L4 The History of Life on Earth

Paleontologist look for clues to understand what happened in the past.

A paleontologist is a scientist who studies the fossilized remains of all kinds of organisms.

Fossils of single-celled organisms date as far back as 3.8 billion years.

A fossil is a trace or imprint of a living thing that is preserved by geological processes.

When an organism is trapped in mud or sediment, the resulting fossil becomes part of that sedimentary layer of rock.

How life forms have changed over time

when extinction occurred

All the fossils that have been discovered worldwide make up the fossil record. The fossil record gives evidence of many of the different organisms that have lived during Earth's long history.

Each fossil gives information about a single organism.

But overall, it helps us to understand larger patterns of change.

Relative dating - determines whether a fossil was formed before or after another fossil

Absolute dating - estimate the age of a fossil in years

Relative Dating : Organism A lived more recently than organism B. OR Organism A is younger than Organism B.

Absolute Dating : Organism A lived between 75 and 130 million years ago.

Over many generations, populations change.

Fossils show the gradual change in limb structure.

Other species have survived for a long time without much change.

An extinction happens when every individual of a species dies.

A mass extinction happens when a large number of species go extinct during a relatively short amount of time.

Gradual environmental changes and catastrophic events can cause mass extinctions.

To keep track of Earth's long history, scientists have developed the geologic time scale.

The geologic time scale is the standard method used to divide Earth's long 4.6-billion-year natural history into manageable parts.

A tool to Earth's history.

Time intervals correspond to significant changes in Earth's history.

Some are defined by mass extinctions or significant changes in the number of species

Others are defined by major changes in the Earth's surface or climate.

The largest divisions of the geologic time scale are eons.

Eons are divided into eras.

Eras are characterized by the type of organism that dominated the Earth at the time.

Each era began with a change in the type of organism that was most dominant.

Eras are further divided into periods.

Periods are divided into epochs.