The History of Life on Earth: Major Events, Eons, and Mass Extinctions

The History of Life on Earth

Overview

The history of life on Earth is marked by major evolutionary events, the emergence and extinction of diverse organisms, and dramatic changes in the planet's environment. This study guide summarizes the key periods, innovations, and mass extinctions that have shaped biological diversity.

Major Events of the Precambrian

Precambrian Eon

The Precambrian encompasses the vast span of time from Earth's formation to the appearance of most animal groups.

• Timeframe: ~4.5 billion years ago (bya) to ~541 million years ago (mya)

• Subdivisions: Hadean, Archean, and Proterozoic eons

• Life Forms: Exclusively unicellular for most of this period

• Atmospheric Conditions: Oxygen was virtually absent from oceans and atmosphere for ~2 billion years, until the evolution of photosynthetic bacteria

The Cambrian Explosion

Rapid Diversification of Life

The Cambrian Explosion marks a period of rapid evolutionary change, where most major animal groups first appeared.

• For almost 3 billion years, life was unicellular, except for brief intervals of small multicellular stages (~1 bya)

• The first animals, such as early sponges, appeared ~700 mya

• ~50 million years later, animals became larger and more complex

• The Cambrian Explosion: The most spectacular evolutionary change in the history of life

Morphological, Physiological, or Behavioral Innovation

Key evolutionary innovations often trigger major diversification events.

• Flowers: Enabled the diversification of angiosperms (flowering plants), now with over 250,000 known species

• Feathers and Wings: Allowed some dinosaurs to fly, leading to the bird lineage (~10,000 species today)

What Triggered the Cambrian Explosion?

Several hypotheses explain the onset of rapid diversification during the Cambrian period.

• Most recent common ancestor of all living animals: May have arisen ~800 mya, before diversification

• Key developmental toolkit genes: Likely evolved before diversification

Hypotheses for the Cambrian Explosion

1. Higher oxygen levels

2. Rise of algae

3. The evolution of predation

4. New niches beget more new niches

5. New genes, new bodies

The Phanerozoic Eon

Overview

The Phanerozoic Eon spans from 541 mya to the present and is divided into three major eras, each characterized by distinct life forms and environmental changes.

• Paleozoic Era: Origin and diversification of animals, land plants, fungi; first appearance of land animals

• Mesozoic Era: Terrestrial environments dominated by gymnosperms and dinosaurs

• Cenozoic Era: Terrestrial environments dominated by angiosperms and mammals

• Each era is divided into periods, which are further divided into epochs

• Constant changes in climate and continental locations occurred during these eras

Paleozoic Era

• Begins with the appearance of most major animal lineages

• Ends with the obliteration of almost all multicellular life forms at the end of the Permian period

• Includes initial diversification of animals, land plants, and fungi

• Land animals first appear

Mesozoic Era

• Begins with the end-Permian extinction events

• Ends with the extinction of dinosaurs between the Cretaceous and Paleogene periods

• Dinosaurs and gymnosperms were the most dominant terrestrial vertebrates and plants, respectively

Cenozoic Era

• Divided into the Paleogene, Neogene, and Quaternary periods

• Mammals and birds were the most dominant vertebrates

• Angiosperms were the most dominant plants

Large-Scale Patterns in Life’s History: Mass Extinctions

Definition and Impact

• Mass extinction: The rapid extinction of a large number of diverse species around the world

• At least 60% of species present are wiped out within 1 million years

• Caused by catastrophic events

• Opposite of adaptive radiation (rapid diversification)

The Big Five Mass Extinction Events

Throughout Earth's history, five major mass extinction events have dramatically reduced biodiversity.

Extinction Event	Approximate Time (mya)	Main Impact
End-Ordovician	~444	Marine species loss
Late Devonian	~375	Marine and terrestrial species loss
End-Permian	~252	Largest extinction; ~90% of species lost
End-Triassic	~201	Marine and terrestrial species loss
End-Cretaceous	~66	Extinction of dinosaurs; ~60–80% of species lost

The End-Permian Extinction

Causes and Consequences

• Largest mass extinction event; ~90% of all species disappeared

• Ongoing research suggests several contributing factors:

1. Siberian traps: Outpourings of molten rock (flood basalts) released heat, CO2, and sulfur dioxide into the atmosphere

2. High atmospheric sulfur dioxide: Caused severe acid rain, devastating plants and dependent organisms

3. Coal fires: Flood basalts ignited widespread coal fires, releasing toxic ash (including mercury) into the air

4. Anoxic oceans: Oceans became lacking in oxygen, fatal to organisms relying on aerobic respiration

5. Sea level drop: Reduced habitat for marine organisms

The End-Cretaceous Extinction

Impact Hypothesis

• The impact hypothesis proposes that an asteroid struck Earth 66 mya, causing the extinction of ~60–80% of multicellular species, including dinosaurs

Evidence for the Impact Hypothesis

• High levels of iridium (indicative of asteroids) found at the Cretaceous–Paleogene (K–Pg) boundary

• A giant crater off the coast of Mexico dates to 66 mya

• Asteroid estimated to be ~10 km across

Consequences of the Impact

• Hot gas fireball spread from the impact site, causing catastrophic wildfires

• The largest tsunami in the last 3.5 billion years, disrupting ocean sediments and circulation

• Extensive acid rain from sulfate-containing rock reacting with water in the atmosphere

• Dust, ash, and soot blocked the Sun, leading to global cooling and a crash in plant productivity

Summary Table: Major Eons and Eras

Eon/Era	Timeframe	Key Features
Precambrian	4.5 bya – 541 mya	Unicellular life, low oxygen, evolution of photosynthetic bacteria
Paleozoic	541 – 252 mya	Diversification of animals, plants, fungi; first land animals
Mesozoic	252 – 66 mya	Dominance of dinosaurs and gymnosperms
Cenozoic	66 mya – present	Dominance of mammals, birds, and angiosperms

Key Terms

• Mass extinction: Rapid loss of a large number of species due to catastrophic events

• Adaptive radiation: Rapid diversification of a lineage into many new species

• Precambrian: The earliest eon, before the appearance of most animal groups

• Phanerozoic: The eon from 541 mya to present, including the Paleozoic, Mesozoic, and Cenozoic eras

• Cambrian Explosion: A period of rapid evolutionary diversification of animal life

Example: Adaptive Radiation vs. Mass Extinction

• Adaptive radiation: Darwin's finches in the Galápagos Islands diversified into multiple species, each adapted to different ecological niches

• Mass extinction: The end-Cretaceous event wiped out the dinosaurs and many other species, opening ecological niches for mammals to diversify

Additional info: The study notes include inferred details from diagrams and tables, such as the timing and impact of extinction events and the major evolutionary innovations.