1. Earth History

2. Eons – largest time division
Eons determined by major changes in the Earth
Most recent Eon started with the appearance of multicellular animals = Phanerozoic Eon
Three Eons make up what was previously know as the Precambrian Period
1) Proterozoic
2) Archean
3) Hadean

3. The Hadean Eon: 4.5 billion years ago
Earth & planets formed from dust and gasses surrounding the sun

4. Heavy elements in planets suggest dust/gas from supernova
We observe other solar systems forming
They form in diffuse nebulae like nebula M16.

5. Electrostatic forces gather dust into large lumps, called planetisimals
Collisions between lumps makes lots of heat
The Earth was molten during the Hadeon Eon.
Steam atmosphere formed as ice comets collided with Earth

6. After most planetisimals had become part of a planet, …
After most planetisimals had become part of a planet, …..fewer collisions
Earth’s temperature dropped below 1,000 ۫C
First rocks formed as crust cooled
The oldest Earth rocks currently known are 3.8 billion years old.
The oldest meteorites and lunar rocks are about 4.5 billion years old
Erosion and plate tectonics have probably destroyed all of the Earth rocks older than 3.8 billion years.

7. The Archaean Eon: 3.8 - 2.5 billion years ago
Began with the formation of solid rock
Volcanic activity added heavier gasses to atmosphere (CO2, N2)
Water condensed
Early oceans formed
First life appeared on Earth

8. Archean Atmosphere What’s missing? (CO2, H2O, N2, CO, H2S, HCl)
No free oxygen O2…..so no ozone either O3
Still present in
modern atmosphere
Rare in
What’s missing?

9. WHEN Did Life Evolve? oldest fossils of life-forms ~3.5 byo
life likely evolved before then, but no fossils
Life must have evolved before there was free oxygen in the atmosphere
O2 breaks down macromolecules
If macromolecules couldn’t form….
…....living cells could not form

10. Archean sediment dark-colored = unoxidized
Archean sediments were deposited before there was free oxygen (O2) in the atmosphere
oxidized (rusted) sediment appears about 2.2 billion years ago in the Proterozoic Eon
The free oxygen was produced by living cells doing photosynthesis
Therefore, life must have evolved before 2.2 bya

11. BIFs (Banded Iron Formations)
thick (100s of feet) deposits
Indirect (non-fossil) evidence for the presence of
cyanobacteria ~ 3.5 bya
Precambrian,
Australia

12. Banded Iron Formations Created
iron dissolves in anaerobic water
when iron bonds to O2 it becomes solid
the solid sinks to the ocean floor is deposited in layers
Layers of oxidized iron are red

13. Dark, Unoxidized Bands Form
When all dissolved Fe is removed, O2 builds up
High O2 concentrations kill the cyanobacteria
Dark sediments are deposited that have no oxidized iron in them

14. Why are BIFs Banded? When runoff added more Fe to the water the entire process stared over again.

15. What Likely Happened O2 made by cyanobacteria at ocean surface
O2 toxic to them
but not if the O2 was removed by bonding with iron (Fe)
Oceans contained dissolved iron

16. BIFs began forming about 3.5 bya = 1st free O2
reached peak about 2.5 bya
deposition ended about 1.8 bya

17. The Proterozoic Eon: 2.5 - .54 billion years ago
End of BIFs……
Cyanobacteria resistant to oxygen poisoning by 1.8 billion years ago (early proterozoic)
Spontaneous formation of new cells no longer possible
Diversification of bacteria
First Eukaryotes appear ( protists)
Ediacaran fauna appear (soft invertebrates)

18. Phanerozioc Eon 540 million years ago -present
540 mya = .54 bya
Began with the Cambrian explosion of multicellular animal species and complex land plants