A Brief Tour of Life. Calendar of Events Event Origin of Earth 1 st Evidence of Life (crystals only produced by organisms) Fossils (Bacteria) Origin of

A Brief Tour of Life

Calendar of Events

EventOrigin of Earth1st Evidence of Life (crystals only produced by organisms)Fossils (Bacteria)Origin of Photosynthesis (Bacteria)Last Banded Iron FormationsOrigin of EukaryotesOrigin of Multicellular Organisms

Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6

Origin of Life

1. Panspermia (pan = expanded; sperm = seed)• Life “seeded” from space• Did not originate on Earth

2. Life Started on Earth — 4 stagesa. Abiotic synthesis of simple organic moleculesb. Assembly of simple molecules into polymersc. Origin of self-replicating molecules (= inheritance)d. Packaging molecules inside membranes

Abiotic synthesis of simple organic molecules

2 possible sources• Extraterrestrial• Meteors• Comets• Evidence: found amino acids inside meteorites

• Synthesis on Earth• Miller-Urey experiment

Miller-Urey Experiment

• Purpose

• Gases

• Spark

• Results

• Conclusion

Test whether abiotic synthesis of organic molecules is possible

NH3, CH4, H2, H2O

Energy source—UV, lightening

After 1 week, brown soup containing simple organic molecules (some amino acids, building blocks of nucleotides)

Abiotic synthesis of simple organic molecules is possible

Origin of Life



Assembly of simple molecules into polymers

• At edges of warm pools, water evaporates• Warm: energy• Evaporation: concentrates molecules• Clays: catalytic• Result: polymers

• Problem? Intense UV• Why intense UV?

• Ozone (O3) absorbs UV• No ozone because no O2

• UV = high energy• Breaks down polymers faster than can form

• Alternatives?• Polymerization in protected areas:

hydrothermal vents, continental volcanoes

Origin of Life



Origin of self-replicating molecules (= inheritance)

• What is the replicator today?• 1st replicator not DNA. Why?

• DNA synthesis requires enzymes (= proteins)• Proteins synthesis requires DNA• Chicken & egg problem: Which came first?

• Conclusion• DNA/Protein system too complex• 1st replicator not DNA, but some simpler system

• What replicator came before DNA?• Unknown• Requirements: Same molecule must …

• Store information for its own synthesis (genetic information)• Catalytic activity to synthesize itself from monomers

• What molecule satisfies these requirements?

DNA

RNA

RNA: Review• Nucleic Acid• Transcription:

DNA RNA• Translation:

RNA Protein

RNA: A Closer Look• Eukaryote Genes Contain

• Exons• Sequences coding for proteins

• Introns• ”Junk” sequences not coding

for protein• Removed before translation

• The Process• DNA transcribed to pre-mRNA

containing exons & introns• Pre-mRNA processed to mature

RNA transcript by …• Removing introns• Splicing exons

The Process in Cells

RNA Satisfies Requirements for Replicator• Stores genetic information

• Where?

• Has catalytic activity• Some RNA molecules catalyze reactions

mRNA self-catalyzes removal of its own introns

Sequence of nucleotides

RNA World• RNA Replicator

• Contains genetic information in its nucleotide sequence

• Catalyzes its own synthesis

• Was RNA 1st replicator?

• Too complex to synthesize abiotically

• But probably preceded DNA

Probably not

Origin of Life



Packaging molecules inside membranes

• When some lipids mixed with water, spontaneously form droplets

• Lipids outside; water inside• Called coacervates

Calendar of Events


Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6

Review: Metabolic Pathways

Catabolic—break down• Cellular Respiration (aerobic)• Glycolysis• Link Reaction• Krebs Cycle• Oxidative Phosphorylation

• Fermentation (anaerobic)

Anabolic—build up (synthesize)• Photosynthesis• Light Dependent Reactions• Light Independent Reactions

Cellular Respiration: Overview

• Cytosol• Glycolysis

• Glucose Pyruvate

• Mitochondrial Matrix• Link Reaction

• Pyruvate Acetyl CoA• Krebs (Citric Acid) Cycle

• Acetyl CoA CO2

• Inner Membrane• Oxidative Phosphorylation

• Most ATP synthesis

• NAD+/NADH connects

Cellular Respiration: Oxidative Phosphorylation• NADH drops off

electrons on ETC• Electrons pass down

ETC, releasing energy• Energy used to pump

H+ to intermembrane space• Electrons move back

to matrix through ATP synthase, making ATP

Fermentation • In absence of O2 cannot break down pyruvate• Synthesize ethanol or

lactate to regenerate NADH

Photosynthesis: Overview

• Light Dependent Reactions• Thylakoid membrane• ETC & chemiosmosis

• Light Independent Reactions• Stroma• Fix carbon

Photosynthesis: ETC & Chemiosmosis• Photon boosts

electrons• Electrons pass down

ETC, releasing energy• Energy used to pump

H+ inside thylakoid• Electrons move back to

stroma through ATP synthase, making ATP

Early Metabolic Pathways

• Early Earth’s atmosphere contained no O2

• Organisms “ate” simple organic molecules made abiotically

• 1st metabolic pathways: • How do we know?• Most widespread in organisms• Inherited from common ancestor

