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Precambrian

Precambrian covers about 4 billion years (and 87%) of Earth history.

Precambrian is divided into 2 eons: – Proterozoic Eon 2.5 - 0.542 billion years ago (or

2500 - 542 million years ago) – Archean Eon 4.6 - 2.5 billion years ago (lower limit

not defined)

Table of time divisions of

Precambrian

Precambrian is not well known or completely understood. Why?

• Precambrian rocks are often poorly exposed. • Many Precambrian rocks have been eroded or

metamorphosed. • Most Precambrian rocks are deeply buried beneath

younger rocks. • Many Precambrian rocks are exposed in fairly

inaccessible or nearly uninhabited areas. • Fossils are seldom found in Precambrian rocks; only

way to correlate is by radiometric dating.

Areas where Precambrian rocks are exposed are shown in yellow, as well as in the red areas in orogenic belts.

Shields and Cratons

Most of what we know about Precambrian is based on studies of rocks from cratons - large portions of continents which have not been deformed since Precambrian or Early Paleozoic.

Shields and Cratons

• The most extensive exposures of Precambrian rocks are in geologically stable regions of continents called shields.

• Example = Canadian shield in North America. Mostly igneous and metamorphic rocks; few sedimentary rocks. Overlying sedimentary rocks were scraped off by glaciers during last Ice Age.

Shields and Cratons

• Stable regions of the craton where shields are covered by sedimentary rocks are called platforms.

• Precambrian rocks are often called basement rocks because they lie beneath a covering of fossil-bearing sedimentary strata.

North American craton, shield, platform, and orogenic belts.

Precambrian Provinces

Various Precambrian provinces can be delineated within the North American continent, based on radiometric ages of rocks, style of folding, and differences in trends of faults and folds.

– Oldest (Archean) rocks are shown in orange.

– Younger (Proterozoic) rocks are shown in green.

Precambrian provinces in North America, with dates

Origin of Plate Tectonics

• By about 4 b.y. ago, the Earth had probably cooled sufficiently for plate formation.

• Once plate tectonics was in progress, it generated crustal rock that could be partially melted in subduction zones and added to the continental crust.

Origin of Plate Tectonics

• Continents also increased in size by addition of microcontinents along subduction zones.

• Greater heat in Archean would have caused faster convection in mantle, more extensive volcanism, more midoceanic ridges, more hot spots, etc.

• Growth of volcanic arcs next to subduction zones led to formation of greenstone belts.

Granulites and Greenstones

The major types of Archean rocks on the cratons are:

– Granulites – Greenstones

Granulites

• Granulites - Highly metamorphosed gneisses (metamorphosed tonalites, granodiorites, and granites) and anorthosites (layered intrusive gabbroic rocks).

Granulites formed from partially melted crust and sediments in subduction zones. Metamorphism altered the rocks to form granulites.

Greenstones• Greenstones - Metamorphosed volcanic rocks and

sediments derived from the weathering and erosion of the volcanic rocks.

Greenstone volcanic rocks commonly have pillow structures, (called pillow basalts), indicating extrusion under water.The green color is the result of low-grade metamorphism, producing green minerals such as chlorite and hornblende.

Greenstones• Mostly found in trough-like or synclinal belts. • Sequence of rock types :

– Ultramafic volcanic rocks near the bottom (komatiites)

– Mafic volcanic rocks (basalts) – Felsic volcanic rocks (andesites and rhyolites) – Sedimentary rocks at the top (shales, graywackes,

conglomerates, and sometimes BIF), deposited in deep water environments adjacent to mountainous coastlines.

Generalized cross-section through two greenstone belts. Note sequence of rock types and relationships between granulites and the greenstones. Granulites are present between greenstone belts.

Earth's Earliest Glaciation

By 2.8 billion years ago, Earth had cooled sufficiently for glaciation to occur. Earth's earliest glaciation is recorded in 2.8 billion year-old sedimentary rocks in South Africa.

Earliest Evidence of LifeThe earliest evidence of life occurs in

Archean sedimentary rocks. – Stromatolites – Microscopic cells of prokaryotes– Algal filaments – Molecular fossils

Stromatolites

• An organo-sedimentary structure built by photosynthetic cyanobacteria or blue-green algae.

• Stromatolites form through the activity of cyanobacteria in the tidal zone. The sticky, mucilage-like algal filaments of the cyanobacteria trap carbonate sediment during high tides.

