Lecture 2 Animal Evolution

Key Concepts

  • 22.2: Large multicellular animal groups arose in parallel, meaning diverse lineages of animals independently evolved or diversified complex multicellularity around similar geological periods, leading to a broad array of animal forms.

  • 22.3 & 22.5: Protostomes vs. Deuterostomes represent two major clades of bilaterian animals, distinguished by fundamental differences in early embryonic development.

Early Animal Evolution

  • The study of early animal evolution examines a diverse set of eukaryotic groups, some of which are closely related to animals:

    • Alveolates (e.g., dinoflagellates, ciliates)

    • Stramenopiles (e.g., diatoms, brown algae)

    • Rhizaria (e.g., foraminiferans, radiolarians)

    • Excavates (e.g., Giardia, Trichomonas)

    • Plantae (plants and green algae)

    • Amoebozoans (e.g., amoeba, slime molds)

    • Fungi (mushrooms, yeasts)

    • Choanoflagellates: These colonial protists are considered the closest living single-celled relatives (sister taxa) to animals due to morphological and genetic similarities.

    • Animals: The diverse kingdom of multicellular organisms.

Monophyletic Nature of Animals

  • Animals are classified as a monophyletic group, meaning that all animals share a single common evolutionary ancestor, and this group includes all of that ancestor's descendants.

  • The precise ancestral lineage of animals and the branching order of early animal groups remain subjects of ongoing scientific debate, largely due to limited fossil evidence and complex genomic data.

  • Two primary hypotheses concerning the identify of the sister taxa (the closest evolutionary relatives) to all other animals are:

    • Sponge Hypothesis: Proposes that sponges, which are morphologically simple (lacking true tissues, nervous systems, muscles, or a gut), are the sister taxa to all other animals.

    • Ctenophore Hypothesis: Proposes that ctenophores (comb jellies), which possess more complex traits like nerves, muscles, and a through-gut, are the sister taxa to all other animals.

Polytomy

  • Polytomy: A node on a phylogenetic tree where more than two lineages descend from a single immediate ancestral lineage. This indicates that the evolutionary relationships among these descendent groups are currently unresolved or rapidly diversified.

Current Models and Hypotheses on Animal Origins
Hypothesis 1: Sponge Sister

  • This hypothesis proposes that nerves, muscles, and a gut evolved once in the lineage after the ancestor of sponges branched off from the ancestor of all other animals. This represents a more parsimonious view regarding the origin of these complex traits.

  • Key characteristics of Sponges: They are sessile filter feeders that lack organized tissues, nervous systems, muscles, and a through-gut.

  • This model suggests the evolution of these complex features occurred de novo in the lineage leading to ctenophores, cnidarians, and bilaterians.

Hypothesis 2: Ctenophore Sister

  • This hypothesis proposes that the ancestor of all animals already possessed nerves, muscles, and a gut.

  • Consequently, the simple morphology of sponges would be explained by the secondary loss of these complex ancestral traits over evolutionary time.

  • This implies a more complex evolutionary history for sponges, involving the degeneration of structures present in their ancestors.

Comparison of Hypotheses

  • Principle of Parsimony: This principle suggests that the simplest explanation, requiring the fewest evolutionary steps (e.g., fewest gains or losses of traits), is generally the most likely hypothesis.

    • Initially, the Sponge Sister hypothesis was considered more parsimonious because it required only a single evolutionary gain of nerves, muscles, and a gut.

    • The Ctenophore Sister hypothesis required one gain followed by a secondary loss in sponges, which was considered less parsimonious by traditional measures.

New Data Supporting Ctenophore Sister

  • Recent robust genomic research, particularly analyses of conserved gene order and genomic architecture (chromosomal similarities), has provided more evidence supporting the Ctenophore sister hypothesis.

  • These studies indicate that sponges share more chromosomal similarities with the majority of other animals (e.g., Bilateria and Cnidaria) than ctenophores do. This suggests that ctenophores diverged even earlier than sponges from the rest of the animal kingdom.

Classification of Animals: Protostomes vs. Deuterostomes

  • One significant way to differentiate major animal groups is based on their early embryonic development, specifically whether they are protostomes or deuterostomes.

  • This classification hinges on several critical developmental processes during gastrulation, the stage where the embryo reorganizes into a multi-layered structure.

Germ Layers in Animal Development

  • Animals develop either with two or three germ layers, which are embryonic tissue layers that give rise to all specialized tissues and organs in the adult animal:

    • Diploblastic (e.g., Cnidarians like jellyfish, sea anemones, and Ctenophores):

    • Develop with only two primary germ layers:

      • Ectoderm: The outermost layer; forms the epidermis (outer skin) and nervous system.

      • Endoderm: The innermost layer; forms the lining of the digestive tract and associated glands.

