• 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.
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.
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
Eons
Eras
Periods
Epochs
Precambrian Time
• Precambrian Time makes up 88% of Earth’s
history, but little is known.
1. 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 softbodied)
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)
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)
Mesozoic Era
Middle Life
245-65mya
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.
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.
Cenozoic Era
(Recent Life)
65mya-today
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)
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)