1. The Fossil Record (Ch 19.1)
Unit 5: Evolution

2. Fossils and Ancient Life
Fossils tell us the story of extinct species (species that have died out)
May organisms have died without leaving a trace which leaves the fossil record incomplete
Fossil range in terms of size, type, and type of preservation
Fossils in Sedimentary Rock
Sediments (sand, silt, or clay) build up over a dead organism that has sunk to the bottom of a body of water
Over time, the layers of sediment are pressed together by the water pressure preserving the organism
What Fossils Reveal
Paleontologists (researchers who study fossils) learn about the structures of organisms, their environment, and the ways in which they lived
They can determine evolutionary relationships among organisms based off the fossil record

3. Dating Earth’s History
Relative Dating
Places rock layers and their fossils in temporal sequence – allows paleontologists to determine whether a fossil is older or younger than other fossils
Index fossils are distinctive fossils that are used to establish and compare the relative ages of rock layers and the fossils they contain
Easily recognized and will occur only in a few rock layers
Example: Trilobites
Radiometric Dating
Relies on radioactive isotopes which decay or break down into stable isotopes to tell the absolute age of fossils
Half-life: time required for half of the radioactive atoms in a sample to decay
Different radioactive isotopes have different half-lives – elements with short half- lives are used to date younger fossils and elements with longer half-lives are used to date older fossils
Carbon-14  used for younger fossils
Potassium-40 / Uranium-238  used for older fossils

4. Geological Time Scale EONS ERAS
The geological time scale is based on both relative and absolute dating. The major divisions of the geological time scale are eons, eras, and periods.
Major changes in the rock layers signified the divisions in the time scale
Radiometric dating was used to verify specific time frames (radioactive isotopes)
Divisions of the Geological Time Scale
Each division has different lengths
Eons are divided into eras which are then divided into periods g
EONS
ERAS
Hadean Eon: the first rocks were formed
Archean Eon: first life appeared
Proterozoic Eon: 2.5 billion years ago – 542 million years ago
Phanerozoic Eon: 542 million years ago – present
Paleozoic
Mesozoic -- Dinosaurs
Cenozoic

5. Geological Time Scale

6. Life on a Changing Planet
Physical Forces
climate  shifts in global temperature will affect living things
Example: wind currents / ocean currents
geological forces  building mountain ranges and moving continents (plate tectonics)
Building mountains, opening coastlines, changing climates, and geological forces have altered habitats of living organisms repeatedly throughout Earth’s history.
Biological Forces
The actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth.
Example: iron deposits and photosynthetic bacteria causes the increase in oxygen in the Earth’s atmosphere