Origins of Life

George Lebo

16 October 2012

AST 2037

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How Did Life Come About?

• First things first: I don’t know!

• Second: Anyone who says they have a proven scientific explanation (currently) is probably selling something!

• That said … there ARE some things we know, and some we strongly suspect

• From them, we can at least TRY to put together a rough sketch of how life probably arose here on Earth

• Let’s do that!

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What do we have to work with?• In the beginning …

• OK, well, not really the beginning. More like:• About 8 billion years after the Big Bang• About 500 million years after the Solar System began to

form• About 4.6 billion years before TODAY

• What was Earth like?• Young, recently-solidified surface• Accretion of material from planetesimals nearing an end

(end of the “Early Heavy Bombardment”• How do we know?

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GREAT IMPACT

• AT THE END OF THE EARLY HEAVY BOMBARDMENT

• GREATER THAN MARS-SIZED OBJECT (THEIA) COLLIDES WITH THE EARTH

• EARTH IS TOTALLY MELTED, ALL LIFE (IF ANY) IS DESTROYED

• MOON FORMED OUT OF THE DEBRIS

• FOLLOWED BY WHAT WE CALL THE LATE HEAVY BOMBARDMENT

Earth: T – 4.6 Billion Yrs• Rocks were just solidifying on surface• How do we know? Age-dating of the oldest known rocks• From what? Radioactive isotope dating• Huh?• First: what’s an isotope?

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Elements and Isotopes• An “element” has a certain number of protons and electrons

• For instance, hydrogen (H) has 1 of each• Oxygen (O) has 8 of each• Carbon (C) has 6 of each

• “Isotopes” of a given element have the same number of protons/electrons, but different numbers of neutrons in the nucleus:

• “Normal” H has 0 neutrons, deuterium has 1 neutron, tritium has 2 neutrons – but ALL are still hydrogen

• O16 is “normal” oxygen, most common – has 8 protons and 8 neutrons (8+8 = 16)

• The positron emitter in PET scans is O15 • O18 is more rare (8 protons + 10 neutrons = 18)• C12 (6+6) is common, C14 (6+8) is rare – and radioactive!!

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Radioactive Decay• Many non- “normal” isotopes are radioactive, and

they “decay” into other elements• This process converts a “parent” to a “daughter”

isotope• This happens on a known timescale called the

“half-life” of the decay (the time it takes for ½ of the parent atoms to decay)

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Radioactive Age-Dating• So … by counting

parent/daughter atoms inside a rock, we KNOW how many half-lives since the rock solidified from magma

• We can measure the atomic half-life in a physics lab (or, even calculate it from quantum physics these days)

• Then, we know HOW OLD the rock is …

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Some Handy Decays

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Parent IsotopeStable Daughter

Product Half-Life

Uranium-238 Lead-206 4.5 billion yr

Uranium-235 Lead-207 704 million yr

Thorium-232 Lead-208 14.0 billion yr

Rubidium-87 Strontium-87 48.8 billion yr

Potassium-40 Argon-40 1.25 billion yr

Samarium-147 Neodymium-143 106 billion yr

Complete Uranium-238 Decay Chain

Earth: T – 4.4 Billion Yrs• Atmosphere & oceans – non-existent!!• How do we know? Rocks formed back then had very little

“volatiles” in them (i.e. H, H2O, O2, etc.)• What happened to volatiles? Solar wind

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How Did We Get Oceans?• From Outer Space! Comet/Ocean Theory:

• Comets (big balls of ice) crash into baby Earth• Crash melts/vaporizes the ice• Once the steam cools, it condenses• The liquid water flows “downhill” and pools together• This makes oceans• Also brings lots of other “volatile” materials

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Deuterium Issue Resolved!• We know from meteors & space probes that the inner Solar

System has more heavy isotopes than the outer Solar System• We think this is due to the solar wind

• Almost all known comets today are in the outer SS• But, back in the day, inner SS would have had comets too

(those are the most likely to hit Earth in the Early Heavy BB!)

• Suggestion: Maybe inner SS comet water would have deuterium abundance like Earth’s ocean water (?)

• In 2005, Gemini Observatory measured deuterium abundance from H20 in asteroid belt comets matches Earth water !!!

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What was our Young Atmosphere Like?

