1. Earth History Origin of Earth

2. Eons – largest time division
Eons determined by major changes in the Earth
Most recent Eon started with the proliferation 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. Electromagnetic forces & Gravity gathers dust into large lumps, called planetisimals
Collisions between lumps makes lots of heat
The Earth was molten during the Hadeon Eon.
Steam atmosphere formed from light elements & water vapor from comets

6. After most planetisimals became fused into planets, …..fewer collisions
Earth’s temperature dropped below 1,000 ۫C
First rocks formed
oldest Earth rocks 3.8 billion years old.
The oldest meteorites and lunar rocks about 4.5 byo
Erosion and plate tectonics destroyed all Earth rocks older than 3.8 by.

7. The Archaean Eon: 3.8 - 2.5 billion years ago
Began with formation of solid rock
Volcanoes added heavier gasses to atmosphere (CO2, N2)
Water condensed
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 ~3.5 byo
life likely evolved before then, but no fossils
Life must have evolved before there was free oxygen
O2 breaks down macromolecules
If macromolecules couldn’t form….
…....living cells could not form

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

11. Early Archean sediment = dark-colored = unoxidized
Archean sediments deposited before there was free oxygen
oxidized (rusted) sediment appears in the Proterozoic Eon
free oxygen was produced by living cells doing photosynthesis
Therefore, life must have evolved before BIFs

12. BIFs (Banded Iron Formations)
Indirect (non-fossil) evidence for the presence of
Cyanobacteria – 3.5 bya
Precambrian,
Australia

13. 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

14. What Likely Happened O2 made by cyanobacteria at ocean surface
Toxic O2 removed by iron until iron runs out
Then O2 builds up & kills off most cyanobacteria
O2 levels drop
Unoxidized dark sediments deposited

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

16. 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)

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