With radiometric dates on the time line that began 4.54 billion years ago, we can place the great events of Earth's history. Most of the story happened before animals with skeletons even existed.
| About this long ago | Event |
|---|---|
| 4,540 million years | Earth forms |
| 4,400 million years | oldest known mineral grains (zircons): early crust and probably liquid water |
| 4,000 million years | oldest known rocks |
| 3,500 million years | oldest widely accepted fossils: stromatolites built by microbes |
| 2,400 million years | oxygen builds up in the air (the Great Oxidation Event) |
| 539 million years | the Cambrian explosion: animals with shells and skeletons spread |
| 445, 372, 252, 201, 66 million years | the five great mass extinctions; the largest at 252, the end of the dinosaurs at 66 |
| 0.3 million years | our species appears |
Why it works
How do we know the air once lacked oxygen? Rocks older than about 2.4 billion years contain river-worn grains of pyrite and uraninite, minerals that oxygen would quickly destroy; younger rocks contain red beds stained by iron oxide. Photosynthesizing microbes (cyanobacteria) slowly released the oxygen, changing the planet's chemistry. Mass extinctions show up as sudden losses of many fossil species at once, across the world, at the same dated layers.
Key idea
Earth's air had almost no oxygen for its first two billion years; oxygen built up about 2.4 billion years ago. Animals with skeletons appeared about 539 million years ago, and five mass extinctions punctuate the record since.
Worked examples
Worked example: Before the animals
What fraction of Earth's history passed before the Cambrian explosion?
- $4,540 - 539 = 4,001$ million years passed first.
- $4,001 \div 4,540 \approx 0.88$: about 88% of Earth's history.
Answer: About 88%.
Worked example: On a 24-hour clock
If Earth's history were one day, how many minutes before midnight did the dinosaurs die out (66 million years ago)?
- Fraction still to come: $66 \div 4,540 = 0.0145$.
- $0.0145 \times 1,440 \approx 21$ minutes before midnight.
Answer: About 21 minutes before midnight.
Common mistake
It is easy to assume the air always had plenty of oxygen. For its first two billion years Earth's air had almost none; animals like us could not have breathed it. Check the rocks: grains of pyrite in ancient river sands could not survive in today's air.