Origins of Life
Why is there such a diversity of living forms yet all
life shows remarkable similarities:
1. genetic code = DNA
same basic code for all forms of life
2. common chemicals & metabolic pathways
eg. 20 amino acids used by all life
3. all made of cells
cell structures are amazingly similar
only two basic forms:
procaryotes and eucaryotes
Darwin provided a mechanism that showed all lfe was interrelated
But., how did it all start?
Darwin called this the “mystery of mysteries”
since Darwins time we have learned how to very
accurately date rocks & fossils by radioactive
decay
Decay System
Half Life
Rubidium Strontium 1.42X10-11 yr-1
Lutetium  Hafnium 1.94x10-11 yr-1
Uranium Lead 1.55 X10-10 yr-1
Potassium  Argon 0.581 X10-10 yr-1
14
C
5568±30 yr3
Useful Timespan
48.8 BY
35.7±1.2BY
4.47BY
1.93BY
from the Fossil record we have learned:
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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all forms of life didn’t appear at the same time
life progressed from small, simple forms to
more complex and larger forms
the early earth was very different from today
Previous ideas on the Origin of Life
Until mid 1800’s it was generally thought that life could
arise by spontaneous generation (600BC-1800’s)
=Nonliving matter had capacity within itself to
turn into certain types of living organisms
eg: moist soil
manure
fruits



toads, snakes, mice
flies
molds
believed there was some “vital force” in the elements
that could transform matter
Late 1600’s: Redi performed experiments that dealt
1s t major blow to this theory
Later: Spallanzani
-- sealed flasks
Pasteur: Final downfall of the idea
showed it included microrganisms
BUT: This left biologists with no plausible hypothesis
for the Origin of Life until fairly recently
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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How quickly did it happen?
some believe that the spontaneous origin of life would
be impossible:
the odds of life arising on the early earth are about the
same as the odds of a tornado going through a
junkyard and randomly assembling a fully functional
jumbo jet, 747
others believe that life began as something very
simple and only after billions of years did it
achieve the great complexity exhibited today
if the origin of life occurred relatively quickly after we
can surmise that conditions were suitable it would
indicate that it did indeed begin very simply and
that natural laws are conducive to its origin given
the suitable conditions and materials; that the
origin of life is inevitable under the proper
conditions
if life didn’t appear on earth for several billion years
afterwards then it would imply that the origin of
life is an extremely unlikely event and may have
implications for finding life on other planets
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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Major Steps Needed for Life to Originate:
1. Suitable Environment
2. Formation of Basic Building Blocks
3. Metabolism & the Formation of Large
Polymers
4. Compartmentalization; ie. Cells
5. Operating Instructions
These are probably not sequential events:
most or all probably occurred at the same time
eg. interstate system: none  fully developed
not: first roads, then gas stations, state police
system, then dealerships, etc
Our thoughts on what the first cell were like can only
be based on what cells are like today
it is possible the the first cells were considerably more simple and
inefficient and perhaps very little like cells around today;
over billions of years more complex and more efficient cells
would have evolved while such early primitive cells went
extinct with out leaving any fossil record
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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1. Suitable Environment
4.6 BY:
the sun is a second or third generation star
even today, a star explodes each second with a
brightness greater than a galaxy
 all matter on earth has been through 1 or 2
previous cycles of “stellar alchemy”
sun and planets form from previous supernova
explosion producing cloud of dust and gas
heavy bombardments by comets and meteorites
(size of Ohio) – each impact would have
caused any liquid water to boil away
completely
when the moon formed by a grazing impact of a mars-sized
body it would have completely destroyed any life that might
have arisen
even today 40,000 tons of space debris fall to earth each
year, mainly as dust
surface temperatures up to 1000-3000º C
no solid ground
no liquid water (no oceans, lakes)
only steam from geysers, volcanoes
4.0 -3.9 BY:
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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intensive bombardment slowed
crust cooled and solidified
cooled enough for liquid water to collect in basins
intense storms
atmospheric water condensed to form oceans
most of this water came from volcanic activity
up to 1/3rd of water on the earth (and in us) may have
come from comets
even today 10 M small comets hit earth each yr
 each comet = wt of 60 compact cars together
if this is same as rate throughout earths history it would
equal the volume of today’s oceans
earth’s early atmosphere was derived mainly
from volcanoes:
contained: H2 O, CO2 , N2 , CO, H2 , H2 S,
HCl, HCN (reducing atmosphere)
[todays atm: 78% N2 , 20% O2 , 4% H2 O, 0.03% CO2 ]
CHON all major atoms were available
early earth still very energetic:
lots of volcanic activity
severe lightning storms
unshielded solar radiation: xrays and UV
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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more radioactive materials
most agree there was a suitable environment
for life by 3.9 BY ago
How quickly did life happen?