Stromatolites

Modern stromatolites, Shark Bay, western Australia

Stromatolites

• More abundant in Proterozoic rocks than in Archean rocks. Examples: – Oldest are 3.5 b.y. old, Warrawoona Group,

Australia's Pilbara Shield – 3 b.y. old Pongola Group of southern Africa – 2.8 b.y. old Bulawayan Group of Australia

Stromatolites• Stromatolites are scarce today because

microorganisms that build them are eaten by marine snails and other grazing invertebrates.

• Stromatolites survive only in environments that are too saline or otherwise unsuitable for most grazing invertebrates.

• The decline of stromatolites is associated with the evolutionary appearance of new groups of marine invertebrates during Early Paleozoic.

Oldest direct evidence of life• Microscopic cells and filaments of prokaryotes.• Associated with stromatolites• Similar to cyanobacteria living today, which produce

oxygen.• Fossiliferous chert bed associated with the Apex

Basalt• Found in Warrawoona Group, Pilbara Supergroup,

western Australia• 3.460-3.465 billion years old

Other evidence of Archean life

• Indirect evidence of life in older rocksFound in banded iron deposits in Greenland.Carbon-13 to carbon-14 ratios are similar to those in present-day organisms.3.8 b.y.

Other evidence of Archean life

• Algal filament fossils Filamentous prokaryotes preserved in stromatolites.Found at North Pole, western Australia;3.4-3.5 b.y. old.

• Spheroidal bacterial structuresFound in rocks of the Fig Tree Group, South Africa (cherts, slates, ironstones, and sandstones).Prokaryotic cells, showing possible cell division; 3.0 - 3.1 b.y. old.

Other evidence of Archean life

• Molecular fossils Preserved organic molecules that only eukaryotic cells produce.

• Indirect evidence for eukaryotes.In black shales from northwestern Australia; 2.7 b.y.

• Origin of eukaryotic life is pushed back to 2.7 b.y.

The Origin of Life

The basic materials from which microbial organisms (i.e., life) could have developed initially. May have arrived on Earth during Archean in meteorites called carbonaceous chondrites, which contain organic compounds.

Life requires these elements:

»Carbon »Hydrogen »Oxygen »Nitrogen »Phosphorus »Sulfur

Each of these is abundant in the Solar System.

Four essential components of life: 1. Proteins - Chains of amino acids. Proteins are used to

build living materials, and as catalysts in chemical reactions in organisms.

2. Nucleic acids - Large complex molecules in cell nucleus. 1. DNA (carries the genetic code and can replicate itself) 2. RNA

3. Organic phosphorus compounds - Used to transform light or chemical fuel into energy required for cell activities.

4. Cell membrane to enclose the components within the cell.

• The earliest organisms developed in the presence of an atmosphere which lacked oxygen. The organisms must have been anaerobic (i.e., they did not require oxygen for respiration).

• Organic molecules could not assemble into larger structures in an oxygenated environment. Oxidation and microbial predators would break down the molecules.

• Because the atmosphere lacked oxygen, there was no ozone shield to protect the surface of the Earth from harmful ultraviolet (UV) radiation.

Origin of amino acids

UV radiation can recombine atoms in mixtures of water, ammonia and hydrocarbons, to form amino acids.

(The energy in lightning can do the same thing.)

Miller Experiment

Lab simulation experiments by S. Miller in the 1950's formed amino acids from gases present in Earth's early atmosphere:– H2,

– CH4 (methane),

– NH3 (ammonia), and

– H2O (water vapor or steam),

along with electrical sparks (to simulate lightning).

Miller Experiment

This was the first laboratory synthesis of amino acids. A liquid was produced that contained a number of amino acids and other complex organic compounds that comprise living organisms. A main requirement was the lack of free oxygen.

Joining Amino Acids to Form Proteins

Amino acids are monomers and have to be joined together to form proteins, which are polymers (or chains).

This requires: – Input of energy – Removal of water

Joining Amino Acids to Form Proteins

How could this occur? 1. Heating (volcanic activity) 2. At lower temperatures in the presence of

phosphoric acid3. Evaporation 4. Freezing 5. Involve water in a dehydration chemical

reaction

Joining Amino Acids to Form Proteins

6. On surface of clay particles, which have charged surfaces, and to which polar molecules could attach. Metallic ions on clays could concentrate organic molecules in an orderly array, causing them to align and link into protein-like chains.

7. On pyrite, which has a positively charged surface to which simple organic compounds can become bonded. Formation of pyrite yields energy which could be used to link amino acids into proteins.

