Eras are described by the kind of life that existed during that time. We're learning about 4 eras:

Precambrian- Few fossils exist from this time, but life began in this era.

Paleozoic- Life became abundant, including fish, insects, amphibians, reptiles, and trilobites. Many organisms went extinct at the end of the Paleozoic era.

Mesozoic - During this era, these organisms appeared: dinosaurs, mammals, birds, and flowering plants. Rocks reveal that many life forms went extinct.

Cenozoic- This is our current era. Fossils show some of our early mammals very large, like the woolly mammoth. The most recent ice age occurred in this era.

(All science is theory, ever-changing due to increase in knowledge through the scientific process. )

Here's a really cool overview of the 4 eras.

More amazingly, the soft parts may even be preserved. They've been found frozen or in tar pits.

This video shows a mutation in the color of mice, and how natural selection has occurred over 1,000 years due to a volcanic change in habitat. These scientists use this evidence to support the theory of evolution. What do you think? Do you reach the same conclusion they did with the evidence?