BIOL 130 Study Guide MT #3 Fall 2025

LUCA and the Age of the Earth

  • LUCA: Stands for the Last Universal Common Ancestor. It refers to the most recent organism from which all organisms now living on Earth descended.

  • Estimated Age of the Earth: Approximately 4.5 billion years.

Definition and Characteristics of Animals

  • Definition of Animals:

    • Multicellular, heterotrophic eukaryotes.

    • Lack cell walls, which is a key feature distinguishing them from plants and fungi.

    • Contain collagen, a unique protein found in the extracellular matrix of animal tissues.

    • Exhibit unique developmental patterns including cleavage, blastula formation, and gastrulation.

HOX Genes and Animal Evolution

  • HOX Genes:

    • Key regulatory genes involved in the development of animal body plans.

    • They help determine the basic structure and orientation of an organism during embryonic development.

Closest Living Relatives of Animals

  • The choanoflagellates are thought to be the closest living relatives of animals.

    • Choanoflagellates are unicellular organisms that share several cell signaling and adhesion genes with animals.

Importance of Porifera in Evolution

  • Porifera (Sponges):

    • They are significant in understanding animal evolution because they represent one of the simplest forms of multicellular life.

    • Their body structure and feeding mechanisms illustrate early pathways of multicellularity and how animal organization might have originated.

Linking Sponges to Choanoflagellates

  • Key Cell Type: Choanocytes are the type of cells in sponges that link them to choanoflagellates.

    • Choanocytes have a collar of microvilli that helps in capturing food particles, similar to the feeding structures of choanoflagellates.

Symmetry in Animals

  • Radiata vs. Bilaterally Symmetrical Animals:

    • Radiata: Have radial symmetry, allowing for multiple symmetrical planes (e.g., cnidarians).

    • Bilaterally symmetrical animals have a single plane of symmetry and are more complex.

Diploblasts vs. Triploblasts

  • Diploblasts: Animals with two germ layers (ectoderm and endoderm) — examples include cnidarians.

  • Triploblasts: Animals with three germ layers (ectoderm, endoderm, mesoderm) — examples include most bilaterally symmetrical animals.

    • The mesoderm is crucial as it forms muscles and most internal organs.

Evolution of the Coelom

  • Coelom: A body cavity lined by mesoderm. It provides space for the development of organs and allows for greater movement and complexity in body structure.

Key Differences: Protostomes vs. Deuterostomes

  • Protostomes:

    • Cleavage patterns exhibit spiral and determinate development.

    • The fate of the blastopore (the opening formed during early embryonic development) develops into the mouth.

  • Deuterostomes:

    • Cleavage patterns exhibit radial and indeterminate development.

    • The blastopore develops into the anus.

Cephalization

  • Cephalization: The evolutionary trend where nervous system and sensory structures become concentrated at the anterior end of the organism (head).

    • It is prevalent in bilateral animals, enhancing their ability to locate food and mates, and avoid predators.

Molecular Phylogenies and Evolutionary Understanding

  • Molecular phylogenies: Utilize genetic data to determine evolutionary relationships, providing insights that may differ from morphological trees based on physical characteristics alone.

    • This helps in understanding ancestry and divergence times more accurately.

Larval Stages and Evolutionary Relationships

  • Larvae: Are sometimes used to infer evolutionary relationships among animals because they can retain ancestral features that are modified in adult forms.

Geologic Time Period and Modern Animal Phyla

  • Most modern animal phyla originated during the Cambrian period (about 541 to 485 million years ago).

Hypotheses for the Cambrian Explosion

  • Proposed hypotheses explaining the Cambrian Explosion include:

    • Ecological Factors: Changes in environmental conditions or ecological dynamics allowed for increased diversity.

    • Genetic Factors: Developmental innovations and genetic changes in early animals may have led to new body plans.

    • Geological Factors: Changes in Earth's environment, including ocean chemistry and plate tectonics, influenced biodiversity.

Differences in Phylogenetic Models

  • Molecular phylogenies differ from traditional morphology-based trees by integrating genetic data to create a more robust understanding of relationships among species, often resolving disagreements seen in morphological trees due to convergent evolution or other factors.