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

The evidence indicates that life The evidence indicates that life formed quickly after the Earth formed quickly after the Earth formed.formed.

Within a few Within a few 100 million 100 million years years

Perhaps as Perhaps as short as 100 short as 100 million yearsmillion years

No Life Life?

How did we get here?

few hundred million years

How did life begin?How did life begin?

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

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

– Organic molecules are building blocks of Organic molecules are building blocks of

life. life.

– Low probability of forming life even if Low probability of forming life even if

repeated several times.repeated several times.

– Intermediate steps of high probability Intermediate steps of high probability

are necessaryare necessary

Chemistry to BiologyChemistry to Biology

Organic Chemistry on Early Organic Chemistry on Early EarthEarth

• In 1920’s, scientists hypothesized In 1920’s, scientists hypothesized that the chemicals in the early that the chemicals in the early atmosphere, fueled by sunlight, atmosphere, fueled by sunlight, would spontaneously create organic would spontaneously create organic moleculesmolecules

• Tested by Miller-Urey experiment Tested by Miller-Urey experiment 1950’s1950’s

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: With the exception Results: With the exception of when oxygen was present, of 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!

Sources of Organic MoleculesSources of Organic Molecules

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

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

chondrites show that they have organic moleculeschondrites show that they have organic molecules

• Chemical reactions near deep-sea Chemical reactions near deep-sea ventsvents– Heat from undersea volcanoes and vents can fuel Heat from undersea volcanoes and vents can fuel

chemical reactions between water and mineralschemical reactions between water and minerals

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 replicatereplicate

• 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

• Both have possible fossil evidence of life on Both have possible fossil evidence of life on

MarsMars

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

• Important new metabolic process evolved Important new metabolic process evolved graduallygradually

• Organisms living close to ocean surface Organisms living close to ocean surface probably developed means of absorbing probably developed means of absorbing sunlight (UV in particular)sunlight (UV in particular)

• Once absorbed, developed method of Once absorbed, developed method of turning it into energyturning it into energy– Modern organisms of purple sulfur bacteria and Modern organisms of purple sulfur bacteria and

green sulfur bacteria much like early green sulfur bacteria much like early photosynthetic microbes, use Hphotosynthetic microbes, use H22S instead of S instead of HH22O for photosynthesisO for photosynthesis

PhotosynthesisPhotosynthesis

• Using water for photosynthesis developed Using water for photosynthesis developed later, perhaps 3.5 billion years agolater, perhaps 3.5 billion years ago

carbon dioxide + water + light (energy) carbon dioxide + water + light (energy) →→ glucose + oxygen glucose + oxygen• First appearing in cyanobacteria (blue-green First appearing in cyanobacteria (blue-green

algae)algae)

• OO22 released into atmosphere released into atmosphere

•Changed the world!Changed the world!

The Rise of Oxygen in the The Rise of Oxygen in the atmosphereatmosphere

• OO22 is highly reactive is highly reactive• All initial OAll initial O22 would react with rock and would react with rock and

minerals in waterminerals in water• OO22 could not accumulate in atmosphere could not accumulate in atmosphere

until surface rock was saturateduntil surface rock was saturated• Rocks 2-3 bill. Yr old (banded iron Rocks 2-3 bill. Yr old (banded iron

formations), show atmosphere had <1% of formations), show atmosphere had <1% of current Ocurrent O22To this day, the To this day, the

majority of oxygen majority of oxygen produced over time is produced over time is locked up in the locked up in the ancient "banded rock" ancient "banded rock" and "red bed" and "red bed" formations.formations.

The Rise of Oxygen in the The Rise of Oxygen in the atmosphereatmosphere

• Clear evidence of OClear evidence of O22 near current levels near current levels appears only 200 million yr ago appears only 200 million yr ago – Find charcoal (fossil fuel)Find charcoal (fossil fuel)– Indicates enough OIndicates enough O22 in atmosphere for fires to in atmosphere for fires to

burnburn

The Oxygen CrisisThe Oxygen Crisis

• Rise of ORise of O22 would have created a crisis would have created a crisis for lifefor life

• OO22 reacts with bonds of organic reacts with bonds of organic materialsmaterials

• Surviving species avoided effects of OSurviving species avoided effects of O22 because they lived or migrated to because they lived or migrated to underground locationsunderground locations– Many anaerobic microbes found in such Many anaerobic microbes found in such

locales todaylocales today

Early EukaryotesEarly Eukaryotes• Fossil evidence dates to 2.1 bill. Yr agoFossil evidence dates to 2.1 bill. Yr ago

• Dates to when ODates to when O22 rising in atmosphere rising in atmosphere

• DNA evidence suggests that prokaryotes DNA evidence suggests that prokaryotes and eukaryotes separated from common and eukaryotes separated from common ancestor much earlierancestor much earlier

