• Fossils are the remains, traces, or imprints of ancient organisms. 

• Fossils can be used in multiple ways to identify how old the rocks around them are.

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How do we organize time?

• The record of Earth’s history from its origin 4.6 billion years ago to the present.

• Broken into smaller pieces based on the fossils contained in rocks.

• You read the record from bottom (oldest) to top (newest)

Geologic Time Scale

Geologic Time is broken down into 4 parts:

- Eon - Largest section of time - the first 3 eons make up 90% of Earth's history.

- Era - Second Largest - each era is hundreds of millions of years.

- Period - Third Largest - tens of millions of years

- Epoch - Smallest - millions of years.

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Breaking Down the Timeline

In Earth's early days, the atmosphere changed a lot. Some of these changes help us to break up geologic time.

​​Atmosphere

​When at least half of ALL the species (plants and animals) on Earth die out. Has happened 5 times.

Mass Extinctions

Events on Earth that cause major change, such as an Ice Age, break up geologic time.

Major Events

​As different families of organisms take over or die out, this creates a break up of geologic time.

​​Organisms

​Breaking it Down by Breaking it Up

Covered in molten rock

The Very Early Earth

​Very Little Water

​No Oxygen

​Lots of Asteroid Impacts

​No Life!!!

​Precambrian Time

• Earth's history between 4.5 billion and 520 million years ago.

• 90% of the history of Earth falls into this time.

• The only life on Earth was microbial (microscopic organisms).

• No animals lived on land because the sun was a deadly laser.

The Cambrian Explosion

• A huge increase in the amount of life on Earth.

• Life is still completely in the oceans and started to move to land.

• The ozone layer forms and protects the Earth from deadly radiation.

• Plant Life begins to flourish on land.

​Scientists use various methods for determining age. The two types of dating scientists use are called Relative Age and Absolute Age

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How do we know how old things are?