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BY2204.Lecture 6
Is this a good
definition of life?
SF Evolution
Life is an
amazing piece
of chemistry
The First Spark of Life
But how did we get from small molecules
floating in the primordial soup to anything as
complex and organised as my niece?
What is life?
Muller 1966 suggested:
4600mya
Earth
formed
Replication
Heredity
= LIFE
5000 million
years ago
4000mya
Life
4000
•  Photosynthesis
starts
Variation
BUT this includes prions,
and excludes viruses!
•  Snowball
earths
The History of Life
•  Oldest
prokaryotes
•  Multicellularity
evolves
•  Cambrian
explosion
3500mya
Oldest
fossil (?)
2700mya
Definitely
Prokaryote
s
•  Life started
•  Earth formed
•  Eukaryotes
appear
3000
3800mya
Oldest
rock
•  Oldest rock
2100mya
Eukaryotes
543mya
Cambrian
Explosion
1500mya
Multicellularity
2000
1000
Now
Snowball Earths
until 1500mya
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The History of Life
4600mya
Earth
formed
4000mya
Life
4000
5000 million
years ago
2700mya
Photosynthesis
starts, producing
oxygen
3500mya
Oldest
prokaryote (?)
Read more about
these in the “Proof of
Life” and “Life’s a
gas” articles
1st
2100mya
Eukaryotes
2000
1000
Snowball Earths
possibly until 650mya
Four stages in the start of life:
1)  How did the “building blocks” of life appear?
Amino Acids to make proteins
Sugars to make RNA’s backbone
Nucleotides to make the code of RNA
Lipids to make cell membranes X (easy!)
2)  How did these join together to make polymers?
3)  How did replication start?
4) How did the replication mechanism get inside a
lipid membrane to form a cell?
During the Hadean period these cooled
to a thin solid crust, like the skin on
cocoa and outgassing occurred.
543mya
Cambrian
Explosion
1500mya
Multicellularity
3/4 of life’s history was single celled!
a) 
b) 
c) 
d) 
Earth formed from an accretion cloud of
dust from the big bang, and was made
of molten rock.
Molten metals sank to the middle leaving
silicates on the surface
3000
3800mya
Oldest
rock
Refer to
yr
notes for
differences
between
prokaryote and
eukaryote cells
What sort of earth gave rise to life?
Now
Unstable and kept hot by
1) continued accretion,
2) meteor landings,
3) radioactive decay of rock
4) UV radiation from the sun.
Even the seas dried up repeatedly!
Stage 1: Origin of Amino Acids, Sugars and
Nucleotides
Earth had a reducing atmosphere
(because no oxygen).
This means chemicals were much more
reactive than now, but how much more?
When scientists thought it was very
reactive, Stanley Miller 1953 did a
very famous experiment exposing
water to that atmosphere, added
lightning …… and got all the building
blocks of life.
Evolution of life thought to be very easy!
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Then the geologists realised the timing
of outgassing was critical to the
composition of the atmosphere
Early, before the metals went to the core
and you get Miller’s very reducing
atmosphere.
Late, after the core was formed, and the
atmosphere was much less reducing so
the chemicals less reactive.
The geologists believe the outgassing
was late.
If you re-do Miller’s experiments with this
late atmosphere, you get…….. Nothing.
So how DO we get the building blocks of
life?
2) Extra-terrestrial
sources of life
Stage 1: Origin of Amino Acids, Sugars and
Nucleotides
We don’t know, but there are
a number of theories.
1) Creation by a
supernatural being.
e.g. Christians’ creation by God,
Aboriginal dreamtime stories, African
Bushman’s subterranean origins of
life, Iroquois creation story in which a
woman from the sky falls to earth
and the animals start terrestrial life
on the back of a giant turtle.
Always remember, these could be true. You can’t prove they
are not. Best to look at options open to science too.
3) Warm Shallow Pond Scenario
Deep sea vents
and estuarine
waters can have
very reducing
conditions
Did meteorites bring in
the building blocks?
Meteorites now often
carry hydrocarbons,
and amino acids,
made in reducing
conditions of space
Rarely survive transit through the atmosphere now, but
thinner atmosphere then.
Need a lot to seed the earth, but a lot of meteorites fell then.
This is quite a popular theory for the origin of aas and sugars.
