Evolution of Brains and Behavior

Evolution of Brains and Behavior

• The study of nervous system development across species can be aided by understanding common ancestry
• Reconstruction of human brain evolution can be achieved by:
  • Study of animals that first developed a nervous system and muscles
  • Observing how the nervous system grew in complexity as the brain evolved to mediate behavior
  • Examining how the human brain attained its current complexity
• Common ancestry uses a forebear from which two or more lineages or family groups arise
• Humans and other apes share common ancestors
• Chimpanzees are more similar to a proposed common ancestor of humans and chimps than modern humans

Origin of Brain Cells and Brain

• Earth formed 4.5 billion years ago, and the first life-forms arose about a billion years later
• Animals evolved the first brain cells roughly 700 million years ago
• Evolution of the first brain occurred around 250 million years ago
• A humanlike brain emerged approximately 6 million years ago
• The modern human brain has only been around for the past 200,000 years
• Large, complex brains appeared very recently when considering evolutionary timescales

The Basics: Classification of Life

• Taxonomy names and classifies species by grouping organisms according to shared characteristics and relationships
• Broadest classification unit: kingdom
• Subordinate groups: phylum, class, order, family, genus, and species
• Humans are in the animal kingdom, chordate phylum, mammalian class, the primate order, the great ape family, the genus Homo, and the species sapiens
• Homo sapiens sapiens means "wise, wise human"
• Cladograms illustrate the evolutionary sequence connecting all living organisms
• Animalia is the most recently evolved kingdom containing species with muscles and nervous systems
• Muscles and nervous systems evolved together to facilitate movement (behavior)

Evolution of Nervous Systems in Animals

• A nervous system is not essential for life, with most organisms doing without one for their existence
• In animals with a nervous system, it evolved through these steps:
• development of neurons and muscles
• formation of a nerve net
• bilateral symmetry
• segmentation
• ganglia
• spinal cord
• brain

Process of Nervous System Evolution

• Brain cells and muscles evolved together, facilitating movement in animals
• Neurons and muscles originated in single-cell animals that developed various ways of moving
• A nerve net is a simple nervous system found in older phyla, like jellyfish and sea anemones
• Nerve nets have no brain or spinal cord structure; they consist of neurons receiving sensory information and connecting to muscles
• The human PNS is reminiscent of the nerve net seen in simpler animals
• Bilateral symmetry is an organization exhibited in more complex animals (such as flatworms)
• The nervous system on one side of the animal mirrors the other
• The human nervous system is bilaterally symmetrical
• Segmentation: the body consists of a series of similar muscular segments
• The human spinal cord and brain display segmentation
• Ganglia: clusters of neurons found in more recently evolved invertebrate phyla
• Ganglia resemble primitive brains and work as command centers
• Encephalization is a characteristic that some phyla possess
• Insects have ganglia that are large enough to be called a brain
• Spinal cord: a single nervous system pathway connecting the brain with sensory receptors and muscles in chordates
• Chordates are named for the notochord, a flexible rod along the back
• In humans, the notochord only exists in the embryo, later encased in bony vertebrae
• Brain: a true brain is displayed in the chordate phylum, which includes amphibians, reptiles, birds, and mammals, which are class members
• Chordates display the greatest degree of encephalization

Chordate Brains

• Humans have the largest brain relative to body size among chordates
• Brains of chordate species are built to a common plan, but display specializations related to behavior
• A cladogram depicts the hypothetical phylogenetic relationships between animal species in a branching diagram
• Branch points (nodes) in a cladogram represent common ancestors
• Chordate cladograms depict seven of the nine classes to which the roughly 38,500 extant chordate species belong
• Variation exists amongst nervous systems of chordates, but there is a common structural pattern
• All chordates display bilateral symmetry, segmentation, and a spinal cord and brain encased in cartilage or bone
• Brain size increase in chordates is related to forms of locomotion
• Chordates have a brainstem, but birds and mammals have a large forebrain and cerebellum
• Large-brained mammals have extensively folded brain structures, increasing surface area within a small skull
• Increased size and folding are seen in primates, enhancing complex movements, manipulation, learning, and social behavior