BIO112 Lec11 Phylogenies and the History of Life 2025

Lecture 11: Phylogenies and the History of Life

The History of Life

  • Two primary tools for analysis:

    • Fossil record

    • Phylogenetic trees

Fossils and Earth's Early Environment

Ancient Fossil Evidence

  • Earth's oldest fossils found in Australia.

  • Microscopic fossils provide evidence for simple cells and bacteria existing in an oxygen-free environment over 3.4 billion years ago.

  • Environmental conditions during this time include:

    • High volcanic activity, cloudy skies, and warm ocean temperatures (40-50°C).

    • Small land masses and significant tidal ranges.

  • Fossil examples:

    • Stromatolites (oldest fossils at 2.8 billion years).

    • Chain of cyanobacterial cells from 1 billion years ago, resembling modern cyanobacteria like Oscillatoria.

How Fossils Form

  1. Intact fossils: Decomposition does not occur.

  2. Compression fossils: Sediments accumulate and compress material into thin films.

  3. Cast fossils: Material decomposes, dissolved minerals form casts.

  4. Permineralized fossils: Slow rotting allows for minerals to infiltrate and harden into stone.

    • Examples of fossils types include:

      • (a) Intact fossil (pollen)

      • (b) Compression fossil (leaf)

      • (c) Cast fossil (bark)

      • (d) Permineralized fossil (tree trunk)

Limitations of the Fossil Record

  1. Habitat bias: Organisms in sediment-rich areas are more likely to fossilize.

  2. Taxonomic and tissue bias: Organisms with durable tissues (bones) decay slower.

  3. Temporal bias: Younger fossils are more common than older fossils.

  4. Abundance bias: Common, widespread species leave more fossil evidence.

Eons and Key Events in Earth's History

Timeline of Key Eons

  • Hadean Eon (4.570 billion years ago):

    • Earth formation; heavy bombardment ends; first oceans form.

  • Archaean Eon (4.000 billion years ago):

    • First photosynthetic cells.

    • Origin of life.

  • Proterozoic Eon (2.500 billion years ago):

    • First evidence of oxygenic photosynthesis.

    • Appearance of first multicellular organisms and sexual structures.

  • Phanerozoic Eon (541 million years ago - present):

    • Major diversification of life, including bilaterally symmetric animals and sponges.

    • Rapid rise of oxygen levels.

Key Organisms and Events by Eon

  • Phanerozoic Eon: First appearances of mammals, birds, flowering plants, and hominins.

  • Proterozoic and Precambrian: Development of eukaryotes and cyanobacteria.

Phylogenetic Trees

Understanding Phylogenetic Trees

  • Phylogeny: The evolutionary history of a group of organisms.

  • Phylogenetic tree: A graphical representation of this history, illustrating ancestor-descendant relationships among taxa.

Anatomy of a Phylogenetic Tree (Table 25.1)

  • Root: The most ancestral branch.

  • Branch: Represents a population over time.

  • Tip: Endpoints representing living/extinct taxa.

  • Node: A point where a branch splits, indicating a common ancestor.

  • Synapomorphy: A shared trait due to common ancestry.

  • Polytomy: A node indicating multiple descendant branches, often due to insufficient data.

Branch Lengths in Phylogenetic Trees (Table 25.3)

  • Represents evolutionary time or genetic differences; can be arbitrary or indicative of relationships.

Summary Tables

Table 25.2: Traits and Groups on Trees

  • Monophyletic group: Contains an ancestor and all its descendants.

  • Paraphyletic group: Includes an ancestor and some, but not all, descendants.

  • Polyphyletic group: Unnatural grouping not including the most recent common ancestor.

Table 25.3: Branch Lengths

  • Length indicates either genetic differences or time, depending on context.