Transcript Mechanisms of Evolution - Bio-Guru

The History of Life on Earth
UNIT 5 - Chapter 25
THE FORMATION OF THE UNIVERSE
The universe is 15 billion years old.
Earth
• The Solar system formed about 5
billion years ago
• Earth formed about 4.5 billion years
ago
• The moon formed soon thereafter
The Moon
• At the time Earth formed 4.5 billion years ago,
other smaller planetary bodies were also
growing.
• One of these hit earth late in Earth's growth
process, blowing out rocky debris.
• A fraction of that debris went into orbit around
the Earth and aggregated into the moon.
• The moon was half the distance it is now to
Earth back then
• It is receding from earth @ 1 inch per year
• The planet’s
surface was
molten at first
• It was bombarded
by meteors and
lightning
• Volcanoes erupted
all over the
surface
Primordial Atmosphere
• The atmosphere consisted of gases like H2, NH3
(ammonia), HCN (hydrogen cyanide), Carbon
dioxide, Methane and trace amounts of O2
• Since there was hardly any O2, there was no O3
(Ozone) layer either
• UV rays beat down on the planet
• No O2 and the planet appeared reddish in color
Primordial Atmosphere
• Carbon dioxide and Methane and are
greenhouse gases and together they kept
the planet from cooling – talk about global
warming!
• The atmosphere on our planet was a
reducing atmosphere.
A reducing environment
Primordial Earth
Extraterrestrial Water?
• Scientific evidence supports the idea
that a hail of comets bombarding the
early Earth could have introduced
water into early earth’s atmosphere.
• Comets born near the orbit of
Jupiter, are made of water (ice) that
matches the isotopic composition of
water in Earth's oceans
• Comets born there might also
contain more complex organic
molecules, the building blocks of life.
Water!
• As the Earth cooled, much of the
atmospheric water condensed and fell as
rain. At first it would evaporate
immediately, then precipitate again.
• Eventually it remained on the ground,
creating large oceans.
• Dissolved in the rain was carbon dioxide,
which formed carbonate rocks such as
limestone and marble.
Oceans
• The CO2 that was dissolved in the rain
was getting reduced to long hydrocarbon
chains in the oceans
• The rain also brought down other
inorganic molecules such as ammonia
(NH3) that were then converted to
organic molecules such as urea and
amino acids in the oceans
Soup’s Up!
• The oceans were rich in minerals and
newly formed organic molecules – a
primordial SOUP!
• As it continued to rain, most of the carbon
dioxide was removed from the
atmosphere, allowing the Earth to escape
from a possible runaway greenhouse
effect and cool down further. (CO2, CH4,
are greenhouse gases)
Can Organic Molecules Form Spontaneously?
• In a famous experiment conducted in 1952,
Stanley Miller and Harold Urey exposed a
mixture of gaseous hydrogen, ammonia,
methane and water (stuff found in the primordial
atmosphere) to an electrical arc for a week.
• At the end of the experiment, the reaction
chamber was coated with a reddish-brown
substance rich in amino acids and other
compounds essential to life.
Close, but no cigar!
• The Miller-Urey
experiment did NOT form
life
• However, it demonstrated
how the reducing
primordial atmosphere
would have been able to
synthesize organic
molecules – the chemical
building blocks of life.
Primordial Catalysts
• In 1957, Sidney Fox demonstrated that amino
acids could be encouraged to polymerize upon
exposure to moderate heat.
• Dry heat and mineral clay acted as chemical
catalysts in a world before enzymes evolved to
perform complex pre-biological chemical
reactions.
• In the presence of dry heat and apatite clay,
amino acids are able to form polypeptide chains
of about 200 amino acids long
First Attempts at Life
Ocean Froth
• The waves crashing on
the shore consisted of
bubbles (froth)
• The “membranes” of the
bubble were either made
of
– hydrocarbon chains/lipids
– Protenoids (chains of
amino acids)
Coacervates/liposomes – lipid
bubbles
• A coacervate is a spherical aggregation of lipid
molecules containing a nutrient-rich interior (early
cytosol?).
