An image of Saturn's ice-An image of Saturn's ice-spewing moon Enceladus spewing moon Enceladus taken from an altitude of 1,700 taken from an altitude of 1,700 kilometers by the Cassini. kilometers by the Cassini. Features the size of a bus are Features the size of a bus are resolvable in this image of the resolvable in this image of the active tiger stripe region of active tiger stripe region of Enceladus. Enceladus.

Although appearing dark in the Although appearing dark in the contrast-enhanced image, the contrast-enhanced image, the surface of Enceladus is surface of Enceladus is covered with some of the covered with some of the brightest ice in the entire Solar brightest ice in the entire Solar System, reflecting about 99 System, reflecting about 99 percent of the light it receives.percent of the light it receives.

Homework #5

Due Tomorrow, 6:00 pm

Exam #2

Wednesday, November 2

Homework #6 will be posted by tomorrow morning. It will be due by

6:00 pm, Monday, Oct. 31

Understanding the history of Understanding the history of life on Earth gives insight life on Earth gives insight

into likelihood and into likelihood and characteristics of life characteristics of life

elsewhereelsewhere

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

• Single celled organismsSingle celled organisms

• 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 ExplosionBegan: ~ Began: ~ 545 million Yr ago 545 million Yr ago Duration: ~ 40 million yearsDuration: ~ 40 million years

• Explosion of life formsExplosion of life forms

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

• Different classifications based on body Different classifications based on body

planplan

• All known body plans made appearance in All known body plans made appearance in

fossil record in a time span of 40 million fossil record in a time span of 40 million

yearsyears– <1% of Earth’s age<1% of Earth’s age

– Animal diversity began Animal diversity began 545 million Yr ago545 million 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 million Yrs for animals to It took only 75 million Yrs for animals to

follow plants out of waterfollow plants out of water

While life has expanded greatly in its While life has expanded greatly in its diversity, this has not always been the diversity, this has not always been the

case: case: mass extinctionsmass extinctions

Mass ExtinctionsMass Extinctions

The Earth has experienced The Earth has experienced five major five major mass mass

extinctionsextinctions in the past in the past half-billion years. half-billion years.

Mass ExtinctionsMass Extinctions• Possible CausesPossible Causes

– ImpactsImpacts• Impact sites found for K-T boundary Impact sites found for K-T boundary

– Chicxulub CraterChicxulub Crater

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

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

Life in the Solar System?Life in the Solar System?

Essentials for Essentials for lifelife

Chemicals / NutrientsChemicals / NutrientsEnergyEnergyLiquid solventLiquid solvent

•Chemical components of cells:Chemical components of cells:– OxygenOxygen– CarbonCarbon– NitrogenNitrogen– HydrogenHydrogen

These elements make up 96% of the These elements make up 96% of the mass of living organisms on Earth.mass of living organisms on Earth.

Where can we expect to find the Where can we expect to find the necessary ingredients/chemicals for necessary ingredients/chemicals for life?life?

• These elements are abundant in the Universe, These elements are abundant in the Universe, except in older star systemsexcept in older star systems

• As long as condensation and accretion occur As long as condensation and accretion occur during a star system’s formation, we can expect during a star system’s formation, we can expect these elements to be presentthese elements to be present

• Organic molecules (building blocks for amino Organic molecules (building blocks for amino acids) have been found in asteroids and cometsacids) have been found in asteroids and comets

What are the energy requirements for What are the energy requirements for life?life?

• SunlightSunlight (photosynthesis)(photosynthesis)

• Consuming organic molecules (ex. eating Consuming organic molecules (ex. eating plants)plants)

• Chemical reactions with inorganic Chemical reactions with inorganic compounds ofcompounds of iron, sulfur, or hydrogeniron, sulfur, or hydrogen

Sunlight?Sunlight?Light is most intense Light is most intense

near the sun (or near the sun (or another star)another star)

Light intensity falls Light intensity falls off with distance off with distance from the Sun/star, from the Sun/star, going as going as 1/(distance)1/(distance)22

Inefficient far from a Inefficient far from a starstar

Where can we expect to find energy for Where can we expect to find energy for life?life?

• Chemical Energy Chemical Energy requires a means requires a means for chemicals to for chemicals to mix & interactmix & interact

• Atmospheres and/or Atmospheres and/or bodies of liquids can bodies of liquids can provide this meansprovide this means

What to use as a solvent?

Liquids exist only in specific temperature and pressure Liquids exist only in specific temperature and pressure ranges. ranges.

On the surface of a planet or moon, temperature alone On the surface of a planet or moon, temperature alone is insufficient for the presence of liquids.is insufficient for the presence of liquids.

• On Earth, all life requires water:On Earth, all life requires water:

Water dissolves organic molecules so they can Water dissolves organic molecules so they can be used for chemical reactions in cellsbe used for chemical reactions in cells

Water transports chemicals into and out of cellsWater transports chemicals into and out of cells

Water is directly involved in many metabolic Water is directly involved in many metabolic reactions within cellsreactions within cells

On Earth, the only other common On Earth, the only other common liquid is molten rock (too hot)!liquid is molten rock (too hot)!

Alternative Liquids for LifeAlternative Liquids for Life(under 1 atmosphere pressure)(under 1 atmosphere pressure)

Substance Freezing

Temperature

Boiling Temperature

Width of Liquid Range

Water (H2O) 0 C 100 C 100 C

Ammonia (NH3) -78 C -33 C 45 C

Methane (CH4) -182 C -164 C 18 C

Ethane (C2H6) -183 C -89 C 94 C

Water remains liquid over broader and higher range of temperatures

The higher temperature range for liquid water allows faster rates of chemical reactions

On colder worlds On colder worlds (e.g., Titan), other (e.g., Titan), other liquids may be liquids may be more common, more common, such assuch as Ammonia NHAmmonia NH33

Methane CHMethane CH44

Ethane CEthane C22HH66

Chemical reactions Chemical reactions are highly are highly temperature temperature sensitive, so on sensitive, so on colder worlds with colder worlds with other liquids, other liquids, reaction rates may reaction rates may be too slow for life.be too slow for life.

ethane/methane lakes on Titan

• Advantages of water:Advantages of water:1.1. Water remains liquid over the widest range Water remains liquid over the widest range

of temperaturesof temperatures2.2. Water is liquid at a higher temperature than Water is liquid at a higher temperature than

the others. High temp makes chemical the others. High temp makes chemical reactions easier.reactions easier.

3.3. Water is less dense when it freezes (ice Water is less dense when it freezes (ice floats), allowing life to survive in the water floats), allowing life to survive in the water under the iceunder the ice

4.4. Ice insulates the water Ice insulates the water underneath it, making underneath it, making it less likely to totally it less likely to totally freeze, promoting freeze, promoting greater climate greater climate stabilitystability

5.5. Water is POLAR. Many Water is POLAR. Many substances can substances can dissolve in water, but dissolve in water, but many do not. Cell many do not. Cell membranes do not membranes do not dissolve in water. dissolve in water. Earth-like cells can Earth-like cells can only survive in water.only survive in water.

Water is the most likely candidate Water is the most likely candidate

liquid as a solvent in life.liquid as a solvent in life.

Searching for liquid water may be the Searching for liquid water may be the

best approach to searching for life. best approach to searching for life.

We do not know if other liquids can We do not know if other liquids can

support life.support life.