Title: How To Study The Origin of Life On Earth
1Astrobiology The Origins of Life Monday, January
29, 2007
2Origins of Life
- today, we know of only one form of life life on
Earth, all of which shares a common origin and
uses DNA molecules as a fundamental means of
reproducing itself - understanding how life originated on Earth may
provide us with clues as to how life might
originate elsewhere in the universe and how we
might recognize it
3Searching for Evidence of Early Life
- there are two direct ways of searching for
evidence of the existence of life during the
earliest part of Earth history - using light and electron microscopy (SEM, ESEM,
TEM) and most recently AFM to search for
fossilized microbes - geochemical measurement, in rocks that are
interpreted as metasediments, of putative organic
traces in the geochemistry. These traces include
clumps of carbon, which may or may not be
biogenic, and biological isotopic fractionation,
both of light elements such as C, N, S, O and D/H
that make up the cell mass, and of the metals
that constitute key housekeeping proteins (Fe,
Cu, Zn). The geochemical approach also involves
micro-scale in-situ analyses of isolated
microfossils from shale and chert samples.
4When Did Life Start on Earth?
- the best estimate of the origin of the universe
is 13.7 Ga - earliest possible emergence of life in the
universe is 12.7 Ga - age of the Earth is 4.55 Ga
- if the assumption that the Hadean period was
relatively cool is correct, then from 4.4-4.3 Ga,
the conditions for prebiotic chemistry and
appearance for life were already met, i.e.,
liquid water, continental crust, etc. - however, the Late Heavy (Meteoritic) Bombardment
between 4.0 and 3.8 Ga likely sterilized the Earth
5Isua Supracrustal Belt
- the oldest known sedimentary rocks are from West
Greenland (Isua Supracrustal Belt) and dated as
no younger than 3.85 Ga (likely older) and
include banded iron formation beds, metacherts,
pillow lava structures, carbonates, and felsic
metasediments
The world's oldest rocks may they contain the
earliest evidence for life on Earth. These rocks
are found in southwest Greenland, including Iron
Mountain of the Isua region.
6Isua Supracrustal Belt
- researchers have studied tiny graphite grains
embedded in different rocks found in the Isua
formations - by measuring the ratio of carbon isotopes, these
researchers determined that the graphite
contained carbon of biological origin - two common and stable isotopes of carbon, 12C and
13C, occur in a mixture in the Earths atmosphere - biological processes build organic molecules with
a higher carbon 1213 d13C ratio than abiotic
processes
7Isua Supracrustal Belt
- and because both isotopes are stable, the ratio
remains in all of lifes products, even after
billions of years - this interpretation, however, remains
controversial as processes such as metamorphism
can change the original carbon isotope ratio
(isotope exchange with carbonates or CO2-rich
fluids and devolatilization reactions)
8BIF, Akilia Island, West Greenland
- the highly metamorphosed quartz-pyroxene rock on
the SW tip of Akilia Island are dated at 3.8 Ga
and has for long been the center of attention
regarding the oldest traces of life on Earth - the BIF was found to contain graphite inclusions
within apatite crystals
9BIF, Akilia Island, West Greenland
- the low d13C of these graphite inclusions
suggested a biologic source material that had
retained its original carbon isotope signature - it has been argued, however, that the protolith
of this rock was not a BIF, but instead a highly
metasomatized ultramafic rock, which does not
represent a marine depositional setting and
would, therefore, not be able to harbour traces
of ancient life - the age of the rock has also been challenged and
that the apatite crystals are much younger than
the surrounding rock matrix
10Apex Chert - Pilbara, Western Australia
- microfossils from the Apex Chert, North Pole,
Australia - these organisms are Archean in age, approximately
3.465 Ga, and resemble filamentous cyanobacteria - the first appearance of cyanobacteria in the
fossil record is a matter of great scientific
interest because they gave rise to the
oxygen-rich atmosphere that allowed the evolution
of higher life forms
11Apex Chert - Pilbara, Western Australia
- researchers have recently suggested, however,
that these are pseudofossils (artefacts) formed
when carbonaceous glass associated with one of
Earths oldest hot springs recrystallized into
spherulitic silica (abiotic processes associated
with hydrothermal activity)
12Hamersley Range, West Australia
- the oldest unambiguous fossil evidence for
cyanobacteria is found in 2.15 Ga rocks of the
Belcher Supergroup, Canada, but biomarkers, which
are complex hydrocarbons (molecular fossils) that
are derived from once living organisms, suggest
the presence of cyanobacteria, methanotrophs or
methylotrophs, and Eucarya in 2.77 Ga rocks of
the Hamersley Basin, Australia.