unfortunately, there is no rock left on the surface of
the earth older than 3.85 BY due to earth’s active geology
 everything older has been lost to weathering and
continental drift
some suggest that the best place to look for signs for the
origin of life are on the moon
some estimate there are thousands of tons of earth
rock on the moon
if we can find rocks older than 3.5 BY we might be able
to find fossils or signs of life in them
oldest “trace fossils” = 3.85 BY, Greenland
1s t true fossils appear 3.5 BY
life must have begun 4.0 to 3.8 BY ago
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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2. Formation of Basic Building Blocks
~4.0-3.9 BY ago there were ideal conditions for
chemical reactions that could produce small
organic molecules:
sugars, amino acids, nucleotides, fatty acids, etc
The comets and meteorites that helped to form the
early earth also brought organic molecules
eg. amino acids are fairly common in these objects
Miller and Urey (1953) and others later modeled the
composition of the early ocean & atmosphere and
used electrical sparks to mimic energy
produced:
all 20 amino acids
several sugars
lipids
purines and pyrimidines
short chains of nucleotides
ATP
short chains of nucleotides
Miller and others (early 1950’s) could produce building
blocks of virtually all small organic molecules
given early atmosphere and energy
these same reactions could also have happened at
deep ocean vents on early earth
but generally cannot occur in the presence of O2
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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3. Metabolism & the Formation of Large
Polymers
even a single living cell consist of 1000’s of highly
organized chemical reactions, all occurring at the
same time
all these reactions together are called metabolism
metabolism consists of 2 main kinds of reactions:
synthesis reactions = building larger molecules
(polymers) out of smaller ones
eg. sugars
amino acids
fatty acids
nucleotides




starches
proteins
lipids
nucleic acids (DNA & RNA)
decomposition reactions = breaking apart large
molecules into smaller ones
synthesis reactions are used for building cell parts,
growth, reproduction, etc
requires energy to to synthesis
decomposition reactions are used to get rid of worn
out parts, to produce building blocks for synthesis
and to produce energy for the synthesis reactions
in living cells today, each of these reactions requires a
specific enzyme (= organic catalyst)
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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 but enzymes are proteins (large polymers);
proteins weren’t around yet
without enzymes, need high temperatures or some
kind of catalyst to do these reactions
there were many high energy sources in these
early days:
lots of volcanic activity
horrendous thunder & lightning
lots of UV radiation (earth had no ozone
shield)
meteorite impacts produce enough energy
to form polypeptides
some believe, given enough time, such reactions were
inevitable on early earth
eg. near volcanic activity?
eg. hydrothermal vents?
also, HCN was common then and is commonly
used as a catalyst in chemistry labs
over millions of years large polymers could
conceivably be constructed
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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the main problem is that such large organic polymers
are unstable and under these intense conditions
would have tended to break down fairly quickly
there had to be some way to make these
polymers more stable???