• The lipids are held together by hydrophobic forces.
• May have acted as a “membrane-bound container” for
life
•
Coacervates can “bud” into two
• They can allow water and chemicals in and out of their
lipid layer (diffusion and osmosis) – this allows them to
grow
Microspheres – protenoid bubbles
• Spherical shells made of polypeptides. Can have a
double layer membrane – much like the lipid bilayer of
modern cells
• Like coacervates, microspheres may have been
important in the development of life, providing a
membrane-enclosed volume which is similar to that of
a cell.
• Microspheres, like coacervates, can grow and allow
diffusion of materials and osmosis.
• The diffusion in and out of organic molecules
and
• The processing of these molecules inside the
coacervates / microspheres
• May have been the evolution of the earliest
metabolism
First Genetic Material
•
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RNA was the first genetic material
The first enzymes were RNA, not proteins
RNA enzymes are called Ribozymes
RNA was eventually able to self-replicate
RNA first developed inside coacervates
Once these coacervates developed RNA
and the ability to replicate, they were
called protobionts or “first life”
First Life?
• Protobionts are organisms that are
considered to have possibly been the
precursors to prokaryotic cells.
• A protobiont is either a coacervate,
liposome or microsphere
• Protobionts exhibit some of the properties
associated with life such as a simple
metabolism and reproduction (budding)
Early Prokaryotes
• With ever-more complex reactions taking place in
protobionts, a point was reached where self-replicating
molecules were formed. One example is the nucleic
acids RNA.
• These molecules have the ability to copy themselves,
and therefore act as information stores. The
protobionts would divide and pass the RNA on to their
“offspring”
Due to random mutations occurring during the RNA
copying process, the protobionts began to evolve
through the process of natural selection.
RNA ACTS AS A
TEMPLATE ON WHICH
POLYPEPTIDES FORM
Polypeptides act as primitive enzymes that aid
replication of all RNA molecules.
• Replication
is eventually
done by
early protein
enzymes,
instead of
ribozymes
Panspermia
•
The theory that earth was
seeded with primitive life
forms from other planets
where life actually arose –
possibly via a meteor or
asteroid from that planet.
•
This merely puts the origin
of life backwards to another
earth-like planet. It might be
possible for very small,
radiation resistant spores to
travel around the universe
under light pressure or for
contamination by space
travel to occur.
The Heterotroph Hypothesis
• The first cells were heterotrophs,
meaning that they obtained their energy
and “food” (such as sulfur compounds)
from their surroundings.
• They did not use nor want O2 in their
environment. In fact O2 was toxic to them.
The Evolution of Autotrophs
• Early on in their existence, the supply of these
resources would have run short, amounting to a
famine.
• This famine exerted extreme evolutionary
pressure on the heterotrophs, leading quite
quickly to the development of cells which were
able to produce their own food.
Life Before Chlorophyll
• These new autotrophs would have performed
chemosynthesis, not photosynthesis. They used
hydrogen sulfide instead of light to make sugar
and other “food”.
• Unfortunately, the supply of hydrogen sulfide is
found only around volcanoes.
• Eventually, some autotrophs evolved chlorophyll
in their membranes and were able to switch to
photosynthesis
Cyanobacteria changed the face of
the Planet
Hooray for Oxygen!
When photosynthesis produced a significant by-product: oxygen.
Oxygen was highly toxic to the early cyanobacteria producing it,
they were forced to excrete it as a gas.
This led to the steady pumping of oxygen into the Earth's
atmosphere.
The by-product of photosynthesis - O2 was poisonous to the early
organisms.
So some died out, others found new habitats away from the
increasing amount of O2 in the atmosphere and still others evolved
(adapted) to use the O2 for metabolism – the aerobic bacteria.
Evidence of Rise in O2 Levels
• Oxygen levels increased
dramatically during the
Proterozoic eon.