Silicified microbial mats, Belcher Supergroup
13Indirect evidence for the activity of
methanogenic Archaea derived from global carbon
isotopic anomalies in the kerogen of 2.8 to 2.5
Ga sedimentary rocks. Biomarker evidence
(2a-methylhopanes) for cyanobacteria. Oldest
known fossils with diagnostic cyanobacterial
morphology from the 2.15-Ga Belcher Supergroup,
Canada, Biomarker evidence (diverse steranes) for
Eucarya. Oldest known fossils with possible
eukaryotic morphology from the 1.87-Ga Negaunee
Iron Formation, Michigan. Previous oldest
sterane biomarkers from the 1.64-Ga Barney Creek
Formation, McArthur Basin, Northern Territory.
Oldest known eukaryotic fossils assigned with
confidence to an extant phylum (Rhodophyta) from
the 1.26 to 0.95 Ga Hunting Formation, Somerset
Island, Canada. Sulfur-isotopic evidence for
mesophilic sulfate-reducing Bacteria from North
Pole, Pilbara Craton, Western Australia.
14When Did Life Start on Earth?
- if life originated on Earth, it did so in a
window of at most 1 Ga (4.55 Ga to 3.5 Ga), most
plausibly 400 Ma (3.9 Ga to 3.5 Ga), and possibly
lt100 Ma (3.9 Ga to 3.85 Ga), if the Greenland
(3.85 Ga) carbon isotopic signal is correct - if life originated elsewhere, the window expands
to 12 to 9 Ga - that full length of time might not be available
on a single planet, but the Earth has provided a
life-friendly environment for at least 3.5 Ga
15Origin of Life
- no consensus for a theory or model of precellular
life - theories of the origin of life fall into two
categories - extraterrestrial origin
- terrestrial origin
16Models for the Origin of Life
17History of the Origins of Life
- abiogenesis is the generation of complex life
from non-living matter
- earlier notions of abiogenesis, i.e., autogenesis
or spontaneous generation (or "Aristotelian
abiogenesis"), held that living organisms are
generated by decaying organic substances, e.g.,
mice spontaneously appear in stored grain or
maggots spontaneously appear in meat
18History of the Origins of Life
- in the 1800s, chemist Louis Pasteur developed the
"germ theory of medicine and argued that people
became ill because they were infected by
invisible things in the surrounding environment
called germs which were not acceptable to
most medical experts of the early 1800s
19Modern Abiogenesis
- in 1871, Charles Darwin made the suggestion that
life may have begun in a "warm little pond, with
all sorts of ammonia and phosphoric salts, light,
heat, electricity, etc. present, that a protein
compound was chemically formed ready to undergo
still more complex changes, at the present day
such matter would be instantly devoured or
absorbed, which would not have been the case
before living creatures were found." - thus, when Earth was ripe for life, the basic
compounds necessary for metabolism might have
existed in surplus with no competition from oxygen
20Modern Abiogenesis
- the modern definition of abiogenesis is concerned
with the formation of the earliest forms of life
on Earth from primordial chemicals, in an
environment regarded as similar to that at the
time shortly after the formation of the Earth
21Current Models of the Origins of Life
- most currently accepted models build in one way
or another upon a number of discoveries
concerning the origin of molecular and cellular
components for life
22Five Steps to the Origins of Life
- plausible pre-biotic conditions result in the
creation of certain basic small molecules
(monomers) of life, such as amino acids. - demonstrated in the Urey-Miller experiment by
Stanley Miller and Harold Urey in 1953 - phospholipids (of an appropriate length) can
spontaneously form lipid bilayers, one of the two
basic components of a cell membrane - the polymerization of nucleotides into random RNA
molecules might have resulted in self-replicating
ribozymes (RNA world hypothesis)
23- selection pressures for catalytic efficiency and
diversity result in ribozymes that catalyse
peptidyl transfer (hence formation of small
proteins), since oligopeptides complex with RNA
to form better catalysts - thus the first
ribosome is born, and protein synthesis becomes
more prevalent - proteins outcompete ribozymes in catalytic
ability, and therefore become the dominant
biopolymer - nucleic acids are restricted to
predominantly genomic use
24Origin of Organic Molecules
We must always recognize that there were both
endogenic and exogenic sources of organic
materials.
