1. History of Life On Earth
McDougall Littell

2. The History Of Life
Fossils provide evidence about the history of life on Earth.
Most fossils are hard body parts or bone. Others form when minerals replace the remains of organisms.
Fossils also include prints left by organisms, such as footprints.
Very rarely, a fossil is the preserved remains of an organism, such as a wooly mammoth in ice or an insect preserved in sap.

6. The History of Life
Relative age is how old a rock or fossil is compared to other rocks or fossils.
Ancient organisms settled to the bottom of a body of water in layers of mud and sand that later formed rock.
A fossil found in a deeper layer of rock is usually older than a fossil found in a layer of rock near the top.

7. The History of Life
Absolute age is the actual age of the rock or fossil.
To measure the absolute age of a fossil, scientists measure its radioactivity.
Over time, radioactive material disappears.
The older the fossil, the less radioactive material it has.

8. The History of Life
The fossil record is all of the information that can be gathered from the fossils in a particular location.
Scientists can then use the record to identify which species lived and died during different periods of time.

9. The History of Life
Scientists can use technology to dig deep into the ground to recover fossils and other information to reconstruct the ancient past.

10. More Complex Organisms Developed Over the Course of Earth's History.
Unicellular organisms, made of only one cell, were the first living things on Earth.
They appeared about 3.8 billion years ago.
The atmosphere then did not have as much oxygen as it does now.
Some of the early organisms added oxygen to the atmosphere.

11. More complex organisms developed over the course of Earth's history.
Multicellular organisms began to live in Earth's oceans about 1.2 billion years ago.
Multicellular organisms have many cells that perform different tasks within the same body.
The fossil record shows that the earliest multicellular organisms were tiny seaweeds.
The earliest animals were similar to jellyfish.

12. The History of Life
Life appeared on land about 500 million years ago. Before that, all living things met their needs while living in water.
Simple plants were the first land-dwelling organisms, getting water from the soil through roots.
The plants in turn provided food and shelter to the first fungi and insects on land.
After insects, amphibians and reptiles began to live on land, followed by birds and mammals.

13. The History of Life
A species becomes extinct when all members of the species have died.
Many species have only been seen as fossils because they became extinct long ago.
A mass extinction is a period when a large number of species become extinct in a very short time.

15. The History of Life
The Permian Extinction occurred about 250 million years ago.
Approximately 90 percent of the species in the ocean, as well as many land-dwelling animals, became extinct.
This mass extinction may have been caused by climate change due to all of Earth's landmasses joining together to form a single, enormous continent.

17. The History of Life
The Cretaceous Extinction occurred about 65 million years ago.
More than half of all the species on Earth, including dinosaurs, became extinct.
This mass extinction may have been caused by a meteorite, a giant space object, colliding with Earth.

18. The meteorite left a
200 km-wide crater
off the Yucatán
peninsula in Mexico

19. The History of Life
Mass extinctions are often followed by the appearance of a large number of new species.
For example, the extinction of dinosaurs may have allowed new species of mammals to develop and thrive.