bio: vertebrates

Introduction to Chapter 19

Welcome to Chapter 19: The last chapter of the course. This chapter covers vertebrates and vertebrate diversity. Reminder: Previous chapter focused on invertebrates, specifically chordates.

Chordates Overview

  • Chordates are a phylum that includes both vertebrates and invertebrates.

  • Key derived characters of chordates:

    • Notochord: A flexible rod running the length of the body.

    • Hollow nerve cord: Situated above the notochord; pivotal for the nervous system in higher organisms.

    • Pharyngeal slits: Openings near the mouth, originally for filter feeding, seen in embryonic stages of higher-order chordates like humans.

    • Post-anal tail: Present in embryonic stages, though not in adult humans.

  • Similarities between various chordate embryos (humans, fish, reptiles, birds) in the embryonic stage highlight evolutionary connections.

Phylogeny of Chordates

Understanding phylogeny will help explore relationships among chordates. The branching points indicate the most recent common ancestors shared by chordate groups. Initial chordates: Lancelets and tunicates. Tunicates are commonly known as sea squirts; they possess chordate characteristics only in their larval stage.

Evolution of Vertebrates

  • Vertebrates: Emerged with the appearance of the vertebral column, a significant derived character.

  • Characteristics of early vertebrates:

    • Lack of jaws (jawless vertebrates).

    • Likely filter feeders with soft cartilage skeletons instead of hard bones.

  • First jawless vertebrates: Included hagfish and lampreys.

    • Hagfish interesting features:

      • Unique anti-predator defense mechanism involves releasing a significant amount of slime when disturbed, confusing potential predators.

    • Lampreys: Parasitic vertebrates with specialized mouthparts for attaching to and feeding off other fish.

Evolution of Jaws

  • Jaws evolved likely as a modification of structural support for the pharyngeal gill slits that all chordates share.

  • This adaptation allowed the gill slits to function properly and gave rise to the jaws seen in jawed vertebrates today.

Lineages of Jawed Vertebrates

  • There are three lineages of jawed vertebrates commonly grouped as fishes:

    1. Chondrichthyes: Sharks and rays, characterized by a cartilage skeleton.

    2. Ray-finned fishes: The majority of fishes with a bony skeleton, showcasing diversity in forms and adaptations.

    3. Lobed-fin fishes: A significant evolutionary bridge between aquatic and terrestrial organisms.

Important Discovery - Tiktaalik

  • The discovery of Tiktaalik, a crucial fossil, highlights important adaptations for life in shallow freshwater environments.

  • Questions to consider while studying Tiktaalik:

    • What sort of body plan was successful in the shallow freshwater swamps?

    • What did the fins on Tiktaalik resemble?

Conclusion

Transition from exploring invertebrates in Chapter 18 to vertebrates in Chapter 19. Focus on the unique traits and adaptations that define vertebrate diversity.

Chapter 19: The Evolution and Diversity of Vertebrates

Introduction

  • In this chapter, we will delve into vertebrates, focusing on their evolution and diversity.

The Discovery of Tiktaalik

  • Tiktaalik is a transitional fossil showcasing both fish and tetrapod characteristics.

  • Tetrapods are organisms that walk on four limbs.

Key Traits of Tiktaalik

  • Features:

    • Flat skull

    • Eyes positioned on the top of the skull

    • A fin skeleton containing bones shared with modern tetrapods.

  • Tiktaalik is believed to be a lobe-finned fish ancestor to tetrapods.

Importance of Transition to Land

  • The evolution from water to land was a significant adaptation in vertebrate history.

  • This transition allowed the exploitation of new resources and evasion from aquatic predators, leading to a mass speciation event known as adaptive radiation.

Phylogeny and Lobe-Finned Fishes

  • Organisms with lobed fins are grouped as tetrapods, giving rise to land-dwelling species.

First Tetrapods: Amphibians

  • Amphibians include salamanders, frogs, and caecilians.

  • Notable adaptations:

    • Ability to live on land but still dependent on moist environments due to high water loss through skin.

    • Use external fertilization, releasing gametes in water for reproduction.

