The History of Life on Earth: Timeline, Fossil Record, and Major Events

The History of Life on Earth

Lecture Overview

This section provides a structured overview of the history of life on Earth, focusing on the fossil and geologic record, dating methods, major eons, continental drift, and mass extinctions. Understanding these topics is essential for grasping the evolutionary processes that have shaped biodiversity.

• A timeline of life's history through the fossil and geologic record

• How rocks and fossils are dated

• The Hadean, Archaean, Proterozoic, and Phanerozoic eons

• Plate tectonics and continental drift

• Mass extinctions and their impacts on life

Timeline of Earth's History

Using the Calendar Metaphor

To conceptualize Earth's 4.6-billion-year history, a calendar year is used as a metaphor:

• Full year = Entire lifespan of Earth (~4.6 billion years)

• Month ≈ 400 million years

• Week ≈ 100 million years

• Day ≈ 12.5 million years

• Hour ≈ 0.5 million years

• Minute ≈ 10,000 years

• Second ≈ 130 years

This analogy helps relate major geologic and biological events to a familiar time frame.

Geologic Time Scale

Eons, Eras, Periods, and Epochs

The geologic time scale is divided into hierarchical units:

• Eons (largest)

• Eras

• Periods

• Epochs (smallest)

Each unit 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 than upper layers.

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

Radiometric Dating

• 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 date a sample, scientists measure the ratio of parent to daughter isotopes and know the half-life.

Key Equation:

$N(t) = N_0 \left(\frac{1}{2}\right)^{t/T_{1/2}}$

Where $N(t)$ is the amount of parent isotope at time $t$, $N_0$ is the original amount, and $T_{1/2}$ is the half-life.

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

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

Major Eons of Earth's History

Hadean and Archaean Eons

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

• Archaean Eon: Origin of life (~3.5–4.0 billion years ago); beginning of atmospheric oxygen due to photosynthetic prokaryotes (e.g., stromatolites).

Proterozoic Eon

• Spans from the oxygen revolution to the Cambrian Explosion (~2.5 billion to ~540 million years ago).

• Major events:

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

  • Origin of multicellular life

  • Emergence of multiple animal phyla

Origin of Eukaryotes

• Development of larger, more complex cells through endosymbiosis (e.g., mitochondria and chloroplasts).

• Evidence for endosymbiosis includes the presence of mitochondrial and chloroplast DNA.

Phanerozoic Eon (Current Eon)

• "Visible life"—marked by abundant animal and plant fossils.

• Began with the Cambrian Explosion (~540 million years ago), a rapid diversification of animal life.

The Cambrian Explosion

The Cambrian Explosion refers to the sudden appearance of fossils resembling modern animal phyla in the Cambrian Period (535 to 525 million years ago). It marks the first evidence of predator-prey interactions and the emergence of complex animal life.

The Geologic Record – Post-Cambrian

• Development of land plants and animals (including fungi)

• Appearance of insects

• Emergence of 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 around 365 million years ago.

• Modern humans appeared only about 195,000 years ago.

Plate Tectonics and the Movement of Continents

• Continental drift is the slow movement of continents over time, altering the size, shape, and number of continents.

• Evidence includes similar fossils found on distant continents.

What Did Earth Used To Look Like?

• Pangaea: A 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 organisms reflects historic movement of continents.

Mass Extinctions

Mass extinctions are periods when large numbers of species become extinct due to rapid and disruptive environmental changes. Earth has experienced five major mass extinctions:

• The Ordovician

• The Devonian

• The Permian

• The Triassic

• The Cretaceous

Mass Extinction Events: The Permian

• Occurred 251 million years ago, marking 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, ending the Mesozoic era.

• Killed most dinosaurs.

• Likely caused by a meteorite impact (Chicxulub crater) and subsequent global cooling due to dust clouds.

Mass Extinctions and the Diversity of Life

• Mass extinctions eliminate many species, but also pave 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.

Summary Table: Major Eons and Events

Eon	Time Frame	Major 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	Oxygen revolution, origin of eukaryotes, multicellularity
Phanerozoic	541 mya–present	Cambrian Explosion, colonization of land, mass extinctions, rise of modern life

Additional info: This summary integrates and expands upon the provided lecture slides, adding definitions, equations, and context for a comprehensive study guide.