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.

• Scientists havedeveloped a model of the history

of life on Earth called thegeologic 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 occurredslowly, over millions of years.• But some times, Earth’s history has been affected by catastrophes such asmassive volcanic eruptions or meteorhitting• These events had a HUGE effect inshaping Earth’s surface and on theevolution of life.

Divisions of the Geologic Time Scale

• Paleontologists divide the geologic timescale 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 TIME4.6BYA-544MYA

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 soft-

bodied)

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 usingPhotosynthesis)

PALEOZOIC ERA(ANCIENT LIFE)

544-245MYA

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 and70% all life on land)

Mesozoic EraMiddle 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 giantmeteor hit Earth creating a humongous dust

cloud and worldwide fires.

Cenozoic Era(Recent Life)

65mya-today

Cenozoic Era65mya-today

• Fossils from the Cenozoic era are closest toEarth’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 EraAge 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)