Transcript History of the World part I

Fossils
How long has Life Existed
on Earth?
Algae
Not
sure!
Earliest fossils:
possibly 3.5 - 4
billion years

Estimate:
Dinosaurs
existed between
230 million
years ago and
65 million years
ago.
Fossil:
any trace of dead
organism
Fossil
The formation of any fossil is a rare event.
 Most fossils form when organisms that die
become buried in sediments (particles of soil
and rock). Layers of sediment cover dead
organisms and after a very long period of time,
they harden into sedimentary rock.

Paleontologist:
A
person who studies fossils
Types of Fossils
1.
Sedimentary or Petrified: forms from
hard body parts where minerals replace
tissue and these minerals eventually
change into rock
Types of Fossils
2.
Mold: a hollow space in sediment in the
shape of an organism or part of an
organism
Types of Fossils
3.
Cast: mold filled with hard mineral; a
copy of the organism
Types of Fossils
Preserved remains
a. Amber - petrified tree sap; usually
small; good because the whole
specimen is preserved
b. Frozen in ice
c. Tar
d.
Fossil Record:
Fossil
evidence of life on earth
Used to study & explain how
new species formed on Earth
(speciation)
Geologic Time Scale:
calendar of life’s history based on radioactive dating; divided into
time spans of eras and periods. (pg. 195) Organism complexity
has increased.
• Precambrian Era – first living things like bacteria, algea, and
fungi
• Paleozoic Era – invertebrate sea animals, land plants, fish, early
amphibians, tropical forests, seed plants, reptiles, mammals
• Mesozoic Era – dinosaurs, mammals, flowering plants
• Cenozoic Era – first monkeys, apes, grasses, humans
How to Date a Fossil

Relative dating: sedimentary rocks form
layers. The layers near the top are
younger, while those farther down are
older. This can only be used when rock
layers have been preserved in their
original sequence. This only tells a
scientist which fossil is older than another.
How to Date a Fossil

Relative dating
How to Date a Fossil

Radioactive dating – rocks that fossils are found near
contain radioactive elements, which are unstable
elements that decay, or breakdown, into different
elements. The half-life of a radioactive element is the
time it takes for half of the atoms in a sample to decay.
Scientists then compare the amount of radioactive
element to the amount that has broken down into the
different element. This allows them to estimate the age
of the rock, and thus the age of the fossil.