Animal Evolution Part 1 Notes

Origin of Animals

  • Animal evolution is covered in two modules.
  • Part one focuses on the origin and characteristics of animals.
  • Topics include:
    • Origin of animals
    • Characteristics of animals
    • Multicellularity
    • Tissue specialization
    • Reproduction
    • Body plans
    • Embryogenesis
    • Development
    • Challenges and adaptations (slides available but not on exam)

Timeline of Life

  • Eukaryotes appeared approximately 2.12.1 billion years ago.
  • Multicellular organisms appeared 1.21.2 billion years ago (protists, not animals).
  • Origin of animals estimated around 800800 million years ago (end of Proterozoic Eon).

Fossil Evidence

  • Oldest animal fossil: Dickinsonia costata (560560 million years old).
    • Soft-bodied, leaf-shaped.
  • Fossil record suggests animal appearance in late Adiacaran period (≈560\approx 560 million years ago).

Steroid Evidence

  • Steroids found in sediments 710710 million years old.
  • Steroids are substances produced exclusively in the animal kingdom.
  • Specific steroids linked to sponges (primitive animals).

Molecular Clock Analysis

  • Suggests animal origin around 800800 million years ago.

Cambrian Explosion

  • Rapid diversification of animal life around 540540 million years ago.
  • Most major animal lineages appeared.

Animal-Choanoflagellate Relationship

  • Animals and choanoflagellates are sister groups.
    • Share a common ancestor not shared by other groups.
  • Common ancestor likely similar to modern choanoflagellates.
  • Sponges diverged early in the animal lineage.
    • Sponge cells (choanocytes) resemble choanoflagellates.

Characteristics of Animals

  • Animalis (Latin): animated, living, breathing.
  • Kingdom of eukaryotes.
  • Monophyletic group (Metazoa).
  • Multicellular.
  • Heterotrophs:
    • Consume organic matter.
    • Ingest and digest food internally (unlike fungi with exoenzymes).

Exceptions to Heterotrophy

  • Emerald Elysia (sea slug):
    • Steals chloroplasts from algae (kleptoplasty).
    • Can live up to 10 months with light alone.
    • Has algal gene for photosynthesis (horizontal gene transfer).
  • Oriental hornet:
    • Harvests solar energy with a photovoltaic sensor (xenterin pigment).
    • Converts light to electrical energy.
    • Diurnal activity pattern (active when sunlight is intense).

Animal Respiration

  • Use aerobic respiration.
  • Use oxygen to break down glucose and other nutrients, creating ATP.
  • Glycolysis:
    • Glucose broken down to CO2 and pyruvate in cytoplasm.
  • Pyruvate oxidized in mitochondria to produce ATP.

Animal Cells and Tissues

  • Cells form tissues.
  • Lack cell walls (unlike plants and fungi).
  • Plasma membrane is the only celluar barrier.
  • Extracellular matrix provides support and cell communication.

Extracellular Matrix

  • Composed of proteins and polysaccharides (collagen).
  • Collagen is the most abundant protein in the animal kingdom, provides mechanical resistance.
  • Cells held together to form tissues (e.g., epithelial, nerve tissue).

Reproduction

  • Reproduce sexually with fusion of gametes.
  • Go through a blastula stage during development.
  • Some can also reproduce asexually.

Movement

  • Animals move around.
  • Some (sponges, anemones) are sessile, but have a motile stage (larval stage).

Origin of Multicellularity

  • Evolved multiple times in protists.
  • Coanoflagellate ancestor was unicellular.
  • Requires:
    • Cell adhesion
    • Cell communication
  • Choanoflagellates have cadherins (cell adhesion molecules).
  • Also have proteins for cellular signaling pathways.

Cadherins and CCD Domain

  • Animal cadherins have a cytoplasmic cadherin domain (CCD).
  • Choanoflagellates lack the CCD domain.
  • Common ancestor passed on cadherin production genes.
  • Genes were modified in favor of multicellularity in animals.

Tissue Specialization

  • Cells develop unique functions for specific tasks.
  • Sponges: simplest animals, early divergence.
    • Filter feeders.
    • Lack true tissues but have different cell types.

Sponge Cell Types

  • Choanocytes:
    • Line the interior walls of sponges.
    • Have collars and flagella (resemble choanoflagellates).
    • Flagella create water current.
    • Absorb particles.
  • Amoebocytes:
    • Move within the mesoglea (gelatinous layer).
    • Transport food and nutrients.

Sexual Reproduction in Animals

  • Diploid phase dominates.
  • Produce haploid gametes by meiosis.
  • Fertilization $\rightarrow$ zygote $\rightarrow$ adult.

