Deuterostome Animals

Key Concepts

  • 31.1 Deuterostomes Include Echinoderms, Hemichordates, and Chordates

  • 31.2 Echinoderms and Hemichordates Are Restricted to Marine Environments

  • 31.3 More Than Half of Chordates Are Also Aquatic

  • 31.4 Tetrapods Diversified Rapidly in Terrestrial Environments

  • 31.5 Humans Evolved among the Primates

Deuterostomes

  • Characterized by three developmental patterns, distinguishing them from protostomes:

    • Radial cleavage: Cells divide in a symmetrical pattern either parallel or perpendicular to the axis of the embryo, allowing for equal-sized blastomeres.

    • Blastopore fate: The blastopore, the first opening that forms during early embryonic development, becomes the anus, with the mouth developing on the opposite side; this contrasts with the protostome, where the mouth forms first.

    • Coelom development: The coelom, or body cavity, develops from mesodermal pockets that bud off from the gastrula cavity, contributing to the anatomical features of these organisms.

  • Triploblastic: All deuterostomes possess three germ layers: ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer), each contributing to various tissues and organs in the organism.

  • Coelomate: They exhibit a true coelom, a body cavity fully lined by mesoderm, providing space for the development of complex organs and organ systems.

  • Common characteristics include:

    • Internal skeletons: Prominent in echinoderms and chordates, offering structural support and enabling movement.

    • Distinct clades arise from deuterostomes:

    • Echinoderms: E.g., sea stars, sea urchins, which have a unique water vascular system aiding in locomotion and feeding.

    • Hemichordates: E.g., acorn worms and pterobranchs, demonstrating a mixture of features from echinoderms and chordates.

    • Chordates: E.g., sea squirts, lancelets, terrestrial vertebrates, characterized by the presence of a notochord and dorsal nerve cord during at least one stage of their life cycle.

Echinoderms (sea urchins) and Hemichordates (segmented worm)

  • Restricted to marine environments, showcasing key adaptations for life in the sea:

    • Adult echinoderms: Exhibit pentaradial symmetry, which enhances their ability to move in various directions effectively. They lack a distinct head structure, aiding in their lifestyle on the ocean floor.

  • Key features of echinoderms (7,500 species include):

    • Pentaradial symmetry: A form of symmetry that allows echinoderms to interact with their environment uniformly from multiple directions.

    • Internal skeleton: Composed of fused calcareous plates, providing a robust framework and protection.

    • Water vascular system: A hydraulic system critical for various functions, including gas exchange, locomotion using tube feet, and feeding. Important structures in this system include:

    • Tube feet: Soft extensions used for locomotion and grasping.

    • Madreporite: The entry point for water into the water vascular system.

    • Ring canal: Circulates water to the radial canals that lead to the tube feet.

Echinoderm Classifications

  • Echinozoans: Include sea urchins and sea cucumbers:

    • Sea urchins: Characterized by a lack of arms, they possess movable spines and some species produce toxins as a defense mechanism against predators.

    • Sea cucumbers: Feature a body with an anterior mouth and posterior anus, using tube feet for anchoring to substrate and exhibiting a more flexible body form.

  • Asterozoans: Include sea stars and brittle stars:

    • Sea stars: Have their gonads and digestive organs located in the arms and utilize tube feet for various functions, including movement and feeding.

    • Brittle stars: Possess flexible arms; they actively use tube feet not for locomotion but for feeding on organic particles in their environment.

Feeding Mechanisms

  • Tube feet: Essential structures in various feeding strategies:

    • Sea lilies: Utilize tube feet located on their arms for filter-feeding, capturing plankton as water passes through.

    • Sea stars: Exhibit specialized feeding behavior by capturing bivalves, exploiting their muscle contractions to feed efficiently.

  • Hemichordates: Exhibit a three-part body plan consisting of:

    • Proboscis: For feeding and attachment.

    • Collar: Connecting the proboscis to the trunk.

    • Trunk: Contains the digestive system and organ systems.

    • Include:

    • Acorn worms (90 species): Typically burrow in sediments, using their proboscis for prey capture and soil suspension feeding.

