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