The Origin and Evolution of Vertebrates

Chapter 34: The Origin and Evolution of Vertebrates

Topics Covered

  • Chordate characteristics and evolution

  • Chordate invertebrates: lancelets, tunicates

  • Chordate vertebrates: fishes, amphibians, reptiles, birds, mammals

  • Gnathostomes

  • Tetrapods

  • Amniotes

  • Mammals

  • Humans and their evolution

Animal Phylogeny

Chordate Characteristics

  1. Bilateral symmetry: Body plan that can be divided into mirror-image halves.

  2. Triploblastic: Development from three germ layers: ectoderm, mesoderm, and endoderm.

  3. Coelomate: Presence of a fluid-filled body cavity (coelom) between the gut and outer body wall.

  4. Deuterostomes: Organism where the mouth does not develop from the blastopore (initial opening) of the embryo.

  5. Cephalization: Concentration of sensory organs and nervous tissue at the anterior end (head).

  6. Complete digestive tract: A digestive system with two openings (mouth and anus).

  7. Closed circulatory system: Blood is contained within vessels.

  8. Most have endoskeleton: Internal skeleton made of bone or cartilage.

  9. Two groups of invertebrates: Remaining chordates are vertebrates.

Phyla

  • Parazoa: Simplest animals (e.g., sponges).

  • Clade Radiata: Includes animals with radial symmetry (e.g., cnidarians).

  • Protostomia: Development where the mouth forms before the anus.

Chordate Phylogenetic Tree

  • Cephalochordata (lancelets)

  • Urochordata (tunicates)

  • Notochord: A long flexible rod underlying the nerve cord. Most vertebrates retain only remnants of the notochord in adults.

  • Myxini: Jawless fish (hagfishes), share a common ancestor with vertebrates.

  • Chordates:

    • Cyclostomes: Jawless fishes such as hagfishes and lampreys.

    • Gnathostomes: Vertebrates with jaws and a mineralized skeleton.

    • Tetrapods: Vertebrates with limbs and digits.

    • Amniotes: Tetrapods with a terrestrially adapted egg.

    • Mammals: Amniotes that produce milk and have hair.

Key Features of Chordates

  • Notochord

  • Dorsal, hollow nerve cord

  • Pharyngeal slits or clefts

  • Post-anal tail

Evolution of Chordates

  • Chordates evolve from ancestors possessing a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail.

  • Vertebrates: Chordates with backbones.

  • Gnathostomes: Vertebrates with jaws.

  • Tetrapods: Gnathostomes with four limbs.

  • Amniotes: Tetrapods with eggs adapted for reproduction on land.

  • Mammals: Amniotes with hair and mammary glands for milk production.

  • Humans: Bipedal mammals with enlarged brains.

Chordate Invertebrates

Lancelets (Cephalochordata)

  • Bladelike shape; marine suspension feeders.

Tunicates (Urochordata)

  • More closely related to other chordates than lancelets.

  • Larval form resembles chordates, but adults draw water through an incurrent siphon, filtering food particles.

  • Adults can shoot water as a defense mechanism.

Vertebrates: Chordates with Backbone

Derived Characters of Vertebrates

  1. Two or more sets of Hox genes: Indicates complexity and evolution of the nervous system and skeletal system (e.g., skull and backbone).

  2. Neural crest: Unique to vertebrates; cells form along the edges of the closing neural tube and disperse to become various structures (teeth, bones, neurons).

Cyclostomes (Agnathans)

  • Jawless fishes with cartilaginous skeletons, no scales, or paired fins.

  • Hagfishes (Myxini): Marine, bottom-dwelling scavengers.

  • Lampreys (Petromyzontida): Free-living or parasitic, feeding by attaching to live fish.

Gnathostomes: Vertebrates with Jaws

Characteristics of Gnathostomes

  1. Jaws: Evolved from skeletal rods that supported pharyngeal slits.

  2. Teeth: To grip and slice food.

  3. Enlarged forebrain: Associated with enhanced sensory capabilities (smell, vision).

  4. Lateral line system: Sensitive to vibrations in aquatic gnathostomes.

Chondrichthyans (Class Chondrichthyes)

  • Cartilaginous fishes with uncovered gill slits (e.g., sharks, rays, ratfish).

Sharks

  • Characteristics: Streamlined body, acute senses (sight, smell, electroreception).

  • Reproductive Strategies:

    • Oviparous: Eggs hatch outside the mother.

    • Ovoviviparous: Eggs retained within the mother.

    • Viviparous: Young develop in the uterus, nourished by placental connections.

  • Conservation Status: Populations endangered due to overfishing, with some areas experiencing a decrease up to 95%.

Osteichthyans (Clade Osteichthyes)

  • Characteristics: Bony fishes with a lateral line system, operculum, and swim bladder.