25Exogenic Origin of Organic Molecules
- the origin of life in Universe was influenced by
organic compounds from space or even a product of
organics present in the nebula - these organic molecules have been detected in
comets, meteorites, and interplanetary dust
particles (IDPs) - much of the infalling material during the era of
bombardment consisted on comets, which contained
not only water but CH4, NH4, methyl alcohol,
hydrogen cynanide and formaldehyde - these molecules along with CO and CO2 provide the
raw materials in building complex organic
molecules
26Red Giant CRL 2688
- A.K.A., the Egg Nebula, this Red Giant 3,000
light-years from us is ejecting material out into
space in cloud of dust and gas moving out from
the star, at 20 km/s - this material is where we encounter our first
organic molecules
27Nebular Chemistry
Nothing survives
Reactions on surfaces (Fischer-Tropsch)
Interstellar ice is stable
28there is a class of molecules that has been found
in meteorites. . .
. . . in fact, you might even know what they
taste like...
29Polycyclic Aromatic Hydrocarbons (PAHs)
- PAHs are stable, complex netwoks of benzene
rings made mostly of carbon, and are everywhere - these sooty molecules may take up 10-15 of all
the galactic carbon! - they are also carcinogens
30Dense Interstellar Clouds
- dusty cold and dark in these clouds the average
temperature is 10 K (-263 C) and nearly
everything freezes out onto microscopic grains
forming ice mantles - UV radiation and cosmic rays bombard the ice,
breaking bonds
radiation
CH3OH
PAH
H2O
CO
CO2
more complex organic molecules
31- in January 2004, a team led by A. Witt discovered
the spectral signatures of the polycyclic
aromatic hydrocarbons anthracene and pyrene in
the UV light emitted by the Red Rectangle nebula
- potentially vital organic molecules for the
formation of life
32- laboratory simulation of PAHs in cometary ice
analogs showed that during irradiation of comets
by a young star, biogenic types of PAHs could be
formed such as quinines - quinones are found in all living organisms on
Earth and thus a part from being the largest
fraction of carbonaceous matter in the universe,
PAH molecules might be important for early life
33Polycyclic Aromatic Hydrocarbons (PAHs)
34Endogenesis Organic Synthesis by Other Energy
Sources
- organic synthesis initiated by impact shock
- Miller-Urey, Uro and Sagan Experiments
- polymerization
- Iron-Sulphur World
35Organic Synthesis by Impact Shock
- laboratory experiments have shown that in
strongly reducing atmospheres, shock waves
emanating from meteoroids either traversing the
Earths atmosphere or during impacts can produce
large quantities of organics - shock heating of reducing gas mixtures can
produce amino acids - however, in the apparently more likely case of a
mildly reducing or intermediate oxidizing
atmosphere, atmospheric shocks were probably of
little importance for direct organic production
36Miller Urey Experiments
- in 1953, Stanley Miller designed an experimental
test for Darwins conception of the origin of
life in ponds or tide pools - the experiment used a highly reduced mixture of
gases (methane, ammonia, hydrogen and oxygen),
which was at that time considered to be the
composition of the prebiotic atmosphere - other less reducing gases produce a lower yield
and variety - Miller discharged an electric spark into a
mixture thought to resemble the primordial
composition of the atmosphere
37Miller Urey Experiments
- a mixture of CH4, NH3, H2O, and H2, thought to
represent Earths prebiotic atmosphere, was
circulated through a liquid water solution and
continuously sparked by an electrical discharge
as an energy source - chemical analysis of the liquid pool revealed the
formation of organic molecules including several
amino acids, which are the building blocks of
life, and fatty acids from inorganic components
38Miller Urey Experiments
- the simple inorganic molecules that Miller placed
