Relate Cause and Effect Why have so few organisms become fossilized Draw Conclusions Many more fossils have been found since Darwin’s day, closing many fossil gaps. How would this information make relative dating more accurate Review Describe three processes that have affected life on Earth

Ch 19 History of Life 19.1 The Fossil Record

Paleontologists Researchers who study fossils to learn about ancient life.

Fossils vary enormously in size, type, and degree of preservation Form only under certain conditions.

Types of Fossils Fossils can be as large and perfectly preserved as an entire animal With hair, skin, scales, or feathers Be as tiny as bacteria, developing embryos, or pollen grains Many are just fragments Trace fossils Casts of footprints, burrows, tracks, or even droppings.

Fossils in Sedimentary Rock Most common Sedimentary rock usually forms when small particles of sand, silt, or clay settles to the bottom of a body of water Covers dead animals as it builds up.

Layers of sediment continue to build up over time Remains are buried deeper and deeper Pressure gradually compresses sediment into rock.

Preserved remains may later be discovered and studied.

Usually soft body structures decay quickly after death Usually only hard parts like wood, shells, bones, or teeth remain Hard structures can be preserved if they are saturated or replaced with minerals.

Soft tissues can be preserved from aerobic decay if buried quickly.

Fossil Information Bone structure and trace fossils, like footprints, indicate how animals moved Fossilized plant leaves and pollen suggest whether the area was a swamp, a lake, a forest, or a desert Comparing body structures in fossils to body structures in living organisms offer clues about how body structures and species have evolved.

Relative Dating Places rock layers and their fossils into a temporal sequence Lower layers of sedimentary rock and fossils are generally older than upper layers.

Index Fossils Distinctive fossils used to establish and compare the relative ages of rock layers and the fossils they contain Easily recognized and will occur in only a few rock layers.

Radiometric Dating Uses radioactive isotopes, which decay, or break down, into nonradioactive isotopes at a steady rate Compares the amount of radioactive to nonreactive isotopes in a sample to determine its age.

Half Life Time required for half of the radioactive atoms in a sample to decay The half-life of potassium-40 is 1.26 billion years.

Carbon-14 Radioactive form of carbon naturally found in the atmosphere that is taken up by living organisms along with “regular” carbon Used to date material that was once alive Half-life of only about 5730 years useful for dating fossils no older than about 60,000 years.

Fossils older than 60,000 years Can’t use Carbon 14 Use other elements to date surrounding rock.

Geologic Time Scale Based on both relative and absolute dating Major divisions of the geologic time scale are eons, eras, and periods.

Changing Planet Building mountains, opening coastlines, changing climates, and geological forces have altered habitats Actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth.

Physical Forces “Heat wave” of the Mesozoic Era, average temperatures were only 6°C to 12°C higher than 20th century Ice ages had average temperatures only about 5°C cooler than 20th century.

Plate Tectonics Theory that explains how solid continental “plates” move slowly above Earth’s molten core (continental drift).

Biological Forces Proterozoic Eon photosynthetic organisms produced oxygen gas and also removed large amounts of carbon dioxide from the atmosphere Provided oxygen for aerobic processes Removed carbon dioxide which cooled the planet.