Jeanne Sept
10/15/04
Earth History
Time Machine
Old Earth !
Age of Planet
~4.5 billion years
Measuring TIME
Changing geography and climate:
an “engine” of macro-evolution
Primate Evolution
Text Chapters 4-5
Life on earth
Age of earth
50 minutes
First life
38 minutes left
First mammals
2 minutes = 200 million years ago
First definite primates
43 seconds = 55 million years ago
First Monkeys & Apes
27 seconds = 35 million years ago
First proto-humans
3.5 seconds
Why important to primate
evolution?
Plate Tectonics
Friction
Plates of
earth’s crust
float on
surface of
molten core
Tectonic activity
Changes in
continents & oceans
– Climate
– Landbridges, etc
Tectonic activity
– Faulting (topography)
– Volcanic highlands
– Sedimentary basins
- Lakes
A105 Fossil Lecture
1
Jeanne Sept
10/15/04
Rift Valley Africa
Stratigraphy
River erodes canyon
& old layers exposed
Sediments accumulated in layers
Very old earth!
Uniformitarianism
Mt St Helens, WA, 1980
Geologist Charles Lyell
•
•
The present is the key
to the past
Explanation in natural
processes
• Sedimentation & burial
Air-fall sediments
Volcanic Ash
A105 Fossil Lecture
2
Jeanne Sept
Ancient layer of volcanic
ash = “tuff”
10/15/04
Laetoli
Ancient Volcanic Ash layer:
tuff
Footprints!
Leaf imprints
A105 Fossil Lecture
3
Jeanne Sept
10/15/04
Footprints
How old are fossils?
How old are the fossils?
Stratigraphic super-position
– Sequence = relative age
Which fossil is older?
A105 Fossil Lecture
4
Jeanne Sept
10/15/04
Chronometric dating of
volcanic sediments
How to date fossils?
Text Ch 4: pp 110-115
Potassium K 39
Chronometric methods=
– measuring “absolute” age
– Radioactive decay
- Some isotopes unstable
- Carbon-14
- Potassium-40
- Rate of decay = steady, clock-like
- “half-life”
(93%)
K 40
(.01%)
Argon Ar 39 & Ar 40
K 40 decays to Ar 40
– K-40  Ar-40 gas trapped in crystals
- K-40 / Ar-40 = time since eruption
- Ar-39 / Ar-40 = time since eruption
– Half-life = 1.3 billion years
K/Ar or Ar/Ar
Laser microprobe
K-40
Measure Ar/Ar ratio in
individual crystals
– Precision
- + 1%
– Range
- >5 billion to ~50,000
years ago
Stable Ar-39
Ar-40
Ratio measures age
How old are fossil
footprints?
Ancient volcanic ash “Tuff”
– K/Ar & Ar/Ar
3,500,000 - 3,700,000
= 3.5 - 3.7 mya
Laetoli (E. Africa)
A105 Fossil Lecture
5