Bernal 1967
Sea was anyway more reducing than it is now, and in estuaries,
clay makes it more so.
Organic molecules form:
HCN and formaldehyde
Sun
HCN
Amino Acids
Formaldehyde
UV
radiation
Sugars
sea
Clay in
estuary
The clay concentrates
these, and UV makes
them react to give sugars
and amino acids (no
nucleotides though!)
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Stage 2: Polymerisation
Pyrite particles in raindrops in clouds
work just like clay in pools, so maybe
polymerisation started in clouds
Need to get concentrations of the
building blocks to allow polymerisation.
Clay can again help hold onto
chemicals.
In boiling mud the amino acids form
blobs of all one type of protein, called
proteinoid microspheres
These float off and cool to
make insoluble stable
“cells” of protein, but with
no cell structure of course.
Similar chains
stick together
Polymerising amino
acids
Boiling clay
Lipid World
Or in puddles where the
building blocks could build
up local concentrations
Proteinoid
microspheres
break off
read more about this in “Origin of Life” reference
Lipids (fats) in water form
themselves into little bubbles
called micelles.
Interestingly, these have
heredity, because when they
split the daughter micelles
contain the same mixture of
lipid molecules as the parent
did.
The mycelles also stick nucleotides to their outsides so help
with the polymerisation process.
This is seen as one route to both ploymerisation and getting
polymer nucleotides into a cell
It’s even been suggested that a very early
snowball earth occurred and the remaining
liquid water concentrated the building blocks.
Stage 3: Replication
(So far just chemistry as no replication, so no natural selection, so not life.)
The RNA world
Once RNA formed, it replicates
spontaneously, and catalyses its
own replication.
So far we can only get this to work for a few base pairs, but
the conditions we should be trying it in are unknown.
Initially all the replication jobs were done by RNA. Now data
storage is done by DNA, and several types of RNA carry the
message, make proteins etc. How we swapped to the current
system is unknown.
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Once replication began, life began, and
natural selection began.
RNA molecules which were better at
replication left more copies of
themselves…. So stood less chance
of going extinct.
Faster replicators grabbed the free
nucleic acids faster = competition.
Mistakes in replication = mutations,
….some of which improved
replication speed = adaptation
All 3 aspects needed for life are present: replication,
heredity (new molecules resemble their parent) and
variation (occasional replication errors occur).
First branch on the tree of life was
between Bacteria, and Archaea
Eukarya branched off Archaea later
and led to Eukaryotes.
The most recent ideas about the origin
of life come from reassessing the
similarities and differences between
these two bacterial lines.
It’s Life… but not as
we know it!
Now replication uses over
30 different proteins to
help, and many types of
DNA and RNA
ALL living things use the SAME system, so the change from
RNA world to this one must have been early.
Stage 4: Cell formation
How the whole system got into a cell, is
entirely unknown. The boundary is lipid, but
how do nucleotides etc. get inside?
What was that first cell like?
Mitchell (1972) suggested
Chemiosmosis = using
energy from food to pump
protons through a
membrane, then allow them
to flow back to release
energy in the cell.
Earned Mitchell the Nobel Prize for Chemistry in 1978
Read New Scientist article “Cradle of Life” to get the full story.
Everyone thought energy from
food or light was used to make
ATP, then split it to release
energy in the cell . But need a
lot of energy all at once to make
an ATP molecule.
But chemiosmosis was ignored initially by scientists looking for
the origins of life, even though
- proton gradients are common in all types of cell e.g. for active
transport across cell membranes.
- much smaller amounts of energy needed to pump out one
proton than to form ATP so could be a useful steppingstone.
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Bill Martin 2009 looked at the problem again.
He argued:
•  Features common to Archaea and bacteria are
likely to be early. These include DNA, RNA
proteins, ribosomes, ATP and a proton gradient
powered enzyme for making ATP
•  Differences between the two lines are likely to have evolved
after the split, so weren’t features of the first live cells.
These include:
DNA replication mechanism
Fermentation (= original way to get energy from food so vital to
our story.)
Cell membranes and cell walls.
So what could the ancestor cell be like if it didn’t have
a cell membrane, and couldn’t eat?
Energy was plentiful – Between the acid sea and
the alkaline vent was a proton gradient readymade, allowing chemiosmosis.
