Earth History

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

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.

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

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.

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

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 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

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

BIFs (Banded Iron Formations) thick (100s of feet) deposits 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

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

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

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

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

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