Download D.1 Origins of Life

Evolution
D.1 Origin of Life on Earth – PART 1
Fundamental questions that
have plagued humankind….
• Where do we come from?
• How did life start on Earth?
• What were our ancestors like millions of years ago?
Problems for starting life
on Earth
• How could the lifeless ball of rock that the planet
Earth was 3.5 billion years ago, become home to
such lush vegetation and a wide variety of
bacteria, fungi, protists, and animals that we see
today?
• There are 4 problems which needed to be
overcome for the life on Earth to exist.
1. Non-Living Synthesis of
Simple Organic Molecules
• Life as we know it is based on organic molecules,
such as amino acids
• Early Earth only had inorganic matter: rocks,
minerals, gases, water….
• Organic chemicals may have been generated on
Earth or they may have been introduced from
space
2. Assembly of these
molecules into polymers
• Organisms are organized!
• Simple organic molecules would have needed to
undergo a process of polymerization to form the
larger more complex organic chemicals required
by cells.
3. Origin of self-replicating molecules
(makes inheritance possible)
• For something to be “alive” it must reproduce on its
own.
• Need a self-replicating molecule
• Only self-replicating molecules are able to undergo
evolution by natural selection
• DNA is the molecule most used for replication of
organisms but it is complex and requires enzymes
for its formation.
• Therefore, it is unlikely that DNA developed very
early.
• Also, to get the proteins required to form from DNA,
RNA is required
4. Packaging these molecules
into membranes
• Water is important to life but tends to depolymerize
molecules
• Many compounds dissolve in water- difficult to
organize into polymers
• The formation of closed membranes is likely an early
and important event in the origin of cellular life
• It allows for the development of an internal
chemistry different from the external environment
Miller and Urey
• Stanley Miller and Harold Urey performed a groundbreaking experiment in 1953 concerning the origin
of life on Earth.
• Scientific evidence suggests that the Earth is ~5
billion years old.
• Formed from a coagulation of dust particles
surrounding the Sun
• In the early life of the planet, the atmosphere
contained hydrogen, water vapour, methane,
ammonia, nitrogen, and hydrogen sulfide- but no
oxygen (O2)
• Thought that synthesis of biological molecules must
have formed in the shallow waters of the ocean s as
the products of chemical reactions between
compounds in the atmosphere and the water.
• Mixture is known as the “primordial soup” (primeval
soup or chemical soup)
• (abiogenesis: the creating for organic matter from
inorganic matter)
Miller and Urey’s
experiment
• Tried to recreate the primordial soup in a glass
sphere based on environmental conditions of the
time.
• Tried introducing gases they believed to be present
at the time (methane, H2, NH3)
• Introduced H2O which has heated to evaporation
and cooled to condense – thereby recreating the
H2O cycle.
• They kept the system at a warm temperature
• Exposed the apparatus to UV radiation (no ozone
layer at the time)
• Generated electric sparks to simulate lightning
Miller and Urey’s
Experiment
• After 1 week:
• 15% of the carbon was now found in organic form
• 13 of the 20 amino acids had formed inside the
primordial soup
• Sugars had formed
• The nitrogenous base adenine had formed
Could comets have brought
organic compounds to Earth?
Panspermia
• Hypothesis that life on Earth may have originated by
the introduction of organic chemicals or even
bacteria via comets
• Comet: small body of rock, dust, and ice that orbits
the Sun
• Geological records show that our planet was
bombarded by a shower of comets and asteroids
about 4 billion years ago (Late Heavy
Bombardment)
• Organic molecules hitchhiking on comets could
survive the impact and the impact could help to
polymerize certain amino acids into polypeptides)
Could life exist on a comet in
the extreme conditions in space?
• Some bacteria and archaebacteria can survive in
extreme environments (Bacterial endospores found in
ice cores in Antarctica)
• Cosmic radiation could provide the energy to form
complex organic molecules
• By studying spectral lines of distant clouds of cosmic dust
particles, astronomers claim to have revealed the
presence of glycine, which is the simplest amino acids.
This suggest organic molecules can form in space
Other possible locations for the
synthesis of organic compounds?
•
•
•
•
Alternating wet/dry conditions?
Near volcanoes?
In deep oceans?
Mars
Alternating Wet Dry
Conditions
• On earth at a seashore or the flood plains of a river
where there is an alternation of wet –dry conditions
• The drying of clay particles could have created
catalyzing reactions and formed early organic
molecules
Stromatolites
• Stromatolites –
formed in shallow
water by trapping
and binding
sediments of biofilms
of microorganisms
• One of the most
ancient forms of life
on Earth
Near Volcanoes
• Although volcanic eruptions can be destructive,
they spew out water vapour, other gases and
various minerals which could be used to form
organic matter
• The rich sources of raw materials plus the warmth of
the volcanic activity could have produced
conditions favourable for the formation of amino
acids and sugars.
In deep oceans
• Organic molecules could have formed around
hydrothermal vents – places where hot water
emanated from beneath the ocean floor.
• Form when cracks in the crust of the seabed expose
sea water to rocks below which are heated by
magma
• The hot water rises and picks up countless minerals
along the way.
• Hydrothermal vents are sometimes referred to as
black smokers because the water coming out of t
them contains so many dark minerals it looks like
smoke.