    • They lack a true mesoderm and therefore lack many complex organ systems.

    • Triploblastic (Most complex animals, including all Protostomes and Deuterostomes):

    • Develop with three primary germ layers:

      • Ectoderm: The outermost embryonic layer; forms the outermost covering (skin, hair, nails), nervous system (brain, spinal cord, nerves), and sensory organs.

      • Mesoderm: The middle embryonic layer; forms the muscles, bones, circulatory system (heart, blood vessels), excretory system (kidneys), reproductive system, and other internal organs.

      • Endoderm: The innermost embryonic layer; forms the lining of the digestive tract (gut), respiratory system (lungs), and many internal glands (liver, pancreas).

Structure of Triploblasts

  • Triploblastic animals are often described as having a “tube within a tube” body plan:

    • The inner tube is the digestive tract (derived from endoderm).

    • The outer tube is the body wall (derived from ectoderm and mesoderm).

  • Embryonic stages progress rapidly from a single zygote to 1 to 32 cells (cleavage), forming a hollow ball called a blastula. Gastrulation then leads to the formation of germ layers and the development of:

    • Archenteron: The primitive gut cavity.

    • Coelom: A fluid-filled body cavity completely lined by mesoderm, providing space for internal organs to develop and move independently, and acting as a hydrostatic skeleton in some animals.

Protostomes vs. Deuterostomes Development (Key Distinctions)

Protostomes

  • Cleavage: Typically undergo spiral cleavage, where cells divide at oblique angles relative to the animal-vegetal axis, resulting in tiers of cells that lie in the furrows of the cells below them.

  • Cell Fate: Exhibit determinate cleavage, meaning the developmental fate of each embryonic cell is fixed very early in development. If an early cell is removed, the embryo will not develop completely.

  • Blastopore Fate: The blastopore (the first opening to form during gastrulation) becomes the mouth of the adult organism. (From Greek: proto- meaning first, stoma meaning mouth).

  • Coelom Formation: The coelom forms via schizocoelous development; a coelomic cavity arises from the splitting of solid masses of mesodermal cells near the blastopore.

Deuterostomes

  • Cleavage: Typically undergo radial cleavage, where cells divide either parallel or perpendicular to the animal-vegetal axis, resulting in tiers of cells that lie directly above each other.

  • Cell Fate: Exhibit indeterminate cleavage, meaning the developmental fate of each embryonic cell is not fixed until a later stage. If an early cell is removed, the remaining cells can often compensate and form a complete, normal embryo (enabling identical twins).

  • Blastopore Fate: The blastopore becomes the anus of the adult organism. The mouth then develops from a secondary opening. (From Greek: deutero- meaning second, stoma meaning mouth).

  • Coelom Formation: The coelom forms via enterocoelous development; the coelom arises from mesodermal outpocketings of the archenteron (primitive gut).

Dorsal-Ventral Patterning Differences

  • Dorsal: Refers to the back side of the organism or the top surface.

  • Ventral: Refers to the belly side of the organism or the bottom surface.

  • Protostomes:

    • Possess a ventral nerve cord (located on the belly side).

    • Typically have a dorsal digestive tract and heart.

  • Deuterostomes:

    • Possess a dorsal nerve cord (located on the back side, like the spinal cord in vertebrates).

    • Typically have a ventral digestive tract and heart.

Examples of Protostomes and Deuterostomes

  • Protostomes include a vast array of invertebrates:

    • Mollusks (e.g., snails, clams, octopuses)

    • Annelids (e.g., segmented worms like earthworms, leeches)

    • Arthropods (e.g., insects, spiders, crustaceans)

    • Flatworms (e.g., planarians, tapeworms)

    • Roundworms (Nematodes)

  • Deuterostomes include a smaller, but highly diverse group:

    • Echinoderms (e.g., sea stars, sea urchins, sea cucumbers)

    • Hemichordates (e.g., acorn worms)

    • Chordates (e.g., vertebrates like fish, amphibians, reptiles, birds, mammals, as well as tunicates and lancelets)

Take Home Messages

  • Large multicellular animal groups have diversified in parallel throughout evolution, leading to vast animal diversity.

  • Current genomic evidence strongly supports ctenophores as the sister taxa to all other animal groups, including sponges. This implies complex traits might have evolved early in the animal lineage and were secondarily lost in sponges.

  • The main differentiation of animals into protostomes and deuterostomes is based on fundamental embryological differences that deeply impact their body plans:

    • Protostomes: Characterized by spiral and determinate cleavage, the mouth forms from the blastopore, coelom forms schizocoelously, and they possess a ventral nerve cord.

    • Deuterostomes: Characterized by radial and indeterminate cleavage, the anus forms from the blastopore (mouth forms secondarily), coelom forms enterocoelously, and they possess a dorsal nerve cord.