This would have spontaneously made
pyrophosphate, a natural analogue of ATP still
found in both archaea and bacteria today.
The pyrophosphate could have driven the formation of amino acids, and
nucleotides.
Temperature gradient between the top and
bottom of the pores concentrate nucleotides at
one end, increasing their likelihood of
ploymerising, forming RNA and proteins.
Convection currents would raise and lower the
temperature continually, which is what you do to
replicate DNA now in the lab.
So evolution starts : replicating chemicals breed and compete in the
pores.
Russell 1990s looking at alkaline hydrothermal vents in the deep
seas. Active ones very rare now but would have been more
common then.
In them, water reacts with certain minerals in the
sea floor, to release hydrogen, alkaline fluids and
heat.
The early ocean was acidic and iron-rich.
When upwelling
hydrothermal fluids reacted
with this seawater they
produced carbonate rocks
riddled with tiny pores and a
“foam” of iron-sulphur
bubbles which catalysed
further reactions.
At some point DNA was also invented
Fatty molecules would also be produced and coat
the iron-sulphur froth to form cell-like bubbles.
Some of the DNA or RNA is likely to have got inside
some of them. But they dissolve if they leave the
vent as their energy source is still there.
To escape the vents and become free-living they needed to make
pyrophosphate themselves, which needs the enzyme
pyrophosphatase – still found in both bacteria and archaea.
They also needed to create their own proton gradient. Allowed them to
pump single protons out of the cell when they had a little energy, then
let them all flood back to make an ATP energy store.
According to the theory this escape from the vents happened twice,
which is why archaea and bacteria have different cell membrane and
cell wall structures.
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Once life had become independent, we had the
Prokaryotes
All the ways of extracting energy from the world were
invented by the prokaryotes
Heterotrophy (= eating other organisms)
Archaea now only survive in extreme environments where
nothing else competes with them
Bacteria are still common, and use a wide variety of ways of
making energy, scattered across their phylogenetic tree.
Photo-autotrophy (=energy
from the sun)
Only one made
oxygen cyanobacteria
Lithotrophy (=energy from
chemical reactions e.g. H2S)
Mixotrophy (= energy from any
combination of the above)
Importance of the Cyanobacteria
Cyanobacteria evolved photosynthesis,
i.e. splitting water to make oxygen.
One group survives today as stromatolites
and thrombolites off Western Australia.
Fossilized ones exist from 2700mya.
Oxygen is very toxic to most other bacteria, so they poisoned
almost everything else. It also made an ozone atmosphere.
The ozone in the
atmosphere protected the
land from UV light and so
made it habitable to lichens.
Summary
We still do not know for sure:
1) How the building blocks of life appeared, especially
the nucleotides
2) How the replication mechanism got inside the cell
membrane
3) How the change to DNA happened
4) When any of this happened, or where.
But we’re finally getting some more
convincing theories!
They helped raise the oxygen levels to today’s levels (eventually
– see “Life’s a Gas” article), allowing larger animals onto land.
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Recommended reading:
If you are interested:
Freeman and Herron “Evolutionary Analysis” 3rd Edition
Chapter 16, Origins of life
Skelton “Evolution, a biological and palaeological approach”
Chapter 16.1 to
16.4 – Origins of life, life and times of early life forms (prokaryotes).
Cambell and Reece “Biology” 6th Edition:
Chapter 26 Origins of life.
New Scientist 17th Oct 2009 “Cradle of Life”
http://www.nick-lane.net/OriginOfLife.pdf - the detail of the alkaline
hydrothermal vents theory
New Scientist 6 Feb 2010 p36-39 “Life’s a Gas” – current
views on how and when the levels of oxygen rose and
allowed complex life.
NB DO NOT learn the chemical reactions, or even write them out. You don’t need
that much detail!
New Scientist 11th July 2009 p38-41 “Dawn of the animals”- The latest
ideas on why eukaryotes took so long to get going, and evidence of preCambrian life.
New Scientist 14 June 2003 p 33-39 “Evolution – Five Big Questions” –
views on the origin of life, mechanisms of evolution, speciation,
predictability of evolutionary outcomes, and religion’s position.
New Scientist 27 Sept 2003 p22-23 “Conference report: Origin of Life” –
Some interesting snippets about the evidence for early life.
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