• Geologists refer to the
"great iron crisis" when the
rising levels of oxygen in
the world's oceans caused
the formation or iron oxide
(Fe2O3), often preserved as
the banded iron formation
(an important commercial
source of iron).
Cyanobacteria changed the
earth’s reducing atmosphere to an
oxidizing one.
Life in the Blue
• 2.3 and 2.4 billion years
ago, oxygen rose sharply in
the Earth's atmosphere and
oceans, during the 'Great
Oxidation Event,
• This was most likely due to
photosynthesis by
cyanobacteria
• The planet and the oceans
now looked blue
Evidence of Cyanobacteria
• Stromatolites
are fossilized
mats of
prehistoric
cyanobacteria
Endosymbiosis led to Eukaryotes
•
Around 1.5 billion years ago, eukaryotic organisms first appeared. Unlike prokaryotic
organisms, these possessed inner membranes around a nucleus of DNA, and also contained
sophisticated organelles such as mitochondria (for aerobic respiration) and chloroplasts (for
photosynthesis). Subsequently, the eukaryotic cells developed into specialized colonies, and
provided the basis for all multicellular life known today.
•
It is hypothesized by scientists today that these eukaryotes were a result of endosymbiosis
between small and large species of bacteria.
Never Again!
• So if life “happened” rather spontaneously
on primordial Earth, why do we not see it
now?
We do not have the same primordial
conditions anymore.
The only way new life can be born now, is
from pre-existing life.
Fossil Record
• Sedimentary rocks are the
richest source of fossils
• Sedimentation is not a
continuous process, it
happens in periods due to
changing levels of seas
and other bodies of water
• As a result of these
sedimentation periods,
sedimentary rocks form
strata
Dating Fossils
(Not what Anna Nicole Smith did)
1. Relative Dating
(no, not what you think)
Because older and simpler fossils reside in
deeper strata, paleontologists can tell the
relative ages of fossils as early, or late,
before or after.
2. Absolute Dating
Paleontologists now use radiometric dating
techniques such as carbon 14, Uranium
238, etc. to get a more precise date.
Radiometric Dating
• All matter contains all isotopes of certain
elements such as carbon 14 and carbon 12
• The radioactive isotopes decay at a fixed rate
once an animal dies, since it is not actively
restoring atoms in its body
• Scientists can measure how much of the parent
isotope has decayed (like C14) and how much
by-product (like N) has accumulated
Half-Life
• The amount of time it takes for 50% of the
radioactive isotopes to decay is called the
isotopes half-life
• For example, carbon-14 has a half-life of 5600
years and Uranium 238’s half life is 4.5 billion
years!
• Carbon 14 has beta decay – where it turns into
Nitrogen 14 by converting one of its extra
neutrons to a proton, an electron and another
subatomic particle)
L and D amino acids
• Organisms only synthesize L-amino acids
• When they die, the L-amino acids convert
to D.
• Measuring the ratio of L and D amino
acids gives paleontologists an idea of how
old the fossil is
History of the
Continental Drift
Periods of Mass Extinctions and
Diversity
• Most famous – Permian extinctions – almost
90% of all the species that existed went extinct
during the Permian period
• This mass extinction took 5 million years (not a
long time considering the age of life itself)
• Continents had fused to form Pangaea, which
disrupted many organisms
• Sudden volcanic activity in Siberia caused global
warming and oxygen deficit
The end of the Cretaceous
period witnessed another
mass extinction - the end of
the age of dinosaurs
• The Chicxulub Crater is
an ancient impact crater
buried underneath the
Yucatán Peninsula in
Mexico.
• The crater is more than
110 miles in diameter,
making it one of the
largest confirmed impact
structures in the world;
the impacting asteroid or
comet that formed the
crater was at least
6 miles in diameter.
Ta Ta, T-Rex
• The end of dinosaurs was a good thing for us
• The dinosaur reign kept mammalian evolution
at bay, because dinos were ferocious
predators
• The only mammals that thrived back then
were tiny, shrew-like ones
• Once the major dinos died out, the mammals
thrived.
THE END