into his apparatus, produced a variety of complex
molecules
- the interactions of these molecules would have
increased as their concentrations increased and
reactions would have led to the building of
larger, more complex molecules
39Miller Urey Experiments
- subsequent experiments substituted ultraviolet
light or heat as the energy source or have
altered the initial abundances of gases - in all
experiments representatives or precursors of all
four organic macromolecular classes amino acids
formed - Miller's (and subsequent) experiments have not
proven life originated in this way, only that
conditions thought to have existed over 3 Ga were
such that the spontaneous (inorganic) formation
of organic macromolecules could have taken place - simple organic molecules are a long way from a
fully functional self-replicating life form
however, in an environment with no pre-existing
life these molecules may have accumulated and
provided a rich environment for chemical
evolution ("soup theory")
40Endogenesis
- while there is still debate on the generality of
the experimental synthetic pathways and on the
stability of the molecules produced most if not
all of the essential building blocks of proteins,
carbohydrates, and nucleic acids can be readily
produced under quite general primitive reducing
conditions - while such experiments have not led to the
synthesis of living things from the basic
materials of the early Earth, they do give
biologists more reasons to believe that the
chemical steps that led to life are not as
difficult to reproduce as was once thought
41Iron-Sulphur World Theory
- is a hypothesis for the origin of life advanced
by Günter Wächtershäuser involving forms of iron
and sulfur - Wächtershäuser claims that an early form of
metabolism predated genetics, i.e., chemical
reactions that produce energy in a form that can
be harnessed by other processes - once a primitive metabolic cycle was established,
it began to produce ever more complex compounds
42Iron-Sulphur World Theory
- key idea of the theory is that this early
chemistry of life occurred not in bulk solution
in the oceans, but on mineral surfaces (e.g. iron
pyrites) near deep submarine vents - carbon-rich molecules could accumulate on iron
pyrite crystals and acquire new carbon atoms from
material ejected by the hydrothermal vents - the first 'cells' would have been lipid bubbles
on the mineral surfaces
43 Pyrite as a Source of Energy and Molecular Order
Polymerization Reaction Adsorption of
glyceraldehyde-3- phosphate to the surface of
pyrite followed by polymerization (Wachterhauser
1988)
44Formation of Pyrite
- CO2 H2 ? HCOOH ?Go 30.2 kJ/mol
- Fe2 2H2S ? FeS2 4H 2e-
- FeS H2S ? FeS2 H2 ?Go -41.9 kJ/mol
- Fe CO3 H2S (aq) ? FeS2 2H2 H2O CO2 ?Go
-61.7 kJ/mol - CO2 FeS H2S (aq) ? FeS2 H2O HCOOH ?Go
-11.7 kJ/mol - Formation of pyrite can serve as a source of
reducing power.
45Iron-Sulphur World Theory
- however, like those who hypothesize that life
began in ponds or tide pools, Wächtershäuser has
no clear way to pass from the building blocks to
living creatures - nevertheless, with his emphasis on the high
temperature origin of life, he may be on the
right track
46From Organic Molecules to Protocells
- there are many different hypotheses regarding the
path from organic molecules to protocells - virtually all biologists now agree that bacterial
cells cannot form from nonliving chemicals in one
step - if life arises from nonliving chemicals,
there must be intermediate forms, "precellular
life - three basic approaches
- genes-first - the early appearance of nucleic
acids - metabolism-first - the evolution of biochemical
reactions and pathways - hybrid models - combine aspects of both
47The Beginnings of Life on Earth
The only premise that all precellular theories
share is that it would be an extremely long time
before the first bacterial cells evolved
How long would it take?
