Evolution of Vertebrates

Evolution of Vertebrates

• Complex multicellular water-dwelling organisms first appeared on earth about 800 million years ago
• 250 million years later, chordates (animals with dorsal nerve cords) evolved
• First chordates with spinal bones (vertebrae) to protect the dorsal nerve cords evolved 25 million years later
• First vertebrates were primitive bony fishes, now there are 7 classes of vertebrates: 3 classes of fish, amphibians, reptiles, birds, and mammals

Evolution of Amphibians

• 410 million years ago, bony fishes started to venture out of the water, gaining two advantages: escaping stagnant pools and accessing terrestrial food sources
• Through natural selection, fins and gills of bony fishes transformed into legs and lungs, giving rise to the first amphibians 370 million years ago
• Amphibians in larval form must live in water, while only adults can survive on land

Evolution of Reptiles

• 315 million years ago, reptiles evolved from a branch of amphibians, laying shell-covered eggs and covered by dry scales
• These adaptations reduced the reliance of reptiles on watery habitats
• Reptiles spend their first stage of life in watery habitats, then live far from water due to dry scales reducing water loss through their water-permeable skin

Evolution of Mammals

• 225 million years ago, a new class of vertebrates evolved from a small reptile line, characterized by females feeding their young with mammary gland secretions
• Females nurture their young in a watery environment within their bodies until they are mature enough to be born
• There are 26 different orders of mammals, including primates, with 16 groups of primates, such as apes and old world monkeys

Emergence of Humankind

• Humans are known as hominins, including 6 subgroups, such as Australopithecus and Homo
• Homo is thought to be composed of at least eight species, seven of which are now extinct
• The first Homo species emerged from Australopithecus 4 million years ago, while Homo sapiens emerged 275,000 years ago
• Australopithecus were about 1.3 meters tall, with small brains and pelvic and leg bones indicating they were upright
• Distinctive features of Homo include larger brain cavities than Australopithecus but smaller than modern humans

Classification of Humans

• Kingdom: animals
• Phylum: chordate
• Class: mammal
• Order: primate
• Family: Hominid
• Tribe: Hominin
• Genus: Homo
• Species: sapiens

Thinking About Human Evolution

• Evolution does not proceed in a single line
• Humans have little reason to claim evolutionary supremacy, given our brief existence
• Evolution does not always proceed slowly, and rapid changes can be triggered by sudden environmental changes
• Fewer than 1% of all known species are still in existence
• Evolution does not progress to preordained perfection
• Not all existing behaviors or structures are adaptive, and some may be incidental by-products (spandrels)
• Not all existing adaptive characteristics evolved to perform their current function
• Exaptation: evolved to serve one function, later co-opted to serve another
• Homologous: structures similar among species
• Analogous: structures similar but with different evolutionary origins
• Convergent: evolution of different species using the same solution as demanded by the environment

Evolution of Human Brain

• There is no clear relationship between overall brain size and intelligence
• Brain size (cerebrum) increased during evolution
• Increase in number of convolutions (folds on cerebral surface) greatly increased the surface area of the cerebral cortex

Understanding Mate Bonding

• Most mate bonds are promiscuous
• Most mammals form polygynous mating bonds
• Polyandry: males contribute to reproduction more than females
• Humans generally form monogamous bonds (one on one)
• Mate bonding in humans is predicted through evolutionary theory
• Men value youth and attractiveness in partners
• Women value power and earning capacity in partners
• Physically attractive women bond with men with high status
• Mate attraction: F = physical attraction, M = display power and resources
• Men are more likely than women to commit adultery

Fundamental Genetics

Mendelian Genetics

• Dichotomous traits: occur in one form or another, never in combination
• True breeding lines: breeding lines where interbred members always produce offspring with the same trait generation after generation
• Dominant trait: appears in all first-generation offspring
• Recessive trait: appears in about one quarter of the second-generation offspring
• Phenotype: an organism's observable trait
• Genotype: traits passed on through genetic material
• Homozygous: identical alleles (BB)
• Heterozygous: different alleles (Bw)

Chromosomes

• Genes are in chromosomes
• Humans have 23 pairs of chromosomes with an allele on each chromosome
• Meiosis (producing gametes): chromosomes divide and one of each pair goes to each of the two gametes that result from cell division
• First stage, chromosomes line up in pairs
• Then crossover occurs, forming a linkage, and break apart to exchange sections
• As a result of genetic recombination, each of the gametes that formed zygotes contains chromosomes that are unique, spliced together recombinations of chromosomes from parents
• After meiosis, mitosis occurs