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
Parent Isotope
Stable 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
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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 Crystal
Radiometrically 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 Acids
Amino Alkanoic Acids
2 Carbon:
Glycine
3 Carbon:
Alanine
b-alanine
Serine
Sarcosine
4 Carbon:
Threonine
a-Aminobutyric Acid
b-Aminobutyric Acid
g-Aminobutyric Acid
a-Aminoisobutyric Acid
b-Aminoisobutyric Acid
N-Ethylglycine
N,N-dimethylglycine
N-Methylalanine
N-methyl-b-alanine
5 Carbon:
Valine
Isovaline
Norvaline
Proline
Methionine
3-Amino-2-ethylpropanoic Acid
3-Amino-2,2-dimethylpropanoic Acid
3-Amino-2-methylbutanoic Acid
3-Amino-3-methylbutanoic Acid
4-Amino-2-methylbutanoic Acid
4-Amino-3-methylbutanoic Acid
Allo-3-amino-2-methylbutanoic Acid
3-Aminopentanoic Acid
4-Aminopentanoic Acid
5-Aminopentanoic Acid
Amino Dialkanoic Acids
4 Carbon:
Aspartic Acid
5 Carbon:
Glutamic Acid
2-Methylaspartic Acid
3-Methylaspartic Acid
Allo-3-methylaspartic Acid
N-Methylaspartic Acid
6 Carbon:
a-Aminoadipic Acid
2-Methylglutamic Acid
7 Carbon:
a-Aminopimelic Acid
Amino Alkanoic Acids
6 Carbon:
Leucine
Isoleucine
Alloisoleucine
Norleucine
Pseudoleucine
Cycloleucine
2-Methyl-norvaline
Pipecolic Acid
2-Amino-2-ethylbutanoic Acid
3-Amino-2-ethylbutanoic Acid*
2-Amino-2,3-dimethylbutanoic Acid
3-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 Acid
2-Amino-2-ethyl-3-methylbutanoic Acid
2-Amino-2-ethylpentanoic Acid
2-Amino-3-ethylpentanoic Acid
2-Amino-2,3-dimethylpentanoic Acid
2-Amino-2,4-dimethylpentanoic Acid
2-Amino-3,3-dimethylpentanoic Acid
2-Amino-3,4-dimethylpentanoic Acid
2-Amino-4,4-dimethylpentanoic Acid
Allo-2-amino-2,3-dimethylpentanoic Acid*
Allo-2-amino-3,4-dimethylpentanoic Acid
2-Amino-2-methylhexanoic Acid
2-Amino-3-methylhexanoic Acid
2-Amino-4-methylhexanoic Acid
2-Amino-5-methylhexanoic Acid
Allo-2-amino-3-methylhexanoic Acid*
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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?
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•
•
•
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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