Define the four geological eras.

1. Precambrian Era: This is the earliest and longest geological era, spanning from the formation of the Earth around 4.6 billion years ago to the beginning of the Paleozoic Era around 541 million years ago. It is characterized by the formation of the Earth's crust, the development of the atmosphere and oceans, and the emergence of early life forms.

2. Paleozoic Era: Following the Precambrian Era, the Paleozoic Era lasted from around 541 million years ago to 252 million years ago. It is known for the diversification of life forms, including the emergence of fish, amphibians, reptiles, and early plants. Major events during this era include the Cambrian Explosion, the colonization of land by plants and animals, and the formation of the supercontinent Pangaea.

3. Mesozoic Era: The Mesozoic Era, also known as the Age of Dinosaurs, lasted from around 252 million years ago to 66 million years ago. It is characterized by the dominance of dinosaurs on land, as well as the emergence of birds, mammals, and flowering plants. Major events during this era include the breakup of Pangaea, the evolution of birds from dinosaurs, and the mass extinction event that wiped out the dinosaurs at the end of the era.

4. Cenozoic Era: The Cenozoic Era began around 66 million years ago and continues to the present day. It is known as the Age of Mammals, as mammals have become the dominant group of animals on Earth. Major events during this era include the diversification of mammals, the evolution of humans, and the ongoing impact of climate change and human activity on the planet.

1. Precambrian era (4.6 billion years ago to 541 million years ago)

2. Paleozoic era (541 million years ago to 252 million years ago)
- Cambrian period (541 million years ago to 485 million years ago)
- Ordovician period (485 million years ago to 444 million years ago)
- Silurian period (444 million years ago to 419 million years ago)
- Devonian period (419 million years ago to 359 million years ago)
- Carboniferous period (359 million years ago to 299 million years ago)
- Permian period (299 million years ago to 252 million years ago)

3. Mesozoic era (252 million years ago to 66 million years ago)
- Triassic period (252 million years ago to 201 million years ago)
- Jurassic period (201 million years ago to 145 million years ago)
- Cretaceous period (145 million years ago to 66 million years ago)

4. Cenozoic era (66 million years ago to present)
- Paleocene epoch (66 million years ago to 56 million years ago)
- Eocene epoch (56 million years ago to 34 million years ago)
- Oligocene epoch (34 million years ago to 23 million years ago)
- Miocene epoch (23 million years ago to 5.3 million years ago)
- Pliocene epoch (5.3 million years ago to 2.6 million years ago)
- Pleistocene epoch (2.6 million years ago to 11,700 years ago)
- Holocene epoch (11,700 years ago to present)