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)