ratio of actual useful proteins to possible
random proteins up to one in something like
10500, barring incredible luck, something like
10500 trials to probably find one
48"Genes First" Models RNA World
- the RNA world hypothesis proposes that RNA was
actually the first life-form on Earth, later
developing a cell membrane around it and becoming
the first prokaryotic cell - hypothesis is supported by the RNA's ability to
store, transmit, and duplicate genetic
information, just like DNA does - RNA can also act as a ribozyme (an enzyme made of
ribonucleic acid) - because it can reproduce on its own, performing
the tasks of both DNA and proteins (enzymes), RNA
is believed to have once been capable of
independent life
49Metabolism First Models
- several models reject the idea of the
self-replication of a "naked-gene" and postulate
the emergence of a primitive metabolism that
could provide an environment for the later
emergence of RNA replication - in 1924, Alexander Oparin suggested the idea of
primitive self-replicating vesicles - basic
organic chemicals might form into microscopic
localized systems (e.g., coacervates) - possible
precursors of cells - from which primitive living
things could develop - more recent variants in the 1980s and 1990s
include Günter Wächtershäuser's iron-sulfur world
theory and models introduced by Christian de Duve
based on the chemistry of thioesters (compounds
resulting from the bonding of sulfur with an acyl
group an alkyl group attached to a carbon-oxygen
double bond) - however, the idea that a closed metabolic cycle,
such as the reductive citric acid cycle proposed
by Günter Wächtershäuser, could form
spontaneously remains unsupported
50Aerosols as Prebiotic Chemical Reactors
- aerosol sizes with significant atmospheric
lifetimes are the same as those of single-celled
organisms - aerosols that are coated with organic surfactant
layers may act as a chemical reactor in the
prebiotic production of polymeric molecular
species
(Dobson et al. 2000)
51Clay Minerals and the Origin of Life
- clay minerals have a very complex crystalline
structure, with a complex pattern of positive and
negative charges - clay minerals can grow, adding new atoms while
maintaining its crystal structure (Genetic
Takeover) - it has been shown experimentally that clay
minerals can catalyze the formation of at least
small amino acids and nucleotide polymers
52Genetic Takeover and the Mineral Origins of Life
- Genetic Takeover
- the original replicator (i.e., life) probably was
not DNA possibly a primitive genetic system such
as clay minerals or crystal medium, which RNA
replaced - a mechanism by which evolution may produce
changes in the genetic substrate of organisms
53Genetic Takeover
- the yellow regions represent phenotypes - G1 is
the primary genetic substrate, and G2 is the
secondary one. Arrows within organisms indicate
paths of genetic expression - a simple organism with genetic substrate G1
produces substance G2 as a component of its
metabolic processes - G2 is inherited - and comes to carry heritable
information. Gradually, G2 displaces G1 as the
primary genetic material for the organism - secondary genetic material arises not as a
modification of the primary one, but rather from
molecules synthesized under its control
54Clay Minerals and Genetic Takeover
- CairnsSmith (1982) proposed that clay minerals
were lifes ultimate ancestor - characteristics of clay minerals
- products of processes of weathering and
diagenesis - main component of soils and sediments
- extensive structural variation
- high affinity for adsorption of H2O and organics
- high ion exchange capacity
- clay minerals could provide the basis for an
evolution through natural selection
55- the tetrahedral layer consists of a 2D network of
corner linked tetrahedras the cation is mostly
Si, with some Al-substitution - the interlayer
space is filled by cations such as K, Na, Ca, and
Mg - the octahedral layer consists of a 2D
network of edge-linked octahedras the octahedral
cation is predominantly Al, Fe, Cr, and Mg
Smectite
i t o t
Illite
i t o t
56Replication of Clay Crystals
Zone A Clay crystals are impervious, deflecting
the flow of nutrients. Zone B Clay crystals
do not adhere and are washed out of
system. Zone C Some clay crystals stick to the
walls but let nutrient solutions flow.
57Polymerization of Organic Molecules onMineral
Surfaces
- it is difficult if not impossible to synthesize
long polymers of amino acids, nucleotides, etc.,
in homogeneous aqueous solutions - minerals with ion-exchange properties (e.g., clay
minerals), however, may have functioned in the
prebiotic synthesis of negatively-charged
polymers - the lattice structure of clay minerals such as
kaolinite may acts as templates for the
organization of organic matter into polymers - polymers are long-chain molecules composed of
many similar or identical molecular subunits,
e.g., starch
58Structural relationship between kaolinite-amino
acid-peptide system
59(No Transcript)
60Clay Minerals and Origins of Life
- it is likely that if clays or like minerals
played a role in the origin of life, aluminum and
silicate would be more widely distributed in
biology - instead, these compounds are rare
61Deep-Hot Biosphere Model
- deep subsurface offers infinite possibilities for
the mixing of water with various chemical and
physical conditions - life in subsurface environments is protected
towards meteoritic impacts, ultraviolet
irradiation, volcanic eruptions and desiccation
and is rich in surfaces with - the oxygen-poor water reacts with iron-bearing
minerals such as olivine and pyroxene to make H2 - bacteria use the H2 and dissolved CO2 to make CH4
and other hydrocarbon molecules needed to build
cell material
62Deep Basins
Life on our planet may have evolved first at
depth, then migrated to the surface as the
environment became more tolerable.
