA accessibility and gene expression.
Epigenetic Mechanisms in Behavioral Changes
These mechanisms are responsible for various behavioral changes related to stress responses, addiction, and adaptations to environmental pressures over generations.