20. Origin Of Animals

University of Alberta BIOL 108: Introduction to Biological Diversity

Topic 20: Origin of Animals - Lecture A2

  • Lecturer: Yan-yin Wang

Protists Leading to Animals

  • Molecular Phylogenetics:

    • Suggests sponges and other animals shared a last common ancestor.

    • Sponges diverged approximately 670 – 700 million years ago (ma).

    • Last common ancestor likely resembled extant protist choanoflagellates from the clade Opisthokont.

    • Reference: Jirsová & Wideman, 2022.

Choanoflagellate Protists

  • Definition:

    • Choanoflagellates are the extant sister group to animals.

  • Characteristics:

    • Small, unicellular, heterotrophic protists.

    • Approximately 140 extant species found in marine and freshwater environments.

    • Unique funnel-shaped collar of microvilli used for feeding.

    • Finger-like projections of the cell membrane capture bacteria.

    • Reference: Brunet & King, 2022.

Evolutionary Link Between Choanoflagellates and Animals

  • Animals are hypothesized to evolve from a choanoflagellate-like ancestor due to:

    • Similar cell morphology.

    • Presence of collar cells (choanocytes) in sponges.

    • Resemblance of animal cells not seen in non-choanoflagellate protists, plants, or fungi.

    • DNA sequence homology supports genetic connections.

    • Reference: Campbell et al., 2024.

Evolution of Multicellularity

  • Eukaryotic Evolution:

    • Eukaryotic cells' evolution allowed for emergence of various forms in protists.

  • Independence of Multicellularity:

    • Multicellularity evolved independently in multiple lineages, leading to complex forms in algae, plants, fungi, and animals.

  • Advantages:

    • Evolution of multicellularity provided advantages such as specialization and increased efficiency in resource utilization.

  • Evidence indicates multicellular eukaryotes emerged around 1.2 billion years ago, with a molecular clock suggesting an earlier origin around 1.7 billion years ago.

  • Whole-body animal fossils known from about 560 million years ago (Late Proterozoic Eon).

  • Reference: Campbell et al., 2024.

Geological Setting - Neoproterozoic Era

  • Timeframe: ~1 billion - 541 million years ago.

  • Events:

    • Rise of planktonic algae contributed to the Neoproterozoic Oxygenation Event (800 – 650 ma).

    • Formation of algal-dominated ecosystems with efficient nutrient and energy transfers.

  • Complex Multicellular Life:

    • Earliest complex multicellular animal fossils found in Ediacaran biota (635 – 541 ma).

    • Characteristics of Ediacaran organisms:

      • Soft-bodied, multicellular, mostly sessile.

      • Most Ediacaran taxa went extinct by the Cambrian Era.

    • Reference: Tajika & Harada, 2019.

Geological Setting - Palaeozoic Era

  • Timeframe: ~541 - 252 million years ago.

  • Cambrian Explosion:

    • Significant diversification of animal life.

    • Rapid appearance of most animal phyla.

    • Some animal taxa, e.g., sponges, existed prior to this event.

    • Emergence of hard body parts such as shells and exoskeletons.

    • Reference: Campbell et al., 2024.

Hypotheses Related to Cambrian Explosion

  • Oxygen Levels:

    • Increased oceanic oxygen supported the development of larger body sizes and higher energy demands.

  • Predation Evolution:

    • Emergence of predatory behaviors resulted in the development of complex body structures for evasion and defense.

  • Hox Gene Role:

    • Evolution of Hox genes modified body plans and segment development, contributing to biodiversity.

    • Reference: Campbell et al., 2024.

Geological Timeline in Animal Evolution

  • Palaeozoic Era:

    • Increased animal diversity negatively impacted by mass extinctions (Ordovician, Late Devonian, End-Permian).

  • Invasions of Land:

    • Invertebrates ventured onto land around 450 million years ago, followed by vertebrates around 365 million years ago.

  • Mesozoic Era:

    • Emergence of dinosaurs and mammals; origin of feathers and powered flight.

  • Cenozoic Era:

    • Mammals occupied numerous ecological niches post-End-Cretaceous mass extinction.

Invertebrates

  • Dominance:

    • Most extant animals are invertebrates, found in nearly all habitats.

    • Size range varies from microscopic to gigantic.

    • Invertebrates are classified as a paraphyletic group.

    • Reference: Campbell et al., 2024.

Shared, Derived Traits of Metazoa

  • Metazoa Traits:

    1. Multicellularity and cell differentiation.

    2. Cell adhesion (extracellular matrix).

    3. Sexual reproduction with sperm and ova.

    4. Development of an embryonic blastula.

  • Eumetazoa Specific Traits:

    1. True tissues, including muscle and nerve tissues.

    2. Gastrulation.

  • Bilateria Specific Traits:

    1. Bilateral symmetry.

    2. Triploblastic organization.

    3. Development of complex organs.