    • Pterobranchs (30 species): Live in secreted tubes in marine environments, showcasing tentacles for feeding and gas exchange.

Chordates

  • Key derived features found at some life stage include:

    • Dorsal, hollow nerve cord: A defining characteristic of chordates, forming the basis of the central nervous system.

    • Notochord: A flexible rod providing structural support; in some species, it is replaced by vertebrae during development.

    • Pharyngeal slits: Modifications in adult forms; in vertebrates, these evolve into important structures such as jaw elements and gill arches.

Vertebrate Chordates

  • Emerging evolutionary trends include:

    • Replacement of notochord: By a vertebral column, enhancing protection and support.

    • Development of advanced brain structures: Encased in a bony or cartilaginous skull, indicating increased complexity in nervous control and sensory processing.

    • Advanced circulatory systems: Including the evolution of a multi-chambered heart for improved blood circulation.

  • Gnathostomes: Evolved jaws from gill arches, leading to a significant diversification of jawed vertebrates. Key groups include:

    • Chondrichthyans: Including sharks and rays, characterized by a cartilaginous skeleton and unique adaptations to their aquatic environment.

    • Ray-finned fishes: Feature swim bladders for buoyancy, allowing them to maintain their position in the water column effectively, leading to ecological dominance in aquatic habitats.

Transition to Land: Tetrapods

  • Lobed-limbed vertebrates: Evolved muscular fins that were critical for supporting weight on land and facilitating transitions from aquatic to terrestrial life. This evolutionary event marks a significant milestone in vertebrate history.

  • Amphibians: These animals are tied to moist environments and go through a metamorphosis from an aquatic larval stage to a terrestrial adult stage; they are divided into caecilians, anurans (frogs and toads), and salamanders, each exhibiting unique adaptations.

  • Amniotes: Represent a group adapted for life on land. Key adaptations include:

    • Amniotic egg: Featuring membranes that protect the embryo and allow for development in terrestrial environments.

    • Waterproof skin: Reducing water loss and enabling survival in dry habitats.

    • Highly efficient kidneys: To regulate water and electrolyte balance, crucial for life on land.

Amniotes and Their Evolution

  • From ancestral amphibians, a split emerged into two major lineages:

    • Synapsids: Leading to mammals, characterized by unique features including a single temporal fenestra in the skull, hair, and mammary glands for nurturing offspring.

    • Sauropsids: Leading to reptiles and birds, displaying a wide diversification of form and function suitable for various ecological niches.

  • The structures of amniotic eggs are crucial for environmental adaptation. They permit advanced developmental stages before hatching, which is essential for survival in fluctuating ecological conditions.

Mammalian Diversity

  • Mammals can be classified predominantly into two main groups:

    • Marsupials: Such as kangaroos and koalas, characterized by giving birth to relatively undeveloped offspring, which continue to develop in pouches.

    • Eutherians: Also known as placental mammals, which possess complex placentas that nourish developing embryos, allowing for longer gestation periods and more fully developed young at birth.

  • Evolutionary adaptations in mammals reflect their exploitation of diverse ecological niches, ranging from aquatic to terrestrial environments, showcased by their varied reproductive strategies, feeding habits, and morphological features.

Evolution of Primates

  • Primate evolutionary history involves various clades diverging over 90 million years ago, leading to adaptations suited for arboreal habitats:

    • Opposable digits: Allowing grasping and manipulation, essential for foraging and navigating within tree canopies.

    • The coexistence of multiple hominin species during evolution reflects a more branched, tree-like model rather than a linear progression, leading to significant cultural developments, including tool usage and complex social behaviors which distinguish modern humans from their ancestors.

Human Evolution

  • The exploration of human evolution through paleoanthropology emphasizes a branching tree model of evolution, contrasting the traditional linear perspective:

    • Misconceptions: Humans did not evolve directly from chimpanzees; rather, both species share a common ancestor from which they diverged.

    • The coexistence and interaction between different hominin species influenced gene flow and social behaviors, demonstrated through fossil evidence and archaeological findings.

    • Studies of transitional species, such as Australopithecus and Homo, highlight significant developments in tool-making, social structures, and cognitive abilities that have shaped human ancestry and development into the modern era.