another question is how any reactions got organized
into something we could call metabolism
one of the most basic, most important, and most
widespread set of reactings in metabolism in
all cells is something called the Krebs cycle or
citric acid cycle
this is a set of 11 reactions between very small
organic molecules
it is central to most other metabolic pathways
in all cells
and it can lead to the formation of amino
acids, sugars, lipids and nucleic acides (all
major kinds of organic molecules)
it is conceivable that if this small pathway was
randomly formed it could have fairly easily
evolved into many of the much more complex
pathways we see in cells today
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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4. Compartmentalization; ie. Cells
Compartmentalization and isolation of chemical
stew
the internal chemical environment must be
isolated from external environment
large organic molecules must be enclosed and
contained or at least collected together in one
place
must have high concentrations of interacting
organic molecules to approximate
metabolism in living organisms
this container must be able to absorb nutrients and get
rid of waste products
= selective permeability
possible kinds of early compartments:
a. protein &/or lipid spheres (proteinoid
microspheres)
lipids, especially phospholipids can form
bilayered membrane similar to cell
membranes
can self assemble into lipid bilayer
can grow
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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can engulf other spheres
produces a selective permeable membrane
some can store energy; ie. membrane
potential
some of the proteins can act as catalysts
b. ocean foam (bubbles)
mixture of organic chemicals
5% of ocean’s surface today is covered with
foam
can collect and concentrate many materials
c. clay particles
could absorb and concentrate them
eg. clay concentrates amino acids
eg. clay also collects iron and zinc which can help to
catalyze such reactions
Iron Pyrite around hydrothermal vents could
also do that
evolving procell would require a controlled and
constant source of energy to synthesize complex
organic molecules
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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sugars are used today to make ATP
must assume ATP was available then
can be made if phosphates are present
Evolution must have started “before” life
actually arose
even without life would get a kind of chemical
natural selection
those droplets that were most stable and best
able to accumulate organic molecules would
grow and split
other droplets would fall apart or fail to grow
and divide
the “environment” would “select” for some
over others
may have happened in tidepools, on clay, at deep
ocean vents??
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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5. Operating Instructions
need instructions = blueprint
must be highly organized
must be reproducible
Origin of heredity
main problem today in understanding origin of life is
explaining how these collections of organic
chemicals could become organized into living self
autonomous, reproducing cells
Life as Information Storage
All life consist of chemicals in very organized patterns
yet laws of nature says that things tend toward
disorder, not order
one way to get an idea of the likelihood of life
appearing is to consider how ordered life is
compared to nonliving matter
 what is needed to make order out of
disorder
we can view the genetic code as “information storage”
of data bytes (1 byte = 8 bits)
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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Tandy 1000
 256 kilobytes 100,000’s bytes
early Pentiums & power PC’s megabytes
millions of bytes
today’s computers
 gigabytes
billions of bytes
eg. DNA in human cells contains
of RAM in each cell
~1 gigabyte
eg. E. coli contains
~1 megabytes
eg. the simplest organism capable of
independent existence has 200 genes;
therefore
~ 60 kilobytes
eg. George Church of Harvard Medical
School calculated that the theoretical
minimum number of genes a cell
needs is 151 (no junk DNA) or
~45 kilobytes
eg. even viruses with 2% as much genetic
info as E coli have
~15 kilobytes
eg. one typical gene contains
~ 300 bytes
based on natural laws, the early pre life earth could
have accumulated only up to ~25 bytes of
information by pure chance
if the first cell needed 50 kilobytes or more of
information it is highly unlikely that this would
have occurred on the early earth by pure chance
in laboratory biologists have succeeded in producing very small DNA
molecules of about 50 nucleotides from a mixture of simple
building blocks
 much less than that neede for even 1 gene
if life on earth had a spontaneous origin there must
have been some intermediate state that sped up
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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the ability to accumulate information content.
In every cell today:
DNA stores the genetic code
RNA uses that code to make either
enzymes for metabolism
or
new copies of DNA for reproduction
DNA
RNA
enzymes
metabolism
new DNA
reproduction
A simpler genetic code?
metabolism is the result of the activity of 10’s of
1000’s of different enzymes
enzymes are some of the largest, most complex
organic molecules in a cell
computer analysis indicates that today’s enzymes
could have originated from a very few, much
simpler enzyme molecules constructed with a
much simpler genetic code than the one in cells
today
A simpler molecule than DNA?
