Chordate and Vertebrate Evolution

Chordate and Vertebrate Evolution Notes

Chordates and Their Unique Traits

Primary Traits of Chordates

1. Notochord: A longitudinal, flexible rod situated between the digestive tube and the nerve cord, providing skeletal support.

2. Dorsal Hollow Nerve Cord: Develops into the central nervous system (CNS).

3. Post-Anal Tail: Extends past the anus and typically contains skeletal elements and muscles; significantly reduced in humans.

4. Pharyngeal Slits: Abundant in the embryonic stage, in some vertebrates modified for gas exchange (gills).

Derived Characters of Chordates

• Dorsal: Positioned on top, emphasizing the chordate's body plan.

• Pharyngeal slits: Supportive structures potentially modified over evolutionary time.

• Hollow nerve cord: Indicates evolutionary development towards the CNS.

Phylogeny and Sister Groups to Chordata

• Chordate Phylogeny:

  • Key characteristics: Dorsal hollow nerve cord, notochord, post-anal tail.

  • Sister groups include Echinodermata and Hemichordata which diverged earlier in evolutionary history.

• Major taxonomic groups within Chordata:

  • Cephalochordata (Lancelets)

  • Urochordata (Tunicates)

  • Vertebrata:

    • Cyclostomata (Hagfishes and lampreys)

    • Gnathostomata (Jawed vertebrates)

    • Placoderms, sharks, and bony fishes.

Invertebrate Chordates

Lancelets (Cephalochordata)

• Larvae feed on plankton and metamorphose into adults that burrow into the substrate.

• Function of Pharyngeal Slits: Serve for feeding and gas exchange.

Tunicates (Urochordata)

• Mobile larvae exhibit clear chordate traits; adults are sessile.

• Known informally as “sea squirts” due to their defensive water jet.

Key Innovations in Vertebrates

General Themes in Evolution

1. Extinction: Many lineages went extinct without direct descendants.

2. Parallel Evolution: Some traits evolved independently across different groups.

3. Trait Loss: Loss of certain traits occurred in different lineages.

First Vertebrates

• Characteristics include a cranium and a spinal column.

• Early vertebrates did not possess jaws, the two existing jawless species are hagfishes and lampreys.

Evolution of the Jaw: Gill-arch Hypothesis

• Gill Arches: Series of bones that support gills; mutations allowed arch formation into jaws, enabling various feeding strategies (e.g., suction feeding, biting).

• Key evidence includes structural similarities and embryonic origins of jaws and gill arches.

Origin of Bony Endoskeleton and Lungs

• Bony Skeleton: Emerged in some cartilaginous fish through bone deposition.

• Lungs: Evolved from paired lungs of early ancestors.

• Swim Bladder: Developed from lungs, providing buoyancy.

Evolution of Limbs in Tetrapods

Tetrapods

• Characteristics: Four limbs for locomotion on land.

• Lungfish Adaptations: Adapted for low-oxygen environments and capable of limited terrestrial movement.

Limbs-From-Fins Hypothesis

• Evolution from fin structures to limbs with digits observed in fossils (e.g., Tiktaalik).

Evolution of the Amniotic Egg

• Components:

  • Amnion: Encases the embryo in fluid for protection.

  • Yolk Sac: Provides nutrients.

  • Chorion: Facilitates gas exchange.

  • Allantois: Assists in waste disposal.

Mammalian Evolution

• Mammals characterized by mammary glands, endothermy, hair, and complex teeth.

• Three lineages: monotremes, marsupials, and placental mammals.

Reptilian Evolution

• Monophyletic class with adaptations like scales, lung breathing, and shelled eggs.

• Major lineages: lizards, turtles, crocodiles, and birds.

Key Takeaways

• Vertebrate evolution is not a simple linear process.

• Some traits evolved independently across various groups; others are unique.

• Evolution emphasizes diversity and adaptation instead of a direct lineage.

Summary of Evolutionary Timeline

• Key milestones from the origin of the solar system to the emergence of complex life forms.

• Understood Transition: Prokaryotes to eukaryotes, single-celled to multicellular organisms, land colonization, and the rise of humans.