63Cold Environments
- organic compounds such as aminoacids have finite
lifetimes in solution,and their stability
decreases markedlyas the temperature increases - at the highest temperatures associated with
hydrothermal vents,amino acids cannot survive at
all - a global ice cover and colder temperatures would
afford significant protection against thermal
degradation - a concentrated mixture of solutes available
during thaws could undergo a burst of chemical
reactions leading to more complex molecules, a
few of which could be on the evolutionary pathway
to life.
64Hydrothermal Vents
- the first cellular life forms may have evolved
inside so-called black smokers at seafloor
spreading zones in the deep sea - many of the basic components of
biochemical/biological systems reflect ancestry
in a hydrothermal system, including metal
proteins, especially those involving
ironsulphur, nickel, molybdenum, copper, cobalt
and zinc, some of which, such as metal-nitrogen
structures - hydrothermal vents systems would have allowed
sulphate reducers to exploit the oxidation
contrast between the atmosphere-ocean system,
open at the top to space, and the more reduced
fluids in contact with mantle derived magma
65A. On land, around a volcano
B. On seafloor, a mid-ocean ridge
66Hydrothermal Vents
- the micro-caverns provide a means of
concentrating newly synthesised molecules,
thereby increasing the chance of forming
oligomers - the steep temperature gradients inside a black
smoker allow for establishing "optimum zones" of
partial reactions in different regions of the
black smoker (e.g. monomer synthesis in the
hotter, oligomerisation in the colder parts) - the flow of hydrothermal water through the
structure provides a constant source of building
blocks and energy (freshly precipitated metal
sulfides)
67Hydrothermal Vents
- the model allows for a succession of different
steps of cellular evolution (prebiotic chemistry,
monomer and oligomer synthesis, peptide and
protein synthesis, RNA world, ribonucleoprotein
assembly and DNA world) in a single structure,
facilitating exchange between all developmental
stages - synthesis of lipids as a means of "closing" the
cells against the environment is not necessary,
until basically all cellular functions are
developed - the synthesis of a lipid membrane
would allow the organisms to leave the
microcavern system of the black smokers and start
their independent lives
68Exogenesis
- proposes that life originated elsewhere in the
universe and was transferred to Earth - panspermia - the theory that microorganisms or
biochemical compounds from outer space are
responsible for originating life on Earth and
possibly in other parts of the universe where
suitable atmospheric conditions exist - panspermia can be either interstellar (between
star systems) or interplanetary (between planets
in the same solar system) - interplanetary transfer of material is well
documented, as evidenced by meteorites of Martian
origin found on Earth - claims that these carry
evidence of extraterrestrial lifeforms have
either been proven unfounded as a result of
terrestrial contamination, misinterpretation, or
hoaxing
69Panspermia Cosmic Ancestry
- Cosmic Ancestry holds that life on Earth was
seeded from space, and that life's evolution to
higher forms depends on genetic programs that
come from space - according to Hoyle Wickramasinghe's expanded
theory of panspermia, there can be no origin of
life from nonliving matter in the finite past - without supernatural intervention, therefore,
life must have always existed
70Panspermia
A low temperature transfer of Martian Meteorite
(ALH84001) from Mars to Earth. Magnetic
microscopy image of ALH84001 the exterior
(upper left) has been remagnetized by the heat of
entry into the Earths atmosphere, while the
interior of the meteorite retains the weaker,
mixed magnetism it acquired on Mars
71A
C
B
north
south
Magnetization of magnetic material. (A) Above
Curie temperature (B) Below Curie point without
external magnetic field (C) Below Curie point
with external magnetic field.
72(Weiss et al. 2000)