Geological Time Scale: Major Events and Periods

Precambrian Time

Covers about 88% of Earth's history.

Precambrian Time

Formation of Earth, first oceans, and atmosphere.

Precambrian Time

First unicellular life (bacteria & archaea).

Precambrian Time

Appearance of photosynthetic organisms → Oxygen buildup (Great Oxygenation Event).

Precambrian Time

First multicellular life towards the end.

What percentage of Earth's history does the Precambrian cover?

About 88% of Earth's history.

What significant events occurred during the Precambrian time?

Formation of Earth, first oceans and atmosphere, appearance of unicellular life, and the Great Oxygenation Event.

When did the first multicellular life appear?

Towards the end of the Precambrian.

What was the Great Oxygenation Event?

The buildup of oxygen in Earth's atmosphere caused by photosynthetic organisms, leading to the extinction of many anaerobic species.

Cambrian Period

"Cambrian Explosion" → Rapid diversification of life.

Cambrian Period

First appearance of many major animal groups (e.g., arthropods, mollusks, early vertebrates).

Cambrian Period

Oceans dominated by trilobites, brachiopods, and early fish.

What is the "Cambrian Explosion"?

A rapid diversification of life where almost all major animal phyla appeared.

What was the dominant group in the Cambrian oceans?

Trilobites, brachiopods, and early fish.

Ordovician Period

First vertebrates with jaws.

Ordovician Period

Early coral reefs form.

Ordovician Period

First land plants & fungi.

Ordovician Period

Ended with a mass extinction due to glaciation.

What was significant about the first vertebrates in the Ordovician period?

They were the first vertebrates with jaws.

What major event ended the Ordovician period?

A mass extinction caused by glaciation.

Silurian Period

First jawed fish appear.

Silurian Period

First land animals (arthropods).

Silurian Period

Vascular plants evolve, allowing colonization of land.

What is a key evolutionary event in the Silurian period?

The first jawed fish and land animals (arthropods).

What evolved in the Silurian period that allowed plants to grow taller?

Vascular plants.

Devonian Period

"Age of Fishes" → Major fish evolution (sharks, bony fish).

Devonian Period

First amphibians transition from water to land.

Devonian Period

First trees and forests.

Devonian Period

Ended with a mass extinction, killing many marine species.

Why is the Devonian period known as the "Age of Fishes"?

It saw the evolution of many fish species, including sharks and bony fish.

What major transition happened in the Devonian period?

The first amphibians transitioned from water to land.

What was a significant evolutionary development in plants during the Devonian?

The appearance of the first trees and forests.

Carboniferous Period

Formation of vast coal forests (swamps filled with giant ferns & trees).

Carboniferous Period

First reptiles evolve.

Carboniferous Period

Oxygen levels were high → Huge insects (e.g., dragonflies with 2.5-ft wingspans).

What is notable about the atmosphere during the Carboniferous period?

High oxygen levels allowed giant insects to evolve, like dragonflies with 2.5-ft wingspans.

What evolved in the Carboniferous period?

The first reptiles.

Carboniferous period

Vast coal forests, which eventually became the coal deposits we use today.

Permian Period

Pangaea supercontinent forms.

Permian Period

Reptiles dominate, mammal-like reptiles appear.

Permian Period

Ends with the Permian-Triassic Mass Extinction ('The Great Dying') → 96% of marine species & 70% of land species wiped out.

Permian-Triassic Mass Extinction

Wiping out 96% of marine species and 70% of land species.

Pangaea

The dominant land form during the Permian period.

Triassic Period

First dinosaurs & mammals appear.

Triassic Period

Pangaea begins to break apart.

Triassic Period

Ended with a mass extinction.

First dinosaurs and mammals

Major groups that appeared in the Triassic period.

Pangaea breaking apart

Major geological event that occurred during the Triassic period.

Jurassic Period

Dinosaurs dominate land.

Jurassic Period

First birds & flying reptiles (pterosaurs) evolve.

Jurassic Period

First flowering plants appear.

Dinosaurs

Dominated the land during the Jurassic period, such as Brachiosaurus and Allosaurus.

First birds and flying reptiles

Major evolutionary development that occurred in the Jurassic period.

First flowering plants

Significant plant evolution that occurred during the Jurassic period.

Cretaceous Period

Dinosaurs continue to dominate.

Cretaceous Period

First modern birds & mammals evolve.

Cretaceous-Paleogene Mass Extinction

Ends with an asteroid impact, leading to the extinction of dinosaurs (except birds).

Cretaceous-Paleogene Mass Extinction

Caused by an asteroid impact, which led to the extinction of dinosaurs (except birds).

First modern birds and mammals

Evolved during the Cretaceous period.

Paleogene & Neogene Periods

Mammals diversify & dominate.

First primates

Evolved during the Paleogene & Neogene periods.

Grasslands

Spread during the Paleogene & Neogene periods.

Key event in the Paleogene & Neogene periods

Mammals began to diversify and dominate the planet.

Major evolutionary step in the Paleogene & Neogene periods

The evolution of the first primates.

Quaternary Period

Ice ages occur.

Quaternary Period

Evolution of Homo sapiens (~300,000 years ago).

Quaternary Period

Modern climate and ecosystems form.

Significant climate event during the Quaternary period

Ice ages, including the most recent one that ended about 10,000 years ago.

Homo sapiens evolution

Approximately 300,000 years ago.

Major evolutionary events during the Quaternary

The rise of humans and the extinction of megafauna like woolly mammoths.

Snowball Earth hypothesis

Earth may have been completely frozen during two events in the Cryogenian period (~720-635 MYA), with life surviving near volcanic hot spots.

Longest geological time period

The Precambrian, which lasted about 4 billion years.

Shortest geological time period

The Quaternary, which spans from 2.6 million years ago to the present day.

Dating ancient rocks and fossils

Using radiometric dating (radioactive decay) and stratigraphy (layering of rocks).