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 rock
2. 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
oceans
4. 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***