Homework #5

Due Tuesday, October 25, 6:00 pm

Exam #2 date

Wednesday, November 2

Exam #3 tentative date

Wednesday, November 30

ExtremophilesExtremophiles

• Life that exists under “extreme” Life that exists under “extreme”

conditions, conditions that until recently conditions, conditions that until recently

were thought to be inhospitable to life.were thought to be inhospitable to life.

ExtremophilesExtremophiles• Volcanic ventsVolcanic vents::

Water temperature Water temperature reaches 400°C (750°F), reaches 400°C (750°F), possible because of the possible because of the large pressurelarge pressure

Black smokers: mixed Black smokers: mixed with volcanic chemicalswith volcanic chemicals

ExtremophilesExtremophiles

• Antarctic dry valleysAntarctic dry valleys:: Microbes in small pockets of water in rocksMicrobes in small pockets of water in rocks

ExtremophilesExtremophiles

• LithophilesLithophiles (rock lovers):(rock lovers): Several kilometers below the surfaceSeveral kilometers below the surface Chemical energy from rocksChemical energy from rocks Carbon from COCarbon from CO22 filtering down filtering down

ExtremophilesExtremophiles• EndosporesEndospores (e.g., anthrax)(e.g., anthrax)

Can lay dormant for long periodsCan lay dormant for long periods Can survive lack of water, extreme heat and Can survive lack of water, extreme heat and

cold, and poisonscold, and poisons Some can survive in vacuumSome can survive in vacuum

Implications for Extraterrestrial LifeImplications for Extraterrestrial Life

Sufficient atmospheric oxygen for eukarya has existed for Sufficient atmospheric oxygen for eukarya has existed for only ~10% of the Earth’s lifetime – complex life not only ~10% of the Earth’s lifetime – complex life not possible before this timepossible before this time

What is the probability that eukarya-like organisms would What is the probability that eukarya-like organisms would develop?develop?

We are more likely to find extremophiles elsewhereWe are more likely to find extremophiles elsewhere

Extremophiles may be the norm, not the exceptionExtremophiles may be the norm, not the exception

The Origin and Evolution The Origin and Evolution of Life on Earthof Life on Earth

When Did Life Begin?When Did Life Begin?

• StromatolitesStromatolites– Living: colonies of bacteria living in outer layer of sedimentary rocksLiving: colonies of bacteria living in outer layer of sedimentary rocks– 3.5 Byr old rocks: almost identical layered structure3.5 Byr old rocks: almost identical layered structure– Inconclusive evidence: sedimentation layering may mimic stromatolitesInconclusive evidence: sedimentation layering may mimic stromatolites

• Fossil evidenceFossil evidence– 3.5 Byr old Australian rock shows “cells”3.5 Byr old Australian rock shows “cells”– Could this form naturally from minerals?Could this form naturally from minerals?– Younger sites: at least two more (3.2-3.5 byr old)Younger sites: at least two more (3.2-3.5 byr old)– Older sites: sedimentary rock too altered to be usefulOlder sites: sedimentary rock too altered to be useful

• 1313C/C/1212C ratioC ratio– Normal abundance ratio 1/89Normal abundance ratio 1/89– Living tissue and fossils show Living tissue and fossils show

less less 1313CC– Some rocks older than 3.85 Some rocks older than 3.85

byr show the low byr show the low 1313C C abundanceabundance

• SterilizationSterilization– Last sterilization: 3.9-4.2 byr Last sterilization: 3.9-4.2 byr

ago (Late Heavy ago (Late Heavy Bombardment)Bombardment)

The evidence indicates life formed The evidence indicates life formed quickly after the Earth became quickly after the Earth became

habitable.habitable.

Within a few 100 million years,

Perhaps as short as 100 million years

The Origin and Evolution The Origin and Evolution of Life on Earthof Life on Earth

How Did Life Begin?How Did Life Begin?

No Life Life?

How did life get here?

few hundred million years

• Simplest organisms today and those dated Simplest organisms today and those dated 3.5 billion years ago are remarkable 3.5 billion years ago are remarkable advancedadvanced

• What are the natural chemical processes What are the natural chemical processes that could have led to life?that could have led to life?

