The Geologic Time Scale

If we condensed the Geologic Time Scale to One Minute when would humans appear?

• Origin of Life in One Minute

Geologic time is divided into:

EONS

ERAS

PERIODS

EPOCHS

WHY DO WE HAVE DIFFERENT NAMES FOR EONS AND ERAS?

Image courtesy of the UC Museum of Paleontology Understanding Evolution: http://evolution.berkeley.edu."

Precambrian Time• 88% of the Geologic Time Scale• 4.6 bya – 540 mya • Almost 4 Billion Years!

The Precambrian is like a book with missing pages.

1. Precambrian rocks are buried under newer layers of rocks (Law of Superposition)

No fossils to identify rocks from this time b/c they have been metamorphosed, eroded, or hidden by overlying rock by now.

There are some areas like the Canadian shield where these rocks are exposed and can be studied

What DO we know about the Precambrian

1. When Earth first formed it was hot and made of lava…it took time for the lava on the surface to cool and harden into rock2. The atmosphere was made of water vapor, carbon dioxide (CO2), and Nitrogen (N)

Earth’s Atmosphere Today:

What DO we know about the Precambrian

3. Torrential rains helped cool the Earth and the low areas filled with water ….this is how we got our oceans4. The first signs of life evolved in the oceans since there was oxygen in water, but none in the air

Hydrogen

• One of the first organisms on Earth to evolve (in the oceans) were a type of prokaryote called cyanobacteria

Precambrian Fossils• During the Precambrian,

there are not many fossils because not much evolution yet

• Most common fossil from cyanobacteria…they made ocean formations called stromatolites– These date

cyanobacteria back to ~ 3.5 bya

Precambrian Fossils

• Other things that happened during the Precambrian include:– Ocean plants evolved – Ocean eukaryotes (organisms with a nucleus)

evolved

***Note: ALL fossils found from the Precambrian are from ancient seafloor since no plants or animals

has yet to evolve onto land***