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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many believe that for life to appear some simpler
genetic material had to be there 1st
 probably RNA appeared 1st
1. RNA is a much simpler molecule than DNA
2. RNA carries the same kind of genetic
information as does DNA
3. RNA is able to replicate itself
4. all life requires RNA to make DNA
5. all life requires RNA to make proteins
(enzymes)
6. RNA can act directly as enzyme or
catalyst
7. some viruses have only RNA as genetic
material and are able to function
8. all components of RNA: sugar, phosphate and N
bases could have been formed under primitive
earth conditions
the packaging of RNA “genes” and their enzyme
products within some kind of container would have
been a significant milestone in the origin of life:
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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would now have had all basic requirements
for life together and interacting
these simple cells could then “evolve” as units
favorable interactions could have been selected,
preserved and perpetuated
 Natural Selection could occur
RNA “life” could have arisen ~ 4BY ago and lasted for
~ 200MY
once DNA appeared, natural selection would have
greatly favored it over RNA
eventually ALL the RNA only cells would have
gone extinct, leaving no trace
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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What were the first cells:
1. 1st cells were Prokaryotes
the earliest fossils we find are prokaryotes
the only fossils we find for the first 2 Billion
years of life are prokaryotes
small, simple, inefficient metabolism, little
internal structure
2. First cells were heterotrophs
all life requires nutrients and energy
all life produces the energy they need by breaking
down sugars and other organic molecules
= respiration
some organisms are able to make their own
sugars
= Autotrophs (=”self feeders”)
other organisms must consume preformed organic
molecules to break down for energy
= Heterotrophs ( ~”feed on others”)
of the two, autotrophs require considerably more
cellular “machinery” to make their own sugars
more genes are required; more complex
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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3.9 BY ago the oceans were thick with 100’s
Millions of years of accumulated organic
chemicals
There were plenty of organic molecules after 100’s
of millions of years of earth history
life didn’t require primary producers
 the planet was awash in “food”
 First cells were simpler heterotrophs
 able to capitalize on this abundance of
nutrients
3. First cells produced energy anaerobically
there was no free oxygen (O2) on the early earth,
either in atmosphere or in oceans
the breakdown of sugars to produce energy
= respiration
respiration can occur with or without O2
 with O2 the process is very efficient
but more complex
requires additional pathways and
enzymes, ie. more genes
= aerobic respiration
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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 without O2 some energy can be made
it is a much simpler process, fewer
enzymes required, fewer genes
= anaerobic respiration
but nowhere near as efficient
( 2 units of energy vs 34 units of energy per
sugar molecule)
first cells were anaerobic heterotrophs who
broke down sugars without the use of O2 gas
they took organic molecules, extracted
energy from them and produced organic
molecules as waste products
 this is the simplest form of energy
production
= fermentation
fermenters excrete acids, alcohols, etc.
(chemicals that contain less energy than the food ingested)
vs aerobic respiration that produces CO2 & H2 O
4. 1st cells probably used RNA as genetic instructions
[what is life?  bacteria and their descendants]
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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Where did life begin?
A. shallow warm water
B. deep ocean thermal vents
C. some other planet or solar system
Evidence:
A. Shallow warm water
on soft clay sediments
habitat closest to suspected energy source needed
for “prelife” events
some of earliest fossils are stromatolites: banded
domes of sedimentary rock and bacterial mats
these form today in shallow salt marshes and
warm lagoons
consist of colonies of bacteria and cyanobacteria in
jellylike secretions interspersed with sediment
layers
produces a banding pattern
B. Deep Ocean Thermal Vents
less exposed seafloor and
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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molecular biology supports idea that earliest life
thrived in hot conditions and may have used
sulfur compounds – common conditions in vent
communities
structure of proteins and genes resembles those
found in bacteria in such habitats today seem
to be precursors to other genes
C. some other planet or solar system
=panspermia; ie. seeded from space
100’s to 1000’s of meteorites and comets
brought organic molecules from space which
could have jump started the process
some suggest bacteria may have traveled
from other places (Mars meteorite-now
disproven)
bacteria, esp spores, can survive very harsh
conditions
eg. some bacteria (Microbispora) that were on the shuttle
Columbia, survived the fiery reentry
but: but only traveled about 1/5th the speed of a
meteorite
still survived extreme heat and impact
that was 6 times faster than anything tested before
Life, Biodiversity, History: Origins of Life, Ziser, Lecture Notes, 2009
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 just defers the problem
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