1. Scientists have
developed a model of the history of life on Earth called the
geologic time scale.
The geological time scale is based on studies of Earth’s geology and the fossil record.

2. Earth’s Geologic Time Scale
• Most of Earth’s changes have occurred slowly, over millions of years. • But some times, Earth’s history has been affected by catastrophes such as massive volcanic eruptions or meteor hitting • These events had a HUGE effect in shaping Earth’s surface and on the evolution of life.

3. Divisions of the Geologic Time Scale
Paleontologists divide the geologic time
scale into different blocks of time.
Largest blocks of time or eons.
Eons are then divided down into smaller blocks of time called eras & periods.
The smallest blocks of time are called epochs.
Eons Eras Periods Epochs

4. Eons Eras Periods Epochs

5. Precambrian time
4.6bya-544mya

6. Precambrian Time
Precambrian Time makes up 88% of Earth’s history, but little is known.
Earth formed 4.6 bya
At first oxygen was absent from the atmosphere.
1st life forms were single celled, like cyanobacteria.
Oceans formed
1st invertebrates (Most life forms were soft-bodied)

7. Precambrian Time
Meteorites crashed on Earth and radiation from the sun bombarded our planet.
Thunderstorms and volcanic
eruptions were nonstop.
*After photosynthesis
started, oxygen becomes a
major gas in Earth’s
atmosphere.
(bacteria, not plants started using
Photosynthesis)

8. Paleozoic Era
(ancient life)
mya

9. Paleozoic Era (ancient life)
Lots of organisms with hard parts, like shells
Warm, shallow seas covered most of Earth
Lots of marine (sea) organisms
1st vertebrates appear
1st land organisms appear
Many mountains formed (Appalachians) & rocks of our continents form!
Pangaea forms
Mass extinction (90% sea life and
70% all life on land)

10. Mesozoic Era
Middle Life
245-65mya

11. Mesozoic Era (middle life)
Also known as the Age of Reptiles or Dinosaurs
Dinosaurs are the most well-known reptiles of the Mesozoic era and dominated Earth for about 150 million
Tectonic plates were moving
VERY fast during this era.
Pangaea breaks apart.

12. Mesozoic Era (Age of the Reptiles)
1st birds
1st mammals (small, mouselike animals)
1st flowering plants
At the end of the Mesozoic era, 65 mya,
geologic evidence indicates that a giant
meteor hit Earth creating a humongous dust cloud and worldwide fires.

13. Cenozoic Era
(Recent Life)
65mya-today

14. Cenozoic Era 65mya-today
Fossils from the Cenozoic era are closest to
Earth’s surface, making them easier to find.
• Therefore, scientists have the most fossil evidence from this era
Earth’s climate cooled& Ice Ages happened!
Mammals evolved adaptations that allowed them to live in many different environments (on land, in water, & in the air)

15. Cenozoic Era Age of the Mammals
Alps, Andes, & Himalayan Mountains formed
Fossil record suggests that human ancestors first appeared about 4mya (Homo sapiens OR modern humans about 140,000 years ago)