17.1 The Fossil RecordFossils and Ancient Life•Paleontologists – scientists who collect and study fossils.

•Fossil – any evidence of an organism that lived long ago

•Fossil record

• Provides evidence about the history of life on Earth

• Includes information about structure of organisms, what they ate, what ate them, and what environment they lived in

• Shows how species, have changed over time.

• Provides incomplete information about the history of life.

•Over 99% of all species that have lived on Earth have become extinct.

How Fossils Form•Most fossils form in sedimentary rock.

•Sedimentary rock forms when rock breaks down into small particles of sand, silt, and clay.

•Fossil formation

• Water carries small rock particles to lakes and seas.

• Dead organisms are buried by layers of sediment, which forms new rock.

• The preserved remains may be later discovered and studied.

Fossil FormationActive Art

Interpreting Fossil Evidence

•Relative dating

• If the rock layers have not been disturbed, the layers at the surface must be younger than the deeper layers

• You cannot determine the actual age of a fossil by using relative dating

• Radioactive Dating

• Use radioactive isotopes in rocks or fossils to determine the exact age

• Radioactive isotopes break down or decay over time, giving off radiation at a specific rate of decay

• Half-life – the length of time required for half of the radioactive atoms in a sample to decay.

• Scientists calculate the age of a fossil based on the amount of remaining radioactive isotopes it contains.

• Two Common Radioactive Isotopes:

• Potassium-40 (half-life = 1.3 billion years)

• Carbon-14 (half-life = 5730 years)

• Carbon-14 begins to decay when an organism dies.

Example: Graph – Radioactive Decay of Potassium-40

Geologic Time Scale

•Geologic Time Scale – a type of calendar that allows scientists to communicate about events that have occurred since Earth was formed

•An era is the largest division in the time scale and each era is subdivided into periods

•The divisions in the time scale are distinguished by the organisms that lived during the time period and the fossil record indicates that there were several occurrences of mass extinction that fall between time divisions

Four Eras

1.Precambrian Era

• Oldest evidence of life on Earth

• 3.5 billion years ago

• Fossils resemble present day species of bacteria called cyanobacteria

• Life existed only in the sea

• 88% of Earth history

2. Paleozoic Era

• Began about 544 million years ago

• Appearance of invertebrates including worms and arthropods

• Ferns and gymnosperms

• Amphibians and reptiles appeared

3. Mesozoic Era

• Began about 245 million years ago

• Divided into 3 periods

• Triassic – mammals and dinosaurs

• Jurassic – lots of dinosaurs & birds appear towards the end

• Cretaceous – flowering plants

4. Cenozoic Era

• Began about 65 million years ago and continues to present

• Mammals began to flourish

• Primates

17-1

BIO-QUIZ

17-11. Which of the following statements about fossils is NOT

true?

a.Most fossils form in sedimentary rock.

b.Fossils occur in a particular order.

c. Only a small portion of fossils are from extinct organisms.

d.Fossils can be used in relative dating of rock formations.

17-12. The fossil record consistently shows evidence that

a.all forms of life have existed in all geologic eras.

b. living organisms have only been on Earth for a short time.

c. living things have changed over time.

d.ancient life-forms are much the same as forms found living today.

17-1

4. Determining the age of a fossil by comparing its placement with fossils in other layers of rock is called

a.carbon-14 dating.

b. fossil-indexing.

c. relative dating.

d.absolute dating.

17-1

5. According to the geologic time scale, geologic time begins with

a.Precambrian Time.

b. the Paleozoic Era.

c. the Quaternary Period.

d. the Cambrian Era.

17.2 Earth’s Early History

• Formation of Earth

• Earth is thought to have formed about 4.6 billion years ago

• Pieces of cosmic debris were attracted to one another over the course of 100 million years.

• The Earth was very hot and volcanoes frequently spewed lava and gases

• Earth’s ancient atmosphere contained hydrogen, methane, ammonium and water vapor

• 4 billion years ago Earth cooled and water in atmosphere to condense

• This led to millions of years of rainstorms

• 3.5 billion years ago the first living organisms appeared in the ocean

• How Did Cells Form?

• The first life forms evolved without oxygen

• Energy from the sun and from lightning triggered chemical reactions to produce simple organic compounds

• This formed a “primordial soup”

• Simple organic compounds link together to form small proteins

• The first life forms were probably heterotrophic prokaryotes that were anaerobic

• Competition for nutrients by heterotrophic prokaryotes led to the evolution of chemoautotrophs

• Example: archaebacteria

• Next photosynthesizing prokaryotes evolved

• Photosynthesis releases oxygen into the air so O2 increased in atmosphere

• Small prokaryote cells were taken in by large prokaryote cells and evolved into eukaryotes

Fig. 21.3, p. 337

tovacuumpump

boiling water

sparkdischarge

liquid water in trap

water containingorganic compounds

water droplets

water in

condenser

electrodes

water out

CH4

NH3

H2O

H2

gases

Earth during its formation, when the moon's orbit was much closer than it is today. If the Earth had condensed into a planet of smaller diameter, its gravitational mass would not have been great enough to hold on to an atmosphere. Without water, life as we know it never would have originated on Earth.