• AssumptionsAssumptions

– Life began under chemical conditions of early Life began under chemical conditions of early EarthEarth

– Life did not migrate to EarthLife did not migrate to Earth

Certain chemical processes are energetically favored given specific circumstances, i.e., in the presence of specific chemical elements, energy sources & liquid(s)

Can the presence of specific elements and energy inevitably lead to the formation of life?

We know it can lead to the building blocks

Simulated conditions thought to exist on the early Earth

Tested for the occurrence of chemical evolution

Considered to be the classic experiment on the origin of life

After one week, Miller and Urey observed that 10–15% of the carbon was now in the form of organic compounds.

Recent re-analysis of Miller's archived solutions found 22 amino acids

Miller-Urey Miller-Urey ExperimentsExperiments

• Different mixes of gases Different mixes of gases used for atmosphereused for atmosphere

• Different energy sources, Different energy sources, like UV (sunlight)like UV (sunlight)

• Results: Results: With the exception With the exception of when oxygen was presentof when oxygen was present, , ALL VARIATIONS PRODUCE ALL VARIATIONS PRODUCE AMINO ACIDS AND COMPLEX AMINO ACIDS AND COMPLEX ORGANIC MOLECULESORGANIC MOLECULES– Proper mix produces less Proper mix produces less

than original experimentthan original experiment– THERE MUST be additional THERE MUST be additional

sources of organic sources of organic material – the atmosphere material – the atmosphere is not sufficient!is not sufficient!

The Origin and Evolution The Origin and Evolution of Life on Earthof Life on Earth

Where Did Life Begin?Where Did Life Begin?

Sources of Organic MoleculesSources of Organic Molecules

• Chemical reactions in atmosphereChemical reactions in atmosphere– Lab experiments show this is probably Lab experiments show this is probably

insufficientinsufficient

• Organic material brought by impactsOrganic material brought by impacts– Chemical analysis of comets and Chemical analysis of comets and

carbonaceous chondrites show that they carbonaceous chondrites show that they have organic moleculeshave organic molecules

• Chemical reactions near deep-sea ventsChemical reactions near deep-sea vents– Heat from undersea volcanoes and vents Heat from undersea volcanoes and vents

can fuel chemical reactions between water can fuel chemical reactions between water and mineralsand minerals

Early Organic ChemistryEarly Organic Chemistry

• No atmospheric oxygenNo atmospheric oxygen– Helps: Oxygen destroys many organic compoundsHelps: Oxygen destroys many organic compounds– Atmosphere is reducing, not oxidizingAtmosphere is reducing, not oxidizing

• Miller-Urey experimentsMiller-Urey experiments

– Can form amino-acid soup from methane (CHCan form amino-acid soup from methane (CH44) and ammonia (NH) and ammonia (NH33) with ) with electric energy (mimicking lightning)electric energy (mimicking lightning)

– Current thinking: early atmosphere was dominated by COCurrent thinking: early atmosphere was dominated by CO22

– Low yieldLow yield– Shallow ponds close to surface -> UV sterilizationShallow ponds close to surface -> UV sterilization

• Alternative sources of amino acidsAlternative sources of amino acids– Extraterrestrial: amino acids are abundant in meteoritesExtraterrestrial: amino acids are abundant in meteorites– Deep sea vents: abundant chemical energy & protected from Deep sea vents: abundant chemical energy & protected from

UVUV

Where Did Life Begin?Where Did Life Begin?Unlikely on landUnlikely on land

– Solar UV radiation: protection today by ozone (OSolar UV radiation: protection today by ozone (O33))– But no atmospheric oxygen in the early EarthBut no atmospheric oxygen in the early Earth– In water: no problem, UV absorbed effectivelyIn water: no problem, UV absorbed effectively

Shallow pondsShallow ponds– First evidence from Miller-Urey experimentFirst evidence from Miller-Urey experiment– Recent evidence: incorrect atmospheric contentRecent evidence: incorrect atmospheric content

ThermophilesThermophiles– DNA evidence shows early thermophilesDNA evidence shows early thermophiles– Have advantage of more chemical energyHave advantage of more chemical energy– Deeper sea vents better protected against Deeper sea vents better protected against

bombardmentbombardment

Chemical reactions near Chemical reactions near deep-sea vents: Considered deep-sea vents: Considered the most likely sites where the most likely sites where life on Earth originatedlife on Earth originated– ““Black smokers”Black smokers”