Asexual Reproduction: Fragmentation

  • Body divides into pieces, each regenerating into a complete adult.
  • Example: Flatworms (planaria).
    • Can regenerate after decapitation.
    • Retain memory after fragmentation and brain regeneration.

Asexual Reproduction: Parthenogenesis

  • Females produce offspring from unfertilized eggs. No genetic material from male.
  • Lack of genetic variation is a disadvantage.
  • Advantage: saves female energy.

Animal Body Plans

  • Morphological and developmental characteristics integral to a living organism.

Symmetry

  • Radial Symmetry:
    • Parts arranged around a central axis.
    • Can be divided into similar halves along multiple planes.
    • Examples: jellyfish, sea anemones.
    • Common in sessile or planktonic organisms.
  • Bilateral Symmetry:
    • Can be divided into two mirror image halves.
    • Ventral (bottom) and dorsal (top) faces, anterior and posterior regions.
    • Examples: flatworms, earthworms, insects, fish, birds, mammals.

Cephalization

  • Concentration of sensory and nervous tissues at anterior end.
  • Associated with bilateral symmetry.
  • Beneficial for active movement and directional locomotion.

Embryogenesis and Development

  • Embryogenesis: transformations from fertilized egg to embryo.
  • Development: stages between embryonic and adult.
  • Genes controlling these processes are similar across taxa.

HOX Genes

  • Homeobox (HOX) genes play an essential role in development of embryos.
  • Determine body plan along head-to-tail axis.
  • Ensure structures form in correct location and with correct characteristics.

Cleavage

  • Diploid zygote undergoes cleavage (series of mitotic cell divisions).
  • Leads to blastula (hollow sphere of cells).

Gastrulation

  • Blastula reorganized into a two- or three-layered embryo (gastrula).
  • Involves invagination of the sphere.

Germ Layers

  • Endoderm: inner layer.
  • Ectoderm: outer layer.
  • Mesoderm (sometimes): middle layer.
  • Arcanteron: cavity inside the endoderm.
  • Blastopore: opening of the archenteron.
  • Blasto cell: space between endoderm and ectoderm.

Diploblastic vs. Triploblastic

  • Diploblastic: animals with radial symmetry, two embryonic layers (endoderm and ectoderm).
  • Triploblastic: animals with bilateral symmetry, three embryonic layers (ectoderm, mesoderm, endoderm).

Germ Layer Fates

  • Ectoderm: epidermis, nervous system.
  • Mesoderm: skeleton, muscles.
  • Endoderm: digestive tract and associated organs.

Coelum

*Body cavity in some triploblastic animals.

  • Filled with liquid and lined by tissue derived from the mesoderm.

Coelum Configurations

  • Coelomates: true coelum, entirely lined with mesoderm.
  • Pseudocoelomates: false coelum, lined by mesoderm on outside, endoderm on inside.
  • Acoelomates: no body cavity, space filled with mesoderm tissue.

Coelum Functions

  • Structural Support: can act as hydrostatic skeleton.
  • Diffusion & Transport: helps transport gasses, nutrients and waste products.
  • Organ Growth: organ can move independently of the body wall.
  • Circulatory system.
  • Allowed for more specialised organs, such as digestive tract and gonads.

Protostome vs. Deuterostome Development

  • Differ in:
    • Mode of cleavage
    • Coelum formation
    • Fate of the blastopore

Cleavage Patterns

  • Protostomes: spiral and determinate cleavage
  • Deuterostomes: radial and indeterminate cleavage

Coelum Formation

  • Protostomes: coelum forms from gaps in mesoderm.
  • Deuterostomes: coelum forms from an enfolding of the archenteron wall.

Blastopore Fate

  • Protostomes: blastopore becomes the mouth.
  • Deuterostomes: blastopore becomes the anus.

Examples

  • Diploblast (hydra): radial symmetry, two layers (no mesoderm).
  • Triploblast Protostome: Coelom forms where the mesoderm splits.
  • Triploblast Deuterostome: sea urchin, coelum forms in the wall of the arcantharon.

Notochord and Neural Tube

  • Chordates: mesoderm develops into a notochord.
    • Flexible rod for support.
    • Some keep it for life, others (vertebrates) lose it.
  • Nervous system develops from mesodermic tissues.
    • Notochord signals ectoderm to form neural plate.
    • Neural plate folds into neural tube.
    • Neural tube develops into central nervous system (brain and spinal cord).

Cephalization

  • Concentration of sensory and neural tissues in anterior end.
  • Associated with head and brain development.
    *Facilitates better environmental scanning, faster reactions and more complex behaviour.