The Geological Timescale (HL) (DP IB Environmental Systems & Societies (ESS))

The Geological Timescale

• The geological timescale provides a framework for understanding Earth's 4.5 billion-year history

• It helps scientists study how life has evolved in response to changing environments over long periods

  • The timescale is essential for understanding the history of the planet and the organisms that have lived on it

Role of fossils in evolution

• Fossils are the preserved remains or traces of ancient organisms

• They are important sources of evidence for understanding how life has changed over time

• By studying fossils, scientists can track the evolution of species and identify when different groups first appeared or went extinct

  • For example, the transition from fish to amphibians can be traced through fossil evidence, showing how certain traits evolved to adapt to land environments

Structure of the geological timescale

• The geological timescale is divided into eons, eras, periods, and epochs

• Each level represents significant stages in Earth's geology and life and changes in these time frames are marked by major geological and biological events

Eons

• There are four main eons in Earth’s history:

  • Hadean Eon (4.5 to 4 billion years ago): The formation of Earth, with a molten surface and no life

  • Archean Eon (4 to 2.5 billion years ago): The appearance of the first simple life forms, such as bacteria

  • Proterozoic Eon (2.5 billion to 541 million years ago): Development of more complex life, including multicellular organisms

  • Phanerozoic Eon (541 million years ago to present): Marked by an abundance of fossils and the rapid evolution of life forms

Eras

• The Phanerozoic Eon is divided into three eras:

  • Paleozoic Era (541 to 252 million years ago): Development of diverse marine life and the colonisation of land by plants and animals

  • The Mesozoic Era (252–66 million years ago), also known as the age of reptiles, which included dinosaurs, was notable for its significant changes in flora and fauna

  • Cenozoic Era (66 million years ago to present): The age of mammals and birds, leading to the rise of humans

Periods and Epochs

• Significant events further divide each era into periods and epochs:

  • For example, the Cretaceous Period (145 to 66 million years ago) ended with the mass extinction of dinosaurs

  • The Pleistocene Epoch (2.6 million to 11,700 years ago) saw the rise of ice ages and the evolution of modern humans

Major geological and biological events

• Changes in the geological timescale are often linked to major events

  • Mass extinctions: These events, such as the Permian-Triassic extinction (around 252 million years ago), greatly reduced biodiversity and allowed new species to evolve

  • Environmental changes: Shifts in climate, land formation, and ocean currents influenced how life adapted and evolved