Earth was formed about 4.6 billion years ago. Earth's long history is divided into units called geologic time, recording the evolution of life and environmental changes.
Basic Knowledge
Geologic time is broadly divided as follows.
① Precambrian Time (4.6–0.54 billion years ago): Formation of Earth, oceans, and atmosphere; emergence of life; production of oxygen.
② Paleozoic Era (0.54–0.25 billion years ago): Cambrian explosion; fish, amphibians, ferns, and trilobites. It ended with the mass extinction of the Permian Period.
③ Mesozoic Era (0.25–0.066 billion years ago): Triassic, Jurassic, and Cretaceous periods. The age of dinosaurs and ammonites. Mammals also appeared.
④ Cenozoic Era (66 million years ago–present): Flourishing mammals, expanding grasslands, and human evolution. The last period, the Quaternary (2.58 million years ago–present), was an age in which glacial and interglacial periods alternated.
Absolute ages are measured by radiometric dating (using uranium, potassium, carbon-14, and other elements).
Precambrian Time（4.6–0.54 billion years ago. The time of life's emergence and oxygen production）
Paleozoic Era（0.54–0.25 billion years ago. The appearance of marine invertebrates and vertebrates）
Mesozoic Era（0.25–0.066 billion years ago. Dinosaurs and ammonites flourished）
Cenozoic Era（66 million years ago–present. The age of mammals and humans）
Quaternary（2.58 million years ago–present. Repeated glacial and interglacial periods）
Radiometric dating（A method of determining absolute age using the half-lives of radioactive elements）
Further Study (Background and Significance)
If Earth's history is compressed into one year, Earth's birth would be January 1, the emergence of life around March, oxygen production (the Great Oxidation Event) around June or July, the Cambrian explosion in mid-November, dinosaur extinction on December 26, and the emergence of humans around 9 p.m. on December 31. Recorded history would occupy only the last few seconds.
In radiometric dating, elements with different half-lives are selected for different purposes. Carbon-14 (half-life 5730 years) is used for dates up to tens of thousands of years ago, potassium-argon (1.25 billion years) for tens of millions to billions of years, and uranium-lead (4.5 billion years) for dating the oldest rocks.
Because humans have greatly changed Earth's environment in recent years, some have proposed calling the present the “Anthropocene”. This gives us an opportunity to consider Earth's history and human responsibility.
- Earth formed = 4.6 billion years ago
- Precambrian Time = 4.6–0.54 billion years ago (about 88% of Earth's history)
- Paleozoic Era = 0.54–0.25 billion years ago (trilobites・fish)
- Mesozoic Era = 0.25–0.066 billion years ago (dinosaurs・ammonites)
- Cenozoic Era = 66 million years ago–present (mammals・humans)
- Quaternary = 2.58 million years ago–present (ice age)
- Carbon-14 method = scale of tens of thousands of years
Points to Note (Easily Confused)
① The boundaries between the Paleozoic, Mesozoic, and Cenozoic eras are marked by mass extinction events (at the ends of the Paleozoic and Mesozoic). ② The Mesozoic periods are ordered Triassic → Jurassic → Cretaceous. ③ The Quaternary is divided into the Pleistocene and Holocene; the Holocene began about 10,000 years ago. ④ Because radiometric dating uses half-life, an element suitable for the target age must be selected.
Practice
- Approximately how many years ago did Earth form?
- In which era did dinosaurs flourish?
- Approximately how many years is the half-life of carbon-14?