Proteinoids

• Proteinoids are protein-like chains produced in the lab by Fox from a mixture of amino acids. Considered to be possibly like the transitional structures leading to proteins billions of years ago.

• Similar proteinoids are also found in nature around Hawaiian volcanoes.

Hot aqueous solutions of proteinoids will cool to form microspheres, tiny spheres that have many characteristics of living cells: – Film-like outer wall – Capable of osmotic shrinking and swelling – Budding similar to yeast – Divide into daughter microspheres – Aggregate into lines to form filaments, as in some

bacteria – Streaming movement of internal particles, as in

living cells

Where Did Life Originate?

Early life may have avoided UV radiation by living: – Deep beneath the water – Beneath the surface of rocks (or below sediment -

such as stromatolites) Life probably began in the sea, perhaps in

areas associated with submarine hydrothermal vents or black smokers.

Evidence for life beginning in the sea near hydrothermal vents:

1. Sea contains salts needed for health and growth. 2. Water is universal solvent, capable of dissolving

organic compounds, producing a "rich organic broth" or primordial soup.

3. Ocean currents mix these compounds, leading to collisions between molecules, leading to combination into larger organic molecules.

Evidence for life beginning in the sea near hydrothermal vents:

4. Microbes at vents are hyperthermophiles that thrive in seawater hotter than boiling point (100oC).

5. These microbes derive energy by chemosynthesis, without light, rather than by photosynthesis (suggests origin in deep water in absence of light).

6. Hyperthermophiles are Archaea, with DNA different from bacteria.

Feeding Life on Earth – Obtaining Nutrients

Examples of types of feeding modes:

1. Fermenters - digest chemicals, such as sugar, in the absence of oxygen, to obtain energy. Produce CO2 and alcohol. Example: Yeast

2. Autotrophs - manufacture their own food.Examples: sulfur bacteria, nitrifying bacteria, and photoautotrophs (such as plants and photosynthetic bacteria) that use photosynthesis

3. Heterotrophs - can't make their own food, so they must find nutrients in the environment to eat. Example: Animals.

Evolution of Early Life • The earliest cells had to form and exist in anoxic

conditions (in the absence of free oxygen).Likely to have been anaerobic bacteria or Archaea.

• Some of the early organisms became photosynthetic, possibly due to a shortage of raw materials for energy. Produced their own raw materials. Autotrophs.Photosynthesis was an adaptive advantage.

• Oxygen was a WASTE PRODUCT of photosynthesis.

Consequences of Oxygen Buildup in the Atmosphere

1. Ozone layer which absorbs harmful UV radiation, and protected primitive and vulnerable life forms.

2. End of banded iron formations which only formed in low, fluctuating O2 conditions

3. Oxidation of iron, leading to the first red beds.4. Aerobic metabolism developed. Uses oxygen to

convert food into energy. 5. Development of eukaryotic cell, which could cope

with oxygen in the atmosphere.

Prokaryotes vs. Eukaryotes • Prokaryotes reproduce asexually by simple cell

division. This restricts their genetic variability. Prokaryotes have shown little evolutionary change for more than 2 billion years.

• Eukaryotes reproduce sexually through the union of an egg and sperm. This combines chromosomes from each parent and leads to genetic recombination and increased variability. Many new genetic combinations. Led to a dramatic increase in the rate of evolution.

Prokaryotes vs. Eukaryotes

Prokaryotes vs. Eukaryotes

The Earliest Eukaryotes Earliest large cells that appear to be eukaryotes appear

in the fossil record about 1.6 - 1.4 b.y. ago (during Proterozoic).

Eukaryotes diversified around the time that the banded iron formations disappeared and the red beds appeared, indicating the presence of oxygen in the atmosphere.

Origin of eukaryotic life was probably around 2.7 b.y., based on molecular fossils.

Endosymbiotic Theory for the Origin of Eukaryotes

• Billions of years ago, several prokaryotic cells came together to live symbiotically within a host cell as protection from (and adaptation to) an oxygenated environment.

• These prokaryotes became organelles. • Evidence for this includes the fact that mitochondria

contain their own DNA.• Example - a host cell (fermentative anaerobe) +

aerobic organelle (mitochondrion) + spirochaete-like organelle (flagellum for motility).

Endosymbiotic Origin of

Eukaryotes

Eukaryotes

The appearance of eukaryotes led to a dramatic increase in the rate of evolution, and was ultimately responsible for the appearance of complex multicellular organisms.