Birth of the universe, solar system and Earth
Scientists think the universe began about 13.8 billion years ago in a very hot, tiny state that expanded. We call this the Big Bang. Stars and galaxies formed later from gas.
About 4.6 billion years ago, a cloud of gas and dust in our galaxy began to collapse and spin. The middle became hot and dense and turned into the Sun. The rest flattened into a disk. Dust stuck to dust and made bigger lumps called planetesimals. Lumps joined to form the planets. This is how Earth formed about 4.54 billion years ago.
Young Earth was very hot. Heavy iron sank to the middle (the core) and lighter rock floated up (the mantle and crust). A giant crash is thought to have made the Moon. As Earth cooled, water vapour turned to rain and the oceans formed, roughly 4.4 to 4 billion years ago.
Fossils and past environments
A fossil is the trace or remains of an ancient living thing kept in rock: a shell, a bone, a leaf print or a footprint.
- Order of layers: sediment piles up flat, so the bottom layer is oldest and the top is youngest. This is relative age.
- Index fossils: animals that lived a short time but spread wide help match layers in far places.
- Clues to the past: sea shells on a mountain mean the land was once under the sea. Coal means a warm swamp forest. Sand grains from wind can mean a dry or cold land.
- Real age: some rocks have unstable atoms that decay at a fixed rate (their half-life). Measuring what is left tells age in years. This is radiometric dating.
From such clues, scientists know Earth went through warm times, ice ages, and several mass extinctions, when many species died out in a short time.
Earth's history in eras
Geologists divide the 4.5 billion years into big blocks called the geologic time scale.
- Precambrian (4.54 billion to 539 million years ago): about 88% of all time. Oceans, first life, oxygen. Mostly tiny life.
- Palaeozoic (539 to 252 million): animals with shells and bones, fish, land plants, first land animals. It ends with the biggest mass extinction.
- Mesozoic (252 to 66 million): age of dinosaurs. It ends with a space rock hitting Earth.
- Cenozoic (66 million to today): mammals spread. Humans (Homo sapiens) appear only about 300,000 years ago.
24-hour clock: if all of Earth's history were one day, animals with shells arrive about 9 pm, dinosaurs about 10:45 pm, and humans in the last 6 seconds.
Why Earth's past matters today
Studying old climates shows how fast and how much Earth can change. Ice cores and rock layers tell us that carbon dioxide and temperature have moved together before. Past extinctions warn us that life can be lost quickly. Coal, oil and gas are made from ancient life, so they are limited.
Key formulas and definitions
- Age of Earth ≈ 4.54 billion years
- Universe ≈ 13.8 billion years old; Sun and solar system ≈ 4.6 billion years
- On a 24-hour clock: 1 hour ≈ 190 million years
- Clock time (hours) = 24 × (1 − years ago ÷ 4540 million)
- Half-life: after one half-life ½ of the parent atoms remain; after two, ¼
- Law of superposition: lower layer = older layer
Worked examples
1. Dinosaurs died out 66 million years ago. How many minutes before midnight is that on the 24-hour clock?
One hour ≈ 190 million years. 66 ÷ 190 ≈ 0.35 hour. 0.35 × 60 ≈ 21 minutes before midnight, about 11:39 pm.
2. A fossil sea shell is found on a mountain. What does it tell us?
That place was under the sea long ago. Later, moving land lifted it up and it became a mountain.
3. In a rock cutting layer A is at the bottom and layer C at the top. Which is older, and which fossils help date a far-away rock?
Layer A is older (law of superposition). Index fossils, which lived a short time but spread wide, in A and in the far rock show that the two layers formed at the same time.
Common mistakes
- Thinking humans and dinosaurs lived together. Dinosaurs ended about 66 million years ago; modern humans are only about 300,000 years old.
- Thinking the top layer is the oldest. In normal layers the bottom is the oldest.
- Mixing the Big Bang (13.8 billion years) with the birth of the Earth (4.54 billion years).
- Thinking Earth was always like today. It was once hot rock, then a world with no oxygen.