Chapter 22.4: The History of Life

Diversity of Life

A result of evolution over geologic time which is studied experimentally using…

1. Nested Similarities/ Comparative Biology

2. Fossil records

• An agreement between these two methods:

  • Eg, Fossil orders match phylogenetic order

  • Fossils: Earliest fish → then mammals → then primates → then humans.

  • Phylogeny: Humans = one branch within primates, nested within mammals, within vertebrates

Nested Similarities/ Comparative Biology

1. From living organism and phylogenesis 

• Lacks extinct forms

Fossil records

1. Direct historical archive

• Limited and incomplete due to biases but offer unique time-based environment insight 

Early Earth

• Earth formed ~4.6 billion years ago

• Oldest animal fossils: 575 million years ago 

• But microbial life existed long before animals — for >3 billion years

Bacteria and Archaea

• Deepest node in the tree of life: The divergence between bacteria and archaea 

• Both diversified before eukaryotes appeared 

  • No oxygen at this point 

Stromatolites

• Layered sedimentary formations made by microbial mats (bacteria)

• Earliest known fossil of life: From 3.5 billion years ago 

  • Indicated diverse microbial ecosystems in shallow seas

• Still form today

Chemical Evidence

• Ancient rocks contain isotopic and molecular traces of biological activity

  • Suggested life originated over 3.5 billion years ago 

• This record no longer works when rocks are too old to preserve clear evidence 

Eukaryotes

• Cells containing a membrane-bound nucleus and other membrane-bound organelles

  • Oxygenation of earth allowed eukaryotic cells with nuclei and organelles to form 

    • Great Oxidation Event: O₂ began accumulating ~2.4 billion years ago from cyanobacteria

      • Allowed for new metabolic and structural diversity 

  • Eukaryotic Origins

    • Archaea contributed to informational systems

    • Proteobacteria became the mitochondrion

    • Possibly a fusion between archael and bacterial cells

  • Eukaryotic Fossils: 

    • Found in rocks that were over a billion years old 1.8-1 Ga

      • Contained unicellular and multicellular forms 

    • Indicated major eukaryotic branches already diverged 

First ANimals

• Late precambrian fossils of oldest animal (575 Ma)

• Fossil indicated: Animals made of a surface covering of cells and interior containing fluid/jelly-like substance 

Cambrian Explosion/ Era/Period: 541-485 Ma

• Rapid diversification of animal life 

  • Increased nutrients → more photosynthesis

  • Increased oxygen → more energy for activity and growth

• When all major animal phylas developed - They appear in fossil record 

Plants Cambrian Explosion/ Era/Period: 541-485 Ma

• Evidence: Spores in rocks since they are resistant to decay 

• Early plants similar to mosses/liverworts.

• Vascular tissues (for water transport and support): ~420 Ma.

• By 360 Ma: forests of large trees, seeds, and complex plant organs.

• Plants transformed land and provided food and structure for other organisms.

Arthropods and Insects Cambrian Explosion/ Era/Period: 541-485 Ma

• First terrestrial animals: scorpion-like arthropods (~430 Ma).

  • Insects evolved by 400 Ma.

• Adaptations: waterproof exoskeletons, air-breathing systems

Fungi Cambrian Explosion/ Era/Period: 541-485 Ma

• Diversified on land alongside plants (~400 Ma).

• Important decomposers and symbionts

Vertebrates Cambrian Explosion/ Era/Period: 541-485 Ma

• Descended from lobe-finned fish.

• Fossils (380–360 Ma) show:

  • Development of limbs, lungs, and skeletal features for land life.

• Mesozoic Era: 252-66 Ma

• The three periods of the Mesozoic Era

1. Triassic Period: The first period, from 252 to 201 Ma

• saw the emergence of the first dinosaurs and mammals.

2. Jurassic Period: From 201 to 145 Ma 

• great diversification for dinosaurs.

3. Cretaceous Period: The final period, from 145 to 66 Ma, 

• The era ended with a major extinction event that wiped out the non-avian dinosaurs and ammonites.

Marine Life: Mesozoic Era: 252-66 Ma

• new corals, diversification of clams, snails, crustaceans

• Some land vertebrates returned to the sea:

Plants and Insects Mesozoic Era: 252-66 Ma

• Angiosperms: Flowering plants 

  • First appeared during this era and became dominant 

• Pollinating Insects: Coevolved with angiosperms to mutualistic relationship 

Dinosaur and Vertebrates Mesozoic Era: 252-66 Ma

• Dinosaurs dominated during this era until their extinction 

• Also saw rise of frogs, mammals, birds, lizards, turtles.

Cenozoic Era: 66 Ma - Present

• Mammalian and Avian Radiation

  • Diversification occurred rapidly after dinosaurs 

  • Opened up resources that were dominantly being used by dinosaurs 

• Development of the modern ecosystem 

• Earth's Changes

Earth's Changes

1. Continental drift → new oceans and mountain ranges

2. COoling Climate: 

• Co₂ levels fell 

• Expansion of grasslands 

• Glaciation began 

3. Ice Age: 2.5 Ma ago

• ce sheets covered northern continents

• Winter/ arctic animals 

  • Extinct due to increase in temp 

4. Human Expansion causing extinction