The Origin and Evolution The Origin and Evolution of Life on Earthof Life on Earth

Chemistry to lifeChemistry to life

Search for Self-Replicating Search for Self-Replicating MoleculeMolecule• Work backward from organisms that live Work backward from organisms that live

todaytoday

• DNA is double-stranded = complicatedDNA is double-stranded = complicated

• RNA obvious candidate, simpler than DNARNA obvious candidate, simpler than DNA– Hereditary informationHereditary information– Can serve as template for replicationCan serve as template for replication– Fewer steps to produce backbone structureFewer steps to produce backbone structure

• Problem: RNA and DNA require enzymes to Problem: RNA and DNA require enzymes to replicate (chicken and egg problem?)replicate (chicken and egg problem?)

• In 1980’s determined that RNA might In 1980’s determined that RNA might catalyze their own replication instead of catalyze their own replication instead of other enzymesother enzymes

• Conclusion: Early Earth-life probably used Conclusion: Early Earth-life probably used RNA to encode its structureRNA to encode its structure

Search for Self-Replicating Search for Self-Replicating MoleculeMolecule

• Early Earth: short strands of RNA-like Early Earth: short strands of RNA-like molecules produced spontaneously partially molecules produced spontaneously partially or completelyor completely

• RNA-like molecules that could replicate faster RNA-like molecules that could replicate faster with less errors soon dominated populationwith less errors soon dominated population

• Copying errors introduced mutations, Copying errors introduced mutations, ensuring the production of many variations ensuring the production of many variations of successful moleculesof successful molecules

• Allowed molecular evolution to continueAllowed molecular evolution to continue

• RNA-world gave way to DNA-worldRNA-world gave way to DNA-world– DNA less prone to copying errorsDNA less prone to copying errors– DNA more flexible hereditary materialDNA more flexible hereditary material– RNA kept some of its original functionsRNA kept some of its original functions

Assembling Complex Organic Assembling Complex Organic MoleculesMolecules• Organic soup was too dilute to favor the Organic soup was too dilute to favor the

creation of complex organic moleculescreation of complex organic molecules

• Lab experiment with possible solution: Lab experiment with possible solution: When hot sand, clay or rock is placed in When hot sand, clay or rock is placed in dilute organic solution, complex molecules dilute organic solution, complex molecules self-assembleself-assemble– Organic molecules stick to surface of clayOrganic molecules stick to surface of clay– Increases density and likelihood of reactionsIncreases density and likelihood of reactions– Strands of RNA up to 100 bases have been Strands of RNA up to 100 bases have been

spontaneously produced this wayspontaneously produced this way

• Other inorganic minerals may have also Other inorganic minerals may have also had a similar rolehad a similar role

• Iron pyrite (fool’s gold)Iron pyrite (fool’s gold)– Positive charges on surface which allows Positive charges on surface which allows

organic molecules to adhereorganic molecules to adhere– Formation of pyrite releases energy which Formation of pyrite releases energy which

could be used as fuel for chemical reactionscould be used as fuel for chemical reactions

Early Cell-like Early Cell-like StructuresStructures

• There are There are advantages to advantages to enclosing enzymes enclosing enzymes with RNA moleculeswith RNA molecules

• Their close proximity Their close proximity increases the rate of increases the rate of reactions between reactions between themthem

• Also, isolates Also, isolates contents from contents from outside worldoutside world

• Lab experiments suggest that Lab experiments suggest that membrane structures existed on membrane structures existed on early Earthearly Earth

• These form spontaneously when…These form spontaneously when…– Cool down warm-water solution of Cool down warm-water solution of

amino acidsamino acids– Mix lipids (fats) with waterMix lipids (fats) with water

Nonliving Pre-Cells have Lifelike Nonliving Pre-Cells have Lifelike BehaviorBehavior• Grow in size until unstable Grow in size until unstable

then split to form a then split to form a ‘daughter’ cell‘daughter’ cell

• Selectively allow other Selectively allow other types of molecules to pass types of molecules to pass in/out of membranein/out of membrane