  • Divided into ray-finned fishes and lobe-finned fishes.

Lobe-Finned Fish (Clade Sarcopterygii)

  • Notable for thick, fleshy fins supported by bones.

  • Examples include coelacanths and lungfishes, which are ancestors of tetrapods.

Tetrapods: Gnathostomes with Limbs

Significant Evolutionary Event

  • Occurred around 365 million years ago when fins evolved into limbs.

Characteristics of Tetrapods

  1. Four limbs: Hands and feet with digits.

  2. Neck: Allows movement of the head.

  3. Fused pelvic girdle: Connected to the backbone.

  4. Absence of gills: Except in some aquatic species.

  5. Ears: Adapted for detecting airborne sounds.

Benefits of Life on Land

  • Escape aquatic predators and access new food resources (e.g., plants, insects).

Origin of Tetrapods

  • Tiktaalik: A transitional fossil showing both fish and tetrapod characteristics.

    • Notable features: flat skull, ribs, neck, presence of both fish and tetrapod traits.

Amphibians (Class Amphibia)

Orders

  1. Urodela: Salamanders and newts (tailed amphibians).

  2. Anura: Frogs and toads (tailless amphibians).

  3. Apoda: Caecilians (legless, nearly blind amphibians).

Lifestyle and Ecology

  • Metamorphosis: Transition from aquatic larvae (tadpoles) to adult forms.

  • Most have moist skin aiding in gas exchange (cutaneous breathing).

  • External fertilization and requirement of moist environments for egg survival.

  • Declining populations due to chytrid fungus, habitat loss, climate change, and pollution.

Amniotes: Tetrapods with Terrestrial Adaptations

Key Features

  1. Amniotic egg: Shelled egg with four extraembryonic membranes.

  2. Skin: Rich in keratin to prevent desiccation.

  3. Rib cage: Expands lungs.

  4. Kidneys: Conserve water.

  5. Internal fertilization: Critical for life on land.

Groups of Amniotes

  • Reptiles (including birds): Ectothermic and characterized by specific skull features (anapsids, diapsids, synapsids).

  • Mammals: Include monotremes, marsupials, and eutherians.

Reptilian Characteristics

  • Turtles: Boxlike shell structure: upper (carapace) and lower (plastron) shields.

  • Lepidosaurs: Includes tuataras and squamates (lizards and snakes).

  • Crocodilians: Closely related to birds; found in warm regions.

Birds (Class Aves)

  • Adaptations for Flight:

    • Keratinized feathers for insulation and aerodynamics.

    • Lightweight skeletal structure.

    • Highly efficient respiratory system with air sacs.

    • 4-chambered heart for efficient oxygenation.

    • Endothermic capabilities.

  • Birds have undergone significant adaptations for various ecological niches, leading to a diversity of forms (e.g., flightless ratites).

Mammals: Characteristics

  1. Mammary glands: Produce milk to nourish young.

  2. Hair: Provides insulation.

  3. Efficient respiratory systems: Support high metabolic rates.

  4. Large brain compared to body size: Supports complex behaviors.

  5. Differentiated teeth: Adapt for varied diets.

Classes of Mammals

  1. Monotremes: Egg-layers (e.g., echidnas, platypus).

  2. Marsupials: Carry young in pouches (e.g., kangaroos, opossums).

  3. Eutherians: Placental mammals; young develop inside the mother.

Evolution of Primate Traits

  • Opposable thumbs and complex social behaviors characterize primates.

  • Differences between anthropoids (monkeys and apes) evolved through adaptive radiations.

  • Distinction between New World and Old World monkeys.

Human Evolution

Characteristics of Humans

  1. Bipedalism: Energy-efficient locomotion.

  2. Larger brains: Capable of language and complex tools.

  3. Reduced jawbones: Shorter digestive tract compared to ancestors.

  4. Cultural development: Artistic expression and tool-making.

Misconceptions about Human Evolution

  • Clarification that early hominins were not chimpanzees but shared a common ancestor with them.

  • Understanding that human evolution is branched with multiple extinct hominin species.

Important Hominin Species

  • Insights from Paleoanthropological studies on various extinct species, such as Australopithecus, Homo neanderthalensis, and Homo sapiens.

Timeline of Hominin Evolution

  • Overview of the evolutionary timeline showing divergence and adaptation of various species throughout the past millions of years.

  • Homo sapiens appeared approximately 200,000 years ago, with all current humans tracing back to African ancestors.

Interbreeding with Neanderthals

  • Evidence of genetic admixture between modern humans and Neanderthals, enhancing our understanding of human evolution.

Conclusion

  • Summary of the chordate lineage from early aquatic forms to land-adapted mammals, including crucial evolutionary developments such as limbs, lungs, and reproductive adaptations.