Precambrian

Life

Earth’s Atmosphere

• Today’s atmosphere and hydrosphere is different than Precambrian

• Today’s atmosphere: – Nitrogen (N2)– Abundant free oxygen

(O2)– Water vapor (H2O)– Ozone (O3)

Earth’s Early Atmosphere• Primitive atmosphere

– He, H in H2O vapor• Blown away (no magnetosphere) or

lost to space (not enough gravity)

– O2 in H2O & CO2– C in CO2– But deficient in O2 & rich in CO2

• Gases from cooling magma– Simple gases – methane (CH4) &

ammonia (NH3)• Atmosphere not conducive to

O2-breathing organisms• Little free O2 in atmosphere

until evolution of photosynthetic organisms – Some oxygen by photochemical

disassociation– Reducing environment changed to

oxygenation one

Precambrian Atmosphere• Evidence for oxygen production and

accumulation in Earth’s atmosphere– Banded Iron Formations (BIF’s)– Red Beds

Banded Iron Formations (BIF’s)

• Occur in rock record about 3.2 Ga—most at2.0-2.5 Ga

• Formed in oceans

• Consist of chert (SiO2) & red bands– Red Bands rich in

iron oxides Fe2O3, Fe3O4

• Record major oxygenation event

PreCambrian BIFs

Origin of BIF’s

• Photosynthesis produced oxygen

– Combined with Fe to produce “rusty rain” in ocean

Red Beds• Similar to BIF’s,

but . .– Terrestrial formations– Lower in Fe

concentration

• Occur in rock record about 2 Ga– Atmosphere at this

time only had 1-2% O2

• Indicate O2 present in atmosphere to “rust” sediments – O & O3 more

effective oxidizing agents

Origin of Red Beds

ferric iron oxides: red beds

• Red beds formed after all reduced iron in ocean had been oxidized

Where did the O2 come from?

• Prokaryotes• Eukaryotes• Ediacaran Fauna

Protein Synthesis• S. Miller, chemist (1953)• Reconstructed “early atmosphere”

– Mixed methane, ammonia, H2 and H2O vapor– Applied electrical charges produced amino acids

Heat, UV radiation, sunlight, radioactivity can do same• Process called abiotic synthesis• Today, only organisms produce amino acids

– Amino acids + organic molecules = protein

Earliest Organisms• Must have had

anaerobic (no O2) heterotrophs– Used organic soup for

food• Free O2 lethal to

anaerobic heterotrophs– Need to adjust to ↑

O2• Cherts important

– Silica gel (volcanism) trapped organisms

• Fig tree chert– S. Africa = 3.1 Ga

• Stromatolite– NW Australia = 3.5 Ga

• 3.85 in Greenland

3.5 Ga Stromatolite

Modern Stromatolite

Archean - Prokaryotes• Cyanobacteria

– Blue-green algae– Single celled, lack nucleus

• Contain DNA, but no membrane-bound organelles• Undergo photosynthesis

Prokaryotes

3.3-3.5 Ga ProkaryoteWarrawoona Group, W. Australia

3.3-3.5 Ga ProkaryoteWarrawoona Group, W. Australia

Archean Eukaryotes• Contain nucleus, DNA and are larger• Membrane-bound organelles• Fig tree has chemical indicators of life

– Pristane/phytanes Chlorophyll products

– C-12 & C-13 Used by photosynthesizing organisms

Eukaryotes

Microfossils, Gunflint Fm, Canada

700-800 Ma Microfossils, Beckspring Dolomite, California

Common Proterozoic Eukaryotes

Metaphytes and Metazoans• 3.5-0.9 Ga small organism

– Single cell or few cells attached

• Next important evolutionary step– Combination of cells to form

macroscopic organism

Metaphytes and Metazoans• Metaphytes

(plants)

• Metazoans (animals)

– First plants = algae– May have multi-cellular algae

– First evidence is trace fossils– Found in late Precambrian – Montana, Canada– Made by large organism

Possible multi-cellular algae, Little Belt Mtns., Montana

Ediacaran Fauna• Soft-bodied fossils in SS, S. Australia

– 1.0”-2.0”, some a few feet– Heterotrophs; previously all autotrophs

Depend on outside food source

– Multi-celled organisms Led to specialized cells Led to organs

– Evidence of systems in organisms

Reconstruction Ediacaran Environment