The History of Life on Earth: Geologic Time, Fossils, and Mass Extinctions

The History of Life on Earth

Introduction

This section explores the major events in Earth's biological history, focusing on the fossil and geologic record, the dating of rocks and fossils, the division of Earth's history into eons, and the impact of plate tectonics and mass extinctions on the diversity of life.

Timeline of Earth's History

Using the Calendar Metaphor

• Calendar Metaphor: The entire 4.6 billion-year history of Earth is represented as a single calendar year.

• Time Equivalents:

  • Month ≈ 400 million years

  • Week ≈ 100 million years

  • Day ≈ 12 million years

  • Hour ≈ 0.5 million years

  • Minute ≈ 10,000 years

  • Second ≈ 130 years

• January 1, 12:00 AM = Beginning of Earth

• December 31, 11:59:59 PM = Present day

Geologic Time Scale

Hierarchy of Time Divisions

• Eons > Eras > Periods > Epochs

• Each division represents significant changes in Earth's geology and life forms.

How Rocks and Fossils Are Dated

Relative and Absolute Dating

• Sedimentary strata reveal the relative ages of fossils (lower layers are older).

• Radiometric dating determines the absolute age of rocks and fossils by measuring the decay of radioactive isotopes.

• Short-term dating (thousands of years): Carbon dating.

• Long-term dating (millions to billions of years): Uranium-lead and other methods.

Radiometric Dating Principles

• A radioactive "parent" isotope decays to a "daughter" isotope at a constant rate.

• Each isotope has a known half-life (the time required for half of the isotope to decay).

• To determine age, need to know:

  • How much parent and daughter isotope is present

  • The half-life of the isotope

Equation:

$N(t) = N_0 \times (1/2)^{t/T_{1/2}}$

Where $N(t)$ is the amount remaining after time $t$, $N_0$ is the initial amount, and $T_{1/2}$ is the half-life.

Major Eons of Earth's History

Hadean and Archaean Eons

• Hadean Eon: Little to no biological activity; Earth was cooling and experiencing heavy asteroid bombardment.

• Archaean Eon:

  • Origin of life (estimated at ~3.5 billion years ago).

  • Beginning of atmospheric oxygen (produced by photosynthetic prokaryotes such as cyanobacteria).

Proterozoic Eon

• Spans from ~2.5 billion to ~540 million years ago.

• Major events:

  • Origin of eukaryotic cells (cells with nuclei and organelles).

  • Origin of multicellular life.

  • Emergence of multiple animal phyla.

Origin of Eukaryotes

• Development of larger organelles (mitochondria and chloroplasts) via endosymbiosis.

• Mitochondrial and chloroplast DNA provide evidence for their symbiotic origins.

Phanerozoic Eon (Current Eon)

• Name means "visible life"; marked by abundant animal and plant fossils.

• Begins with the Cambrian Explosion (~540 million years ago): rapid diversification of animal life, especially those with hard skeletons.

The Cambrian Explosion

• Sudden appearance of fossils resembling modern animal phyla (535–525 million years ago).

• First evidence of predator-prey interactions.

• Earlier animal phyla include sponges, cnidarians, and mollusks.

The Geologic Record – Post-Cambrian

• Development of land plants and animals (including fungi).

• Appearance of insects and vascular plants.

The Colonization of Land

• Arthropods (jointed feet) and tetrapods (four feet) are the most widespread and diverse land animals.

• Tetrapods evolved from lobe-finned fishes (~345 million years ago).

• Human lineage of tetrapods evolved around 6–7 million years ago; modern humans appeared only ~195,000 years ago.

Plate Tectonics and Continental Drift

• Earth's continents have changed position, size, and shape over billions of years due to continental drift.

• Evidence includes similar fossils found on distant continents.

What Did Earth Used To Look Like?

• Pangaea: Supercontinent that existed ~250 million years ago.

• Split apart ~135 million years ago, leading to shallow seas, coral reefs, and diversification of life.

Consequences of Continental Drift

• Opens new habitats and can lead to adaptive radiations.

• Changes in climate as continents move north or south.

• Separation of land masses can lead to allopatric speciation (formation of new species due to geographic isolation).

• Distribution of fossils and living groups reflects historic movement of continents.

Mass Extinctions

• Most species that have ever lived are now extinct.

• Extinctions can be caused by environmental changes, often disruptive and global in scale.

• Earth has experienced five major mass extinctions:

  • Ordovician

  • Devonian

  • Permian

  • Triassic

  • Cretaceous

Mass Extinction Events: The Permian

• Occurred 251 million years ago; defines the boundary between the Paleozoic and Mesozoic eras.

• Caused extinction of about 96% of marine animal species.

• Triggered by massive volcanic activity, global warming, and ocean acidification.

Mass Extinction Events: The Cretaceous

• Occurred 66 million years ago; marks the boundary between the Mesozoic and Cenozoic eras.

• Killed most dinosaurs.

• Evidence suggests a meteorite impact (Chicxulub crater) caused dust clouds, blocked sunlight, and disrupted climate.

Mass Extinctions and the Diversity of Life

• Mass extinctions eliminate many species, paving the way for adaptive radiations (rapid evolution of new species to fill ecological niches).

• Lineages with novel features can be lost, but survivors diversify and dominate new environments.

Table: Major Eons and Key Events

Eon	Time Frame	Key Events
Hadean	4.6–4.0 bya	Formation of Earth, cooling, heavy bombardment
Archaean	4.0–2.5 bya	Origin of life, first prokaryotes, beginning of atmospheric oxygen
Proterozoic	2.5 bya–541 mya	Origin of eukaryotes, multicellular life, oxygen revolution
Phanerozoic	541 mya–present	Cambrian explosion, colonization of land, mass extinctions, rise of modern life

Summary

• Earth's history is divided into eons, eras, periods, and epochs, each marked by significant geological and biological events.

• Dating methods such as radiometric dating allow scientists to determine the age of rocks and fossils.

• Plate tectonics and continental drift have shaped the distribution and evolution of life.

• Mass extinctions have repeatedly reshaped the diversity of life, leading to new adaptive radiations.

Additional info: Some explanations and context have been expanded for clarity and completeness, including the table summarizing eons and key events.