• Unbreathable!• Mostly carbon monoxide (CO), carbon dioxide (CO2),

nitrogen (N2) and water vapor (H2O)• How do we know? Rock chemistry from that time period

shows these compounds

• But … no O2

• Note: free oxygen is very “aggressive” in forming chemical bonds and does bad things to many chemicals (i.e. iron rusts!)

• So … even a little O2 would be pretty obvious in these rocks it just wasn’t there!

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Summary So Far

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Zircon CrystalRadiometrically Dated (They contain

traces of Uranium and Thorium)

Acasta Gneiss Rock Formation(Gneiss means Metamorphic Rock)

Oldest Known Exposed Rock(Dated by embedded Zircons)

Discovered in 1989 in the Acasta River near Great Bear Lake

Then … Life Appears• First fossil cells found in rocks

at about T – 3.7 to T – 3.5 Billion Years!

• Tiny little things• Not O2 breathers like us (none

around!)• Probably CO2 breathers• Modern cyanobacteria look a lot

like these fossils AND they are CO2 breathers

• Suggests that the first (fossil) life may have been cyanobacteria (?)

• “Cyano” means blue

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HOW Did Life Appear?

• It must have formed SOMEHOW!• What do we need?

• Atmosphere – got one!• DNA or something like it – not obviously there (!)• COULD DNA form back then?• Need amino acids, sugars, phosphates DNA

building blocks• Could THEY form?

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Urey-Miller Experiment

• Basic idea:• Take a bunch of chemicals as

known to present in the early atmosphere & ocean

• Put them in a chemistry lab setup with circulating gases

• Simulates “Primordial Soup”• Zap the whole thing with

electric discharge (like lightning!)

• See what happens …

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POTENTIAL PROBLEMS WITH MILLER-UREY RESULTS

• ATMOSPHERE OF EARLY EARTH MAY NOT HAVE CONTAINED AS MANY REDUCING CHEMICALS AS ORIGINALLY THOUGHT.

• BOTH LEFT-AND RIGHT-HANDED AMINO ACIDS (MOSTLY GLYCINE AND ALANINE) WERE MADE.

• THERE WOULD HAVE BEEN LITTLE LIGHTNING ON THE EARLY EARTH

• THE EARLY EARTH ENVIRONMENT WAS HOSTILE TO AMINO ACIDS. ONCE THEY WERE CREATED THEY FACED DESTRUCTION.

1.Random addition of energy is destructive, not creative.

2. Photo-dissociation of water by UV in the early atmosphere would have injected oxygen into it.

3. UV would break up the amino acids as quickly as they were made.

4. In the early-Earth oceans the created amino acids would have suffered thermal decay at the temperatures (T~25C) they were thought to have.

Urey-Miller: Results• What did they find?

• Amino Acids!!! (Lots of them!)

• More specifically:• 13 amino acids used in life; (both L- and R- type)• Sugars• Lipids• About 10-15% of the carbon ended up in protein

structures like this

• Problem: We now believe that the atmosphere at that time was not as reducing as originally thought.

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Meteor Aminos• The Murchison Meteorite is a big chunk of space rock

(Found near Murchison, Victoria, Australia, 7/28/69)• Chemical analysis shows: Amino Acids!• It is another source of aminos!• Diversity?

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List of Murchison Amino AcidsAmino Alkanoic Acids

2 Carbon:Glycine

3 Carbon:Alanineb-alanineSerineSarcosine

4 Carbon:Threoninea-Aminobutyric Acidb-Aminobutyric Acidg-Aminobutyric Acida-Aminoisobutyric Acidb-Aminoisobutyric AcidN-EthylglycineN,N-dimethylglycineN-MethylalanineN-methyl-b-alanine

5 Carbon:ValineIsovalineNorvalineProlineMethionine3-Amino-2-ethylpropanoic Acid3-Amino-2,2-dimethylpropanoic Acid3-Amino-2-methylbutanoic Acid

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3-Amino-3-methylbutanoic Acid4-Amino-2-methylbutanoic Acid4-Amino-3-methylbutanoic AcidAllo-3-amino-2-methylbutanoic Acid3-Aminopentanoic Acid4-Aminopentanoic Acid5-Aminopentanoic Acid

