Animal Evolution Detailed Notes

Animal Evolution

Cambrian Explosion and Species Diversity

  • The radiation of animals (metazoans) started ~550 million years ago during the Cambrian explosion.

  • Estimates of extant animal species range from 3 million to 10 million; approximately 1.4 million species have been described.

  • Vast majority of descriptions are insects and nematodes.

Ancestry of Animals

  • Ancestors of animals originated from a lineage of single-celled protists in the opisthokont clade.

  • This lineage also contributed to the evolution of fungi.

Key Traits of Animals

  • Monophyletic clade with characteristics:

    • Multicellular eukaryotes that develop from a single zygote.

    • Most animals are diploid (with some exceptions).

    • Ingestive heterotrophs, some having autotrophic symbionts.

    • Most possess internal digestion (not seen in fungi).

    • Generally motile and possess muscle and nervous tissues.

Exceptions in Animal Characteristics

  • Not all animals have the same features:

    • Some are sessile (fixed in place).

    • Absence of a nervous system in some species.

    • Some may lack a gut entirely.

  • Traits such as movement are not exclusive to animals (e.g., moving plants).

  • Not all multicellular eukaryotes are considered metazoans (plants, fungi).

Evidence for Shared Ancestry

  • Evidence supporting shared ancestry among animals includes:

    • Fossil records.

    • Conserved embryonic development patterns.

    • Similar morphology and physiology.

    • Protein structure comparisons.

    • Molecular data (gene sequences).

Animal Body Plans and Symmetry

Body Plan Overview

  • Defined as the form and arrangement of organ systems with integrated functions.

  • Diversity of body plans results from a common set of developmental genes, known as Hox genes.

Body Symmetry in Animals

  • Body symmetry is a critical aspect of animal morphology:

    • Asymmetry: seen in sponges.

    • Spherical Symmetry: common in protists.

    • Radial Symmetry: features a main oral-aboral axis; found in diploblastic groups like cnidarians and ctenophores.

    • Bilateral Symmetry: present in bilaterians, aids in a moving lifestyle; clear anterior-posterior (head-tail) orientation.

Evolution of Cephalization

  • Bilateral symmetry is associated with cephalization:

    • Concentration of nervous tissue in the anterior end (formation of a brain).

    • First encounters with stimuli happen at the head where sensory organs are located.

Embryonic Development Patterns

  • Animals classified by embryonic development:

    • Sponges: no germ layers.

    • Diploblasts (e.g., cnidarians): two germ layers (ectoderm, endoderm).

    • Triploblasts (bilaterians): include a mesoderm layer, allowing for diverse tissues.

Increasing Complexity of Animal Forms

  • Evolution generally shows increased complexity in animal forms, with notable exceptions like tapeworms.

  • Examination of increasing complexity in various phyla:

    • Porifera (sponges)

    • Annelida

    • Cnidaria

    • Mollusca

    • Platyhelminthes

    • Arthropoda

    • Nematoda

    • Echinodermata

Examples of Phyla:

Phylum Porifera (Sponges)

  • Sponges are ancient, lacking tissues, organs, or organ systems; function as living water filters.

  • Water is filtered through choanocytes (collar cells) within the body cavity (spongocoel).

  • Similarities between choanoflagellates and choanocytes suggest evolutionary ties.

Phylum Cnidaria

  • Cnidarians are diverse radially symmetric diploblasts, including jellyfish and corals.

  • Exhibit dimorphic life cycles: a sessile polyp and motile medusa.

  • Key characteristic is the nematocyst, a stinging organelle for capturing prey.

Phylum Platyhelminthes (Flatworms)

  • Acoelomate bilaterians with a branched gastrovascular cavity; can be free-living or parasitic.

  • Display slight cephalization and possess a ladder-like nervous system.

Phylum Nematoda (Roundworms)

  • Pseudocoelomate bilaterians with a complete digestive system, lacking cephalization.

  • Notable for diversity and include both free-living decomposers and parasites.

Phylum Annelida (Segmented Worms)

  • Coelomate bilaterians with metamerism; exhibit peristaltic movement.

  • Complete digestive system; shared segmental ganglia in the nervous system.

Phylum Mollusca (Mollusks)

  • Highly diverse, soft-bodied animals with basic anatomy including muscular foot and visceral mass.

  • Includes three significant groups: gastropods, bivalves, cephalopods.

Phylum Arthropoda (Arthropods)

  • Most abundant and diverse animals, particularly insects.

  • Exhibits segmentation and possess a chitinous exoskeleton, jointed appendages, and undergo molting.

  • Major groups include Crustaceans, Hexapods, Myriapods, and Chelicerates.

Phylum Echinodermata (Echinoderms)

  • Exhibit bilateral symmetry as larvae; radial symmetry as adults post-metamorphosis.

  • Characterized by a water-vascular system and calcareous plates.

Classification of Bilaterians

  • Bilaterians classified based on body cavity:

    • Coelomates: enclosed body cavity lined with mesoderm.

    • Acoelomates: no body cavity.

    • Pseudocoelomates: partially lined body cavity.

  • Grouped into two major clades: protostomes and deuterostomes, with the majority being protostomes.