• OO22 played a key role in eukaryote evolution played a key role in eukaryote evolution– Cells can produce energy more efficiently using Cells can produce energy more efficiently using

aerobic metabolism than anaerobic metabolismaerobic metabolism than anaerobic metabolism– Adaptations of aerobic organisms could develop Adaptations of aerobic organisms could develop

adaptations that required more energy that adaptations that required more energy that would be available for anaerobic organismswould be available for anaerobic organisms

The Cambrian ExplosionThe Cambrian Explosion

• Animal branch of the tree of lifeAnimal branch of the tree of life

• Different classifications based on Different classifications based on

body planbody plan

• All known body plans made All known body plans made

appearance in fossil record in a time appearance in fossil record in a time

span of 40 million yearsspan of 40 million years– <1% of Earth’s age<1% of Earth’s age

– Animal diversity began 545 mill. Yr agoAnimal diversity began 545 mill. Yr ago

Colonization of LandColonization of Land

• Life flourished where liquid water Life flourished where liquid water existedexisted

• Life on land was more complicatedLife on land was more complicated– Had to develop means of collecting solar Had to develop means of collecting solar

energy above ground and nutrients belowenergy above ground and nutrients below

• Life in shallow ponds or edges of lakesLife in shallow ponds or edges of lakes– Water evaporatesWater evaporates

– Natural selection favored that which Natural selection favored that which could withstand periods of droughtcould withstand periods of drought

• DNA evidence suggests that plants DNA evidence suggests that plants

evolved from an algaeevolved from an algae

• It took only 75 mill. Yrs for animals to It took only 75 mill. Yrs for animals to

follow plants out of waterfollow plants out of water

Mass ExtinctionsMass Extinctions

Mass ExtinctionsMass Extinctions

• Possible CausesPossible Causes– ImpactsImpacts

• Impact sites found for K-T boundaryImpact sites found for K-T boundary

•Suspected for Permian extinction 245 mill yr Suspected for Permian extinction 245 mill yr agoago

– Active volcanismActive volcanism•Climate changeClimate change

– External influence for copying errorsExternal influence for copying errors• Increase in solar particles or radiation hitting Increase in solar particles or radiation hitting

surfacesurface

•Local supernovaLocal supernova

Primate EvolutionPrimate Evolution

• Monkeys, apes, lemurs and humans have Monkeys, apes, lemurs and humans have common ancestor that lived in treescommon ancestor that lived in trees

• Tree lifeTree life– Limber arms for swinging between branchesLimber arms for swinging between branches– Eyes in front of head for depth perceptionEyes in front of head for depth perception– Offspring would be born more helpless than Offspring would be born more helpless than

other animalsother animals

Emergence of HumansEmergence of Humans

• Did NOT evolve from gorillas or Did NOT evolve from gorillas or monkeysmonkeys

• Share a common ancestor that lived Share a common ancestor that lived just a few million years agojust a few million years ago

• 98% of human genome is identical to 98% of human genome is identical to genome of the chimpanzeegenome of the chimpanzee

• 2% difference in genome separates the 2% difference in genome separates the success of humans verses chimpssuccess of humans verses chimps– Also indicates evolution of intelligence is Also indicates evolution of intelligence is

complex complex

Emergence of HumansEmergence of Humans

Emergence of HumansEmergence of Humans

• After hominids diverged from chimps and After hominids diverged from chimps and gorillas, evolution has followed a complex gorillas, evolution has followed a complex pathpath

• Numerous hominids species existed, some Numerous hominids species existed, some during the same time periodduring the same time period– All humans are the same speciesAll humans are the same species

• First skull fossils that are identical to modern First skull fossils that are identical to modern human skulls dates to 100,000 yr oldhuman skulls dates to 100,000 yr old

• Our ancestors shared the Earth with Our ancestors shared the Earth with NeanderthalsNeanderthals– Went extinct 35,000 years agoWent extinct 35,000 years ago

Emergence of HumansEmergence of Humans

Cultural and Technological Cultural and Technological EvolutionEvolution• Have not undergone biological evolution in Have not undergone biological evolution in

40,000 years40,000 years– Mutation rates are slowMutation rates are slow

• Dramatic cultural changesDramatic cultural changes– Transmission of knowledge between generationsTransmission of knowledge between generations

• Spoken to written word, thousands of yearsSpoken to written word, thousands of years

• agricultureagriculture

• Technological evolutionTechnological evolution– Result of coupling between science and Result of coupling between science and

technologytechnology– About 100 years between industrial revolution to About 100 years between industrial revolution to

landing on the Moon and generating weapons of landing on the Moon and generating weapons of mass destructionmass destruction