Animal Evolution 2/26

Objectives of the Lecture

  • Review of animal phylogeny and terminology.

  • Discussion on characterizing features that define animals.

  • Examination of evolutionary timeline and factors influencing diversification and extinction.

  • Exploration of 5 major extinction events affecting animal evolution.

Overview of Animal Phylogeny

  • Review of what constitutes an animal:

    • Heterotrophic with internal digestion.

    • Multicellular with differentiated cells and tissues.

    • Typically mobile at some point during their life cycle.

    • Presence of a nervous system (with sponges as an exception).

  • The monophyletic group includes all animals sharing a common ancestor.

  • Evolution of nervous systems is complex, involving independent evolution in different lineages.

Clades and Developmental Patterns

  • Distinction between two major clades:

    • Proto-stomes: Blastopore develops into mouth.

    • Deuterostomes: Blastopore develops into anus.

  • Speciation events are not always bifurcated due to conflicting evidence about the relationships among early animal groups.

Evolutionary Timeline Overview

  • Common Ancestor of Animals: Divergence with choanoflagellates around 900 million years ago.

  • Slow initial diversification of animal forms leading into the Cambrian explosion:

    • Cambrian explosion occurred around 540 million years ago, marking a rapid increase in animal diversity with many phyla appearing in the fossil record.

    • Introduced the theory of punctuated equilibrium in evolution.

Factors Contributing to Cambrian Explosion

  • Increase in global oxygen levels.

  • Availability of calcium in seawater contributing to skeletal evolution.

  • Warm, shallow marine environments creating diverse habitats.

  • Evolution of Hox genes enabling body plan diversification.

Post-Cambrian Evolution

  • Ordovician Period (485-420 million years ago):

    • Evolution of the modern spinal column allowing the emergence of bony fish.

    • Early land colonization by arthropods and plants around 470 million years ago.

Early Fish Evolution

  • First fishes appeared in the Ordovician:

    • Early fish lacked bony jaws; transitioned to bony jaws in late Ordovician.

    • Major lineages: Chondrichthyes (cartilaginous fishes) and Osteichthyes (bony fishes).

  • Devonian Period:

    • Known as the golden age of placoderms (extinct armored fish).

    • Lobe-finned fishes emerge, leading to the eventual evolution of tetrapods.

Transition to Terrestrial Life

  • Lobe-finned fishes transitioned to land colonization, leading to the evolution of amphibians during the late Devonian.

  • Early amphibians retained aquatic lifestyles but adapted over 80 million years for terrestrial life.

Important Clades to Note

  • Sarcopterygii: Lobe-finned fishes including coelacanths, lungfish, and all tetrapods.

  • Amniotes: Ancestral lineages of mammals and reptiles developed during the Pennsylvanian period.

Evolutionary Trends

  • Observation of increasing complexity in evolution:

    • Centralization: Nervous systems evolve towards more central control structures.

    • Specialization: Sensory organs localized towards the anterior region of organisms for environmental interaction.