• Store energy in the form of Store energy in the form of electric voltageelectric voltage

Brief SummaryBrief SummarySynthesis of organic precursor molecules

Origin of self-replicating RNA

Origin of enclosed pre-cells

Origins of true cells with RNA genome

Evolution of modern cells with DNA genome

Migration of Life to Earth?Migration of Life to Earth?“Panspermia”“Panspermia”

Proposal: “Seeds of life” Proposal: “Seeds of life” exist everywhere around exist everywhere around the universethe universe

Life on earth started when Life on earth started when these ‘seeds’ came here, these ‘seeds’ came here, probably by a meteor. probably by a meteor.

It also suggests that these It also suggests that these seeds are taken to other seeds are taken to other habitable places in the habitable places in the universe.universe.

Panspermia argument:Panspermia argument:

Against:Against:– No atmosphere or water in spaceNo atmosphere or water in space– Solar and stellar radiation hazards in spaceSolar and stellar radiation hazards in space

For:For:– organic material is everywhere, and some organic material is everywhere, and some

bacteria can withstand large amounts of bacteria can withstand large amounts of radiation and go dormant under low radiation and go dormant under low atmospheric conditionsatmospheric conditions

– Fact: amino acids are found in some meteoritesFact: amino acids are found in some meteorites– Question is not “could” but “did” life originate Question is not “could” but “did” life originate

elsewhereelsewhere

Panspermia: 2 schools of thoughtPanspermia: 2 schools of thought

• School 1School 1: life did not evolve as easily : life did not evolve as easily as imagined on early Earth in as imagined on early Earth in timescales we’ve determinedtimescales we’ve determined

• School 2School 2: life evolved easily and was : life evolved easily and was everywhere with suitable conditionseverywhere with suitable conditions

• Problem: entire solar system was under Problem: entire solar system was under

heavy bombardment at the same time - heavy bombardment at the same time -

hard to form life quickly in another hard to form life quickly in another

location in Solar System location in Solar System

• Other possibility: interstellar migrationOther possibility: interstellar migration

• Problem: rock to be ejected out of its own Problem: rock to be ejected out of its own

system, then fall into ours and hit the tiny system, then fall into ours and hit the tiny

planet of Earth - very dificultplanet of Earth - very dificult

School 1School 1: : life did not evolve as easily life did not evolve as easily as imagined on early Earth in as imagined on early Earth in timescales we’ve determinedtimescales we’ve determined

• Earth was not first planet with Earth was not first planet with suitable conditionssuitable conditions

• Migration of life from another planet Migration of life from another planet (say Mars) dominated before early (say Mars) dominated before early life on Earth couldlife on Earth could– We’re Martians!!!!We’re Martians!!!!

School 2School 2: life evolved easily and was : life evolved easily and was everywhere with suitable conditionseverywhere with suitable conditions

• Martian meteoritesMartian meteorites

• Fossil evidence of life on Mars or Fossil evidence of life on Mars or

geochemical structure? geochemical structure?

Early Evolution and Rise of Early Evolution and Rise of OO22• First organisms had simple metabolismFirst organisms had simple metabolism

• Atmosphere was OAtmosphere was O22 free, life must have free, life must have been anaerobicbeen anaerobic

• Probably chemoheterotrophsProbably chemoheterotrophs– Obtained nutrients from organic materialObtained nutrients from organic material– Obtained energy from inorganic materialObtained energy from inorganic material

Modern archaea appear to be close to the root of the Modern archaea appear to be close to the root of the tree of lifetree of life

Obtaining energy from chemical reactions involving Obtaining energy from chemical reactions involving hydrogen, sulfur and iron compounds (all abundant hydrogen, sulfur and iron compounds (all abundant on early Earth)on early Earth)

Early EvolutionEarly Evolution

• Natural selection probably resulted in Natural selection probably resulted in rapid diversificationrapid diversification

• Modern DNA has enzymes that reduce the Modern DNA has enzymes that reduce the rate of mutationsrate of mutations

• RNA is not so lucky, more likely to have RNA is not so lucky, more likely to have copying errorscopying errors

• Higher mutation rate in early evolution Higher mutation rate in early evolution than nowthan now