Chapter 17 Section 2Earth’s Early History

Objectives:-Describe how conditions on early Earth were different from conditions today-Explain what Miller and Urey’s experiments showed-State the hypotheses that have been proposed for how life first arose on Earth-Identify some of the main evolutionary steps in the early evolution of life

Key ConceptEarth’s Early Atmosphere Contained:

Hydrogen CyanideCarbon Dioxide

Carbon MonoxideNitrogen

Hydrogen SulfideWater

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The First Organic Molecules• Miller and Urey’s Experiment

Mixture of gases simulatingatmosphere of early Earth

Condensationchamber

Spark simulatinglightning storms

Watervapor

Liquid containing amino acids and other organiccompounds

Cold water cools chamber, causing droplets to form.

large organic molecules form tiny bubbles called proteinoid microspheres.

Key Concept

Miller & Urey’s Experiments Suggested: How Mixtures Of The Organic Compounds Necessary For Life Could Have Arisen From Simpler Compounds Present On A Primitive Earth

Original Atmospheric Components Not Accurate

(Re-Analysis of Chemical Composition of Early Rocks)

Corrected Experiments Have Produced Cytosine & Uracil

Two RNA Bases

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The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration

• Endosymbiotic Theory

Mitochondrion

Aerobicbacteria

Nuclear envelopeevolving

Ancient Prokaryotes

Plants and plantlike protists

Primitive PhotosyntheticEukaryote

Primitive AerobicEukaryote

Ancient AnaerobicProkaryote

Chloroplast

Animals, fungi, andnon-plant like protists

Photosynthetic bacteria

Origin of Eukaryotic Cells

• Lynn Margulis – Boston U. 1960’s–Evidence•Mitochondria & Chloroplasts Contain

DNA & Ribosomes Similar To Bacterial DNA & Ribosomes–They Reproduce By Binary Fission

Evolution of RNA & DNA

• Still Unknown However:–RNA Sequences Have Been Found That:• Help DNA Replicate• Transcribe DNA• Translate Proteins• Catalyze Chemical Reactions• Duplicate Themselves

–Perhaps RNA Came First

Free Oxygen

Microfossils - 3.5 Billion Years Old–Prokaryotes–Anaerobic (No O2 In Atmosphere)–Photosynthetic Bacteria Arose• 2.2 Billion Years Ago• O2 Continuously Released Into The

Atmosphere

Free Oxygen

First It Bound Iron In The Oceans

–Iron Oxides Settled To The Bottom Of The Oceans• Formed Great Bands Of Iron That We

Mine Today

Free Oxygen

Next O2 Accumulated In The Atmosphere

–O2 Increased–Methane & Hydrogen Sulfide

Decreased–Atmosphere Turned Blue

Free Oxygen

Is Highly Reactive

Deadly To Anaerobes

First Aerobic Organisms Arise

Free Oxygen

Key ConceptThe Rise Of Oxygen In The Atmosphere

Drove Some Life Forms To Extinction, While Other Life Forms Evolved New, More Efficient Metabolic Pathways That Used Oxygen For Respiration

Variations in CO2 and O2

C = CambrianO = OrdovicianS = SilurianD = DevonianC = CarboniferousP = PermianTr= TriassicJ = JurassicK = CretaceousT = Tertiary

“0” on time scale = Today

Sexual Reproduction & Multicellularity

• Shortly After Forming Multicellular Organisms Sexual Reproduction Began

– A Few Hundred Million Years Later, Multicellular Organisms Arose And Exploded

– Accelerated Genetic Variation