Amino Dialkanoic Acids

4 Carbon:Aspartic Acid

5 Carbon:Glutamic Acid2-Methylaspartic Acid3-Methylaspartic AcidAllo-3-methylaspartic AcidN-Methylaspartic Acid

6 Carbon:a-Aminoadipic Acid2-Methylglutamic Acid

7 Carbon:a-Aminopimelic Acid

Amino Alkanoic Acids

6 Carbon:LeucineIsoleucineAlloisoleucine

NorleucinePseudoleucineCycloleucine2-Methyl-norvalinePipecolic Acid2-Amino-2-ethylbutanoic Acid3-Amino-2-ethylbutanoic Acid*2-Amino-2,3-dimethylbutanoic Acid3-Amino-2,3-dimethylbutanoic Acid*4-Amino-3,3-dimethylbutanoic Acid*3-Amino-3-methylpentanoic Acid*4-Amino-2-methylpentanoic Acid*4-Amino-3-methylpentanoic Acid*4-Amino-4-methylpentaoic Acid*3-methylamine-pentanoic Acid*4-Aminohexanoic Acid*

7 Carbon:2-Amino-2,3,3-trimethylbutanoic Acid2-Amino-2-ethyl-3-methylbutanoic Acid2-Amino-2-ethylpentanoic Acid2-Amino-3-ethylpentanoic Acid2-Amino-2,3-dimethylpentanoic Acid2-Amino-2,4-dimethylpentanoic Acid2-Amino-3,3-dimethylpentanoic Acid2-Amino-3,4-dimethylpentanoic Acid2-Amino-4,4-dimethylpentanoic AcidAllo-2-amino-2,3-dimethylpentanoic Acid*Allo-2-amino-3,4-dimethylpentanoic Acid2-Amino-2-methylhexanoic Acid2-Amino-3-methylhexanoic Acid2-Amino-4-methylhexanoic Acid2-Amino-5-methylhexanoic AcidAllo-2-amino-3-methylhexanoic Acid*

Meteorites: Source for Life?• Note: L/R evenly made here too• Is this the source?• Probably not:

• Not that much amino abundance, and the compounds are stuck inside a rock

• To get enough on Earth, need lots of bombarding

• (but that melts rocks and destroys aminos)

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Got Aminos, etc. – Now What?

• Then, need to put them all together in polymer chains• “Polymerization” of the Primordial Soup• How … ??

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Polymerization• In order to polymerize organic compounds, we would need:• Stable environment

• No big temperature variations• No major mechanical shaking

• Lots of surface area• Points for the various organic compounds to attach

• Perhaps a pattern to it• Provides chemical/physical energy advantage for pattern

formation in the polymer too

• Where do we find that?

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Clays• Naturally-occurring silts made

from silicates• Clay in water can provide

steady temperature and protect anything inside from shaking/waves

• Tend to crystalline-like structures (patterns) with HUGE surface area

• Known to assist (“catalyze”) organic reactions in labs

• Could they be the place

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Life’s Little Irony• Stereotypical Creationist to Stereotypical Evolutionist:

God did it.arrogant You’re fool!• Stereotypical Evolutionist to Stereotypical Creationist:

You’re an arrogant fool!

• Question to both: How do you make humans?

• Creationist: God did it.shape God scooped up some clay, molded it to human, and breathed on it.

• Evolutionist: Well, see … first you get yourself some nice clay …

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Direct jump to DNA?• Maybe … but that is a lot of change of complexity in one hop!• RNA is simpler than DNA• Some critters (i.e. some viruses) seem to run on RNA-only• But … they seem to be dependent on DNA-bearing hosts for

survival (??)• At least opens the possibility of “RNA world” life, which then

evolved into more complex “DNA-world” we all know and love today

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Another theory• Panspermia: Proposed by Fred Hoyle who first

proposed the Steady State Theory of cosmology• Life is commonly present out in space, and was

carried to Earth as spores trapped in meteors• But … radiation issues make this seem a little less

likely

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Summary• Earth of Way Back When was different• We can tell from chemical and radio-isotope analysis of rocks• Water and other volatiles may have come from comet impacts• Life formed a long time ago – about 3.5 Billion Yr or so• We know from fossils• We don’t know exactly how, but …

• We know we had the right elements• Those elements + lightning make amino acids• DNA may have originated from these acids in a clay matrix

• Next question: How did things get from Then to Now??

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