Decline of Amphibians

  • Amphibians are sensitive to pollution, leading to global population declines.

  • They serve as indicators of environmental health.

Evolution of Reptiles

  • Reptiles marked the next evolution with the appearance of the amniotic egg, allowing for reproduction on land.

  • Their scaly skin prevents desiccation and is made of keratin.

Dinosaurs and Their Dominance

  • Dinosaurs, a prominent group within reptiles, dominated terrestrial environments during the Mesozoic Era.

  • Most dinosaurs became extinct, except for birds, after a mass extinction event likely caused by a massive asteroid.

Birds: Descendants of Dinosaurs

  • Birds evolved from small, bipedal dinosaurs called theropods.

  • Important fossil: Archaeopteryx, possessing features of both birds and dinosaurs, helps reveal the evolutionary link.

Adaptations of Birds

  • Birds have adaptations for flight, including endothermy, which allows them to generate their own body heat, contrasting with exothermic reptiles that rely on external heat sources.

Mammals

Introduction to Mammals

  • Mammals have the important adaptation of producing milk through mammary glands.

  • They arose during the Mesozoic era when dinosaurs dominated the earth but were initially small and rat-sized.

  • The mass extinction event that killed the dinosaurs allowed mammals to diversify.

Temperature Regulation

  • Ectotherms (e.g., reptiles and dinosaurs) rely on external heat sources, leading to fluctuating body temperatures and limited activity.

  • Endotherms (mammals) use metabolic energy to maintain constant body temperatures, allowing them to be active at night and divide resources with dinosaurs.

Key Derived Characters of Mammals

  • Mammals are characterized by:

    • Production of milk.

    • Presence of hair.

    • A high metabolic rate due to endothermy.

    • Generally larger brains for their body size.

Groups of Mammals

  1. Monotremes: Egg-laying mammals, including the platypus and echidnas.

  2. Marsupials: Have a placenta, brief gestation, and give birth to tiny, underdeveloped young that continue to develop in a pouch on the mother's body.

  3. Eutherians (Placental Mammals): Give birth to fully developed young. They represent the evolutionary progression of mammals.

Evolution of Humans

Introduction to Human Evolution

  • The human story begins with our primate heritage.

  • Primates include lemurs, tarsiers, monkeys, and apes, arising as a small group of arboreal mammals.

Adaptations of Primates

  • To succeed in arboreal environments, primate ancestors had adaptations including:

    • Forward-facing eyes

    • Long grasping limbs

    • Depth perception

    • Limber joints and grasping hands and feet with flexible digits.

Phylogeny of Primates

  • Three major groups of primates:

    1. Lemurs, lorises, and bush babies

    2. Tarsiers

    3. Anthropoids (including monkeys, apes, and humans)

  • Humans' closest living relatives are chimpanzees, but several extinct species known as hominins are more closely related than chimpanzees.

Understanding Hominins

  • The field of paleoanthropology studies human evolution and hominins.

  • Human evolution did not occur in a straight line.

  • Many hominin species coexisted over time, and fossil evidence indicates their relationships.

  • The longest fossil record is likely associated with Homo erectus and Homo habilis.

Key Characteristics of Hominins

  • Humans and their closest relatives, chimpanzees, share similarities but differ significantly:

    • Bipedalism (walking upright)

    • Larger brains.

  • Bipedalism arose before increases in brain size.

Differentiating Hominin Species

  • The genus Homo includes species such as Homo ergaster and Homo neanderthalensis (Neanderthals), known to have larger brains than modern humans.

  • Neanderthals lived in Europe, exhibited complex behaviors, and had similar DNA to modern humans, indicating interbreeding.

Myths About Human Evolution

  1. Humans are not descendants of chimpanzees: Humans share a common ancestor with chimpanzees, which was likely ape-like.

  2. There is no linear path from an ape-like ancestor to humans: Human evolution reflects a complex phylogeny with numerous hominins.

  3. Bipedalism did not evolve with a large brain: Evidence shows that upright posture evolved before brain size increased, debunking the myth of an advanced, brainy ancestor walking on all fours.