Glycolysis & Fermentation

Photosynthesis

• After a while, “free lunch” over—organic molecules used up• Organisms needed source of food

• Idea: make food (organic compounds) from energy in sunlight• 2nd metabolic pathway:

• How do we know? • Fossilized Photosynthetic Prokaryotes• Banded Iron Formations

Photosynthesis

Fossilized Photosynthetic Prokaryotes

Fossilized Stromatolites (New York State)

Modern Stromatolites (Shark Bay, Australia)

made by cyanobacteria

Photosynthetic Bacteria (Cyanobacteria)

Cyanobacteria Toxic Cyanobacterial Bloom

Historical Atmospheric Oxygen Levels

• Photosynthesis began 3.5 Gya• But atmospheric O2 levels did

not begin to rise until 1.5 Gya

• Why the delay?• Where did the O2 produced

between 3.5-1.5 Gya go?

Banded Iron Formations• How Do We Know Early Earth’s

Atmosphere Contained No O2?• Fe2+ dissolved in oceans• In presence of O2, Fe2+ Fe3+

• Precipitates as Fe2O3 (rust, insoluble)• Produced banded iron formations

• So, Fe2+ absorbed all O2 as it was produced

• How much Fe2+? Enormous amount—continued for 1.5-2 Gy!

Calendar of Events


Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6

Last Banded Iron Formations

• So, when last Fe2+

precipitated, atmospheric O2 levels began to rise• 1.5 Gya• Great Oxygenation Event!

Great Oxygenation Event

• Good, right?• All organisms at that time anaerobic (lived in absence of O2)• O2 highly reactive• Kills living organisms

• Result: catastrophic extinction

New Opportunities

• O2 imposed strong selection pressure• Any organism that tolerated O2 = at

evolutionary advantage

• Solution: detoxify O2 • 4H+ + 4e- + O2 (toxic) 2H2O (non-toxic)• Familiar?• So, ETC evolved to detoxify O2

electron transport chain of cellular respiration

New Opportunities

• 4H+ + 4e- + O2 2H2O releases LOTS of energy• Some bacteria coupled this

release of energy with ETC & ATP synthesis• Where from?

• 3rd metabolic pathway:

Photosynthesis

Cellular Respiration

Calendar of Events


Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6

Origin of Eukaryotes

• History of life on Earth: >60% just prokaryotes• How did eukaryotes arise?• 2 theories• Autogenous Model• Endosymbiotic Model

Autogenous Model

• Auto = self; gen = produce, birth• Idea: Cell membrane folds in to form inner membrane structures• Used to explain origin of• Nuclear membrane• Endomembrane system (smooth ER, rough ER, Golgi apparatus, vesicles)

• Supporting evidence?• Nuclear membrane, ER, & cell membrane continuous• Nuclear membrane = double membrane (2 bilayers)

Endosymbiotic Model

• Endo = inside; sym = together; bio = living• Idea: • Large prokaryote engulfed

smaller prokaryote (endocytosis)• Smaller prokaryote survived =

parasite• Over time, two organisms

developed mutual dependence

Endosymbiotic Model• Smaller prokaryote = heterotroph (cellular respiration)• Became “leaky” to ATP• Mutual dependence

• Large cell provides home, sugar, O2

• Small cell provides ATP

• Organelle?

• Smaller prokaryote = autotroph (photosynthesis)• Became “leaky” to sugar• Mutual dependence

• Large cell provides home• Small cell provides sugar (food)

• Organelle?

Mitochondrion

Chloroplast

Endosymbiotic Model: Supporting Evidence?Chloroplasts & Mitochondria have …• DNA• Only organelles (except nucleus) with DNA• DNA similar to prokaryote DNA

• Not membrane-bound (in nucleus)• Single chromosome• Circular (eukaryote DNA = linear) (Fig 1)• Naked DNA = no histone proteins (eukaryote DNA = with histones) (Fig 2)

• As expected if were free-living prokaryotes

Fig 1

Fig 2

Endosymbiotic Model: Supporting Evidence?Chloroplasts & Mitochondria have …• Ribosomes• Contain own ribosomes• Ribosomes similar to prokaryote ribosomes

• Smaller ribosomes (larger ribosomes in eukaryotes)• (Remember, both eukaryote and prokaryote

ribosomes contain large and small subunits; however, the whole ribosomes is smaller in prokaryotes.)

• As expected if were free-living prokaryotes

Endosymbiotic Model: Supporting Evidence?Chloroplasts & Mitochondria have …• Double Membranes• Membrane chemical composition• Outer• Inner

• As expected if engulfed by endocytosis

: like prokaryote: like eukaryote

Origin of Eukaryotes: Summary

• 2 Models• Which is correct?

Documents

A Brief Tour of Life. Calendar of Events Event Origin of Earth 1 st Evidence of Life (crystals only produced by organisms) Fossils (Bacteria) Origin of