Earth formed about 4.6 billion years ago (4.54 billion, more exactly). To organize that much time, geologists divide it into eons, eras and periods. The boundaries mostly mark big changes in the life seen in fossils, often mass extinctions, and radioactive dating of volcanic layers gives them ages in years. Relative dating puts the events in order.
| Time span | Began about | Life |
|---|---|---|
| Precambrian (Hadean, Archean and Proterozoic eons) | 4,540 million years ago | Single cells for most of it; the first simple many-celled life near its end |
| Paleozoic era | 539 million years ago | Animals with shells; fish; the first land plants, insects and four-legged animals |
| Mesozoic era | 252 million years ago | Dinosaurs, the first mammals and birds, flowering plants |
| Cenozoic era | 66 million years ago | Mammals spread and diversify; humans appear about 300,000 years ago |
Key idea
Earth's 4.6-billion-year history is divided into eons, eras and periods by big changes in life. Most of that time, the Precambrian, came before animals with hard parts; humans arrived only in the last sliver.
Why it works
Deep time is too long to picture directly, so squeeze it. If all of Earth's history were one calendar year, the Precambrian would last until mid-November, the dinosaurs would vanish on about December 26, and humans would show up around 11:25 p.m. on December 31. The Precambrian is about 88% of the whole story: $(4{,}540 - 539) \div 4{,}540 \approx 0.88$.
Worked example: Which era?
A fossil of a plant-eating dinosaur is found in rock about 150 million years old. Which era is that?
- The Mesozoic runs from 252 to 66 million years ago.
- 150 million years is between them.
Answer: The Mesozoic era
Worked example: On the calendar
If Earth's 4.6-billion-year history were one year (365 days), how many days before the end of the year did the dinosaurs (other than birds) die out, 66 million years ago?
- Fraction of history since then: $66 \div 4{,}600 \approx 0.0143$.
- Days: $0.0143 \times 365 \approx 5.2$ days before the end, about December 26.
Answer: About 5 days (December 26)
Common mistake
Thinking people lived alongside dinosaurs. The last dinosaurs other than birds died out about 66 million years ago, and our species appeared about 300,000 years ago, more than 65 million years later. Check the time scale: dinosaurs are Mesozoic; humans are the last moment of the Cenozoic.