• There are entire communities living around these
vents that we did not know about before
• Ex: meter long white an red tube worms that absorb
the minerals from the water and transfer it to
symbiotic bacteria (the bacteria then makes food
for the worm)
• Proves that life can exist at the bottom of the ocean
– despite lack of sunlight
• This environment could be suitable for the formation
of biological polymers
Mars
• As mentioned before, life could have formed extra
terrestrially on a nearby planet or in space.
• While Earth was still too hot, Mars – being smaller
and further from the Sun – would be cooler and
prebiotic evolution could have occurred there
• These organic molecules could have been blasted
from the surface of Mars by an asteroid and comet
impacts
• Meteorites from Mars (possibly containing fossilized
bacteria) have been found in Antarctica
D1 Origins of
Life
Part 2
The role of RNA in Early Life
• In order to transmit hereditary traits to the next
generation, most organisms today store their
genetic code in the form of DNA
• DNA replication requires enzymes, and since the
prebiotic world didn’t have enzymes, it is unlikely
that double stranded DNA was the means of
inheritance in the first organisms.
• RNA can replicate itself without the aid of enzymes
• Thus, RNA may be the early nucleic acid for
hereditary.
Ribozymes
• Small sequences of RNA can act like enzymes –
these are called RIBOZYMES
• Ribozymes can act on themselves and other RNA
sequences
• In lab tests, ribozymes can perform the reactions of
RNA replication
Protobionts
• In the millions of years following the creation of
organic compounds in the primordial soup, these
compounds became more complex.
• Amino acids, monosaccharides, nucleic acids
would have undergone polymerization
• This process may have
occurred in shallow
rock pools, particularly
those where organic
compounds had
accumulated by
absorption on the
surface of clay
particles
• WHY?
• When clay dries out and is heated, as many as 200
amino acids can spontaneously join together in
polypeptide chains.
• In the right conditions, these chains can form
proteinoid microspheres – tiny bubble-like structures
(like a vesicle)
• They could establish and maintain a chemistry
inside which is different from the surrounding
environment
• Coacervate - a microscopic sphere that forms from
lipids in water.
• Forms spontaneously due to the hydrophobic forces
between the water and lipid molecules.
• Also can maintain an internal chemical
environment different from the surrounding
environments.
• Coacervates can be selectively permeable
Coacervates (lipids)
• Although they are not living organisms, proteinoid
microspheres and coacervates are a significant
step toward the formation of cells.
• They solve the problem of protecting polymers from
their destructive environments.
• Could be primitive versions of the first cell
membranes
• PROTOBIONTS – the first precursors to cells, were
likely coacervate droplets which included
polynucleotides (DNA or RNA)
• (remember our cell membranes are lipid based)
• Overtime, true cell membranes evolved and other
characteristics of cells developed.
o Cellular respiration
o Asexual reproduction
Where did all the oxygen
come from?
• 1/5 of the air you are breathing right now is oxygen.
• However, there was none at all present 4 billion
years ago.
• The earliest life forms on Earth were bacteria and
they lived in an environment with an atmosphere of
mostly CO2
• Thus, early life forms were anaerobic cells
• These single-celled organisms would consume
organic molecules (i.e. simple sugars) that were
forming from chemical reactions on Earth
• The more they reproduced, the more food that was
consumed.
• After million of years, their population would have
reached such large numbers that food began to
be scarce.
• In this food shortage, bacteria that could make
their own food would have an advantage.
• ~3.5 billion years ago, bacteria (that is believed to
be related to today’s cyanobacteria)developed
the ability to photosynthesize.
• Must have contained a form of chlorophyll
• Development of photosynthesis was one of the
most significant evens in the history of Earth
• Gives bacteria a source of energy (sunlight) to
survive
• Created a mass pollution of the atmosphere
o Pollution of oxygen!!!
• Oxygen gas is toxic to the kinds of bacteria which
preceded photosynthetic ones, so this pollution
would have eventually killed off large populations
of anaerobes.
• Anaerobic bacteria that survived would live in mud
of places protected from the new oxygen-rich
atmosphere.
• The ability of an organism to make its own food
gives it a distinct advantage over those that
cannot.
• As a result, photosynthetic bacteria proliferated
and produced more and more oxygen
Endosymbiosis
• 3.8-2 bya, bacteria (prokaryotic cells) were the only
organisms on Earth
• The first fossils of cells with a nucleus (eukaryotes) is
from around 2 bya.
• How did prokaryotes develop into eukaryotes?
• Endosymbiosis is the most popular theory
Endosymbiotic Theory
• Organelles that are found inside eukaryotic cells
today were once independent prokaryotic cells.
• They were engulfed by a bigger prokaryotic cell.
• Rather than being digested, the prokaryotes were
kept alive inside the host cell in exchange for their
services
Endosymbiotic Theory
• The host cell would provide protection for the
smaller prokaryotic cell
• The engulfed cell would be beneficial to the host if
it was photosynthetic (providing food) for the host
or able to metabolize food efficiently and produce
energy for the host.
• Explains how membrane bound organelles such as
chloroplasts and mitochondria became part of
eukaryotic cells.
Problems with
Endosymbiosis
• The ability to engulf another cell and have it survive
in the cytoplasm does not guarantee that the host
cell can pass it on to its offspring the genetic code
to synthesize the newly acquired organelle
• When chloroplasts or mitochondria are removed
from a cell, they cannot survive on their own.