Earth History Origin of Earth

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

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

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

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

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.

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

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?

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

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

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

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

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

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

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

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)

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