Geologic Time Scale
When was the Earth formed? When did the first life forms appear? And at what time did the first
multicellular life form? Scientists have developed answers to these very difficult questions
through many years of work. To answer these questions scientists have had to figure out the
age of geologic events by dating rocks, as well as fossils.
The ages of the Earth, and early life, are very big numbers. The age of some life forms
and geologic events go back billions of years. When geologists and paleontologists talk about
these large units of time they refer them to as geologic time. Geologic time consists of large
blocks of time, in hundreds of millions or even billions of years.
In order to divide geologic time into workable segments, geologists have developed
something called a geologic time scale. The geologic time scale breaks down geologic time into
segments based on changes in geologic events or changes in life on Earth. These different
segments can be visually represented on a geologic time line.
The largest segment of time is an Eon. Eons are divided into smaller segments called Eras. Eras
are further subdivided into Periods. And the smallest unit of time on the Geologic Time Scale is
an Epoch.
In this activity, you will fill in some of the significant organisms and events during each period of
the Geologic Time Scale found on the back of this paper. You may use your textbook or go the
Enchanted Learning link found on the Geologic Time Scale page of our Haiku class.
In constructing an evolutionary history, geologists and evolutionary biologists find it important
to know the geologic range of organisms. This is the time when an organism first appears in the
fossil record and when it becomes extinct.
Index fossils are fossils made from organisms that were abundant during certain segments of
geologic time. Often these fossils can provide an estimated date of a layer of rock and are used
to identify a certain period of time.
On the back of this paper, begin by coloring each Eon a different color. Then, color each Era a
different color. Color the periods the same color as the Era they came from.
Then, place the following events in their proper time periods.
 Modern humans appear
 The Earth forms
 First land plants
 First multicelluar
organisms
 First fish

First eukaryotes
 First insects
 Age of fish
 First dinosaurs
 First prokaryote
 First mammals
 Age of reptiles
 Primates appear
 First flowering pants
 Extinction that wiped
out 95% of all species
 First grasses
 Fist reptiles
 First organisms with
hard parts
 Fist birds

First amphibians
 Age of trilobites
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Age of mammals
Extinction that killed the
dinosaurs
Oxygen starts to
accumulate in the
atmosphere
Coal Swamp Forests
Age of Amphibians
Hominids appear
Seaweed appears
First winged insects
Dogs appear