Overview
This lecture gives a timeline of Earth's history, highlighting major planetary events, the emergence of life, mass extinctions, and human evolution up to the present day.
Formation of Earth and the Solar System
- The solar system formed about 4.6 billion years ago from a collapsing cloud of gas and dust.
- The birth of the Sun triggered the accumulation of matter into planets, with rocky planets forming closer to the Sun.
- Early Earth was volcanically active and had a primitive atmosphere of hydrogen sulfide, methane, and carbon dioxide.
- A Mars-sized planet named Theia collided with Earth, forming the Moon.
Early Earth and the Origin of Life
- Initially, Earth had no oceans, but around 3.8 billion years ago, it cooled enough for water to condense into oceans.
- The first microscopic life appeared about 3.7 billion years ago.
Development of Continents and Atmosphere
- The earliest continents, or "katons," formed as land rose from the ocean.
- Around 2.4 billion years ago, cyanobacteria began photosynthesis, producing oxygen and changing Earth’s atmosphere.
- Increased oxygen led to Earth's first ice age.
Supercontinents and Changing Life Forms
- Continents repeatedly merged and broke apart, forming supercontinents like Vaalbara, Rodinia, and Pangaea.
- The Cambrian explosion (540–485 million years ago) saw a rapid diversification of animal life with hard body parts.
- Major mass extinctions, such as the Ordovician-Silurian and the Permian "Great Dying," wiped out large amounts of life.
Rise of Land Life and Dinosaurs
- Trees and the first land animals appeared between 420–350 million years ago.
- Dinosaurs arose around 240–230 million years ago and dominated until their extinction 66 million years ago due to an asteroid impact.
Evolution of Humans
- Early human ancestors appeared around 6 million years ago, with upright walking beginning 4 million years ago.
- Use of tools developed about 3 million years ago; controlled use of fire emerged 800,000 years ago.
- Homo sapiens became the only surviving human species between 40,000 and 15,000 years ago.
Recent Human History and Modern Challenges
- The Agricultural Revolution began 10,000 years ago with the rise of farming societies.
- The Industrial Revolution, about 250 years ago, transitioned society from rural and agricultural to urban and industrial.
- Human population grew rapidly, surpassing 8 billion today.
- Climate change now threatens biodiversity and could lead to another mass extinction.
Key Terms & Definitions
- Supercontinent — A landmass composed of most or all of Earth’s continents joined together.
- Cambrian Explosion — A period of rapid diversification of life with the appearance of many major animal groups.
- Mass Extinction — A widespread and rapid decrease in the amount of life on Earth.
- Cyanobacteria — Microorganisms that produce oxygen through photosynthesis.
- Industrial Revolution — The transition to new manufacturing processes and urbanization, starting in the 18th century.
Action Items / Next Steps
- Review key mass extinction events and their causes.
- Study the timeline of major evolutionary milestones.
- Prepare to discuss current challenges facing Earth, especially climate change.