macroevolution: part iv
DESCRIPTION
Creation Of The Universe Based on studies of the modern universe, it is estimated that the universe began in a single instant 13-15 billion years ago. In that instant all existing energy appeared and exploded outward from a single point. This is known as the Big Bang. Be ready and willing to endure a bit of “Sheldon” any time you say “The Big Bang Theory”. After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons. While protons and neutrons combined to form the first atomic nuclei only a few minutes after the Big Bang, it would take thousands of years for electrons to combine with them and create electrically neutral atoms. The first element produced was hydrogen, along with traces of helium and lithium. Giant clouds of these primordial elements would coalesce through gravity to form stars and galaxies, and the heavier elements would be synthesized either within stars or during supernovae. Graphic: http://www.environmentalgraffiti.com/ecology/mother-earth-when-volcanoes-ruled-the-world/580TRANSCRIPT
Macroevolution: Part IV
Origin of Life Creation Of The Universe
Based on studies of the modern universe, it is estimated that the
universe began in a single instant billion years ago. In that
instant all existing energy appeared and exploded outward from a
single point. This is known as the Big Bang. Be ready and willing
to endure a bit of Sheldon any time you say The Big Bang Theory.
After its initial expansion from a singularity, the Universe cooled
sufficiently to allow energy to be converted into various subatomic
particles, including protons, neutrons, and electrons. While
protons and neutrons combined to form the first atomic nuclei only
a few minutes after the Big Bang, it would take thousands of years
for electrons to combine with them and create electrically neutral
atoms. The first element produced was hydrogen, along with traces
of helium and lithium. Giant clouds of these primordial elements
would coalesce through gravity to form stars and galaxies, and the
heavier elements would be synthesized either within stars or during
supernovae. Graphic: Creation Of The Universe
After its initial expansion from a singularity, the Universe cooled
sufficiently to allow energy to be converted into various subatomic
particles, including protons, neutrons, and electrons. While
protons and neutrons combined to form the first atomic nuclei only
a few minutes after the Big Bang, it would take thousands of years
for electrons to combine with them and create electrically neutral
atoms. The first element produced was hydrogen, along with traces
of helium and lithium. Creation Of The Universe
Giant clouds of these primordial elements would coalesce through
gravity to form stars and galaxies, and the heavier elements would
be synthesized either within stars or during supernovae. Creation
of the Solar System and the Earth
Our sun formed about 5 billion years ago. In our universe there
were clouds of dust and asteroids (large space rocks).Collisions
between the asteroids formed larger and larger asteroids which
increased their gravitational pull finally forming planets.
Formation of Earth is estimated at 4.6 billion years ago. Even
after Earth formed, it was constantly bombarded by meteorites. An
early atmosphere formed that contained water vapor, carbon dioxide,
gaseous hydrogen and nitrogen, with little or no oxygen gas. The
oldest rocks found containing iron showed no oxidation (rusting).
Oxidation is found in more recent rock formations. Ask students the
significance of the observation in the last bullet. The presence of
iron without any rusting indicates no oxygen gas was present. Later
rocks show the presence of oxygen gas in red bands of iron that has
oxidized, like those in the image in the slide. Graphic: Campbell
Radioisotopes Can Date Rocks and Fossils
Radioactive isotopes decay in a predictable pattern over a long
period of time. A half-life is the amount of time it takes for
exactly one half of a certain amount of radioactive material to
undergo radioactive decay and form a new substance. Carbon-14 or
14C decays into 14N. The half-life of 14C is 5,700 years.All living
organisms have the same ratio of 14C to 12C, but the amount of 14C
begins to decay once the organism dies and is no longer a part of
the food chain. Determining the amount of 14C in a fossil indicates
its age. Potassium-40 has a half-life of 1.3 billion years and
uranium-238 has a half-life of 4.5 billion years. Emphasize that
the atmosphere has a fixed amount of 14C and 12C and that plants
fix both 14C and 12C through photosynthesis.Wehave established the
ratio of these two forms of carbon in all living plants and this
ratio remains constant throughout the food chain. Therefore, the
ratio of 14C and 12C is the same for all living organisms. Once the
organism dies, carbon is no longer being added to its tissues, thus
the 14C begins to decay into 14Nwhile 12C is a stable isotope and
does not decay at all. SO, we assume the known ratio for living
plants as the ratio that must have been present when the organism
died. Since we know both the starting ratio and the current ratio
of 14C and 12C we can compare them and determine how long ago that
organism died. This specific technique is only valid for fossils up
to about 60,000 years old. For older fossils, we must use isotopes
with longer half-lives, such as those listed in the slide. Graphic
Campbell Possible Steps in the Origin of Life
Shown are the steps necessary to create lifeas we know it These are
the steps that could have occurred in the transition from non-life
to life, sometime called the protobiont hypothesis. This is the
most commonly accepted hypothesis for the origin of life on Earth.
The following slides will examine each of these steps individually.
Students should understand the evidence that suggests the
plausibility of each step. A protobiont is an aggregate of
abiotically produced organic molecules surrounded by a membrane or
a membrane-like structure. Protobionts exhibit some of the
properties associated with life, including simple reproduction,
metabolism and excitability, as well as the maintenance of an
internal chemical environment different from that of their
surroundings. It has been suggested that they are a key step in the
origin of life on earth. Experiments by Sidney W. Fox and Aleksandr
Oparin have demonstrated that they may be formed spontaneously, in
conditions similar to the environment thought to exist on an early
Earth. These experiments formed liposomes and microspheres, which
have membrane structure similar to the phospholipid bilayer found
in cells. Early Atmosphere is Anaerobic
Oxygen is a very corrosive gas. It oxidizes or breaks down many
molecules. If oxygen was present in the early atmosphere, life as
we know it would not exist. When the earth formed, it was extremely
hot with many volcanic eruptions. Point out that oxygen gas is
formed as a product of photosynthesis, thus photosynthesis was not
occurring before life was formed. Oxygen gas is necessary for
combustion and oxidation reactions.If oxygen were in the air
molecules would have been destroyed as fast as they were being
synthesized Stanley Miller and Harold Urey used methane (CH4),
ammonia (NH3), water (H2O), and carbon dioxide (CO2) to simulate
the atmosphere of early Earth. (see next slide) Steam and ice from
meteorites provided Earth with water. Experimental Design: The
origin of life on this planet
The Miller-Urey experiment demonstrated the abiotic synthesis of
organic compounds. Water(H2O),methane(CH4), ammonia(NH3),
andhydrogen (H2) were all sealed inside a sterile array of glass
tubes and flasks connected in a loop, with one flask half-full of
liquid water and another flask containing a pair of electrodes.
Historical note:Originally, Miller reported that 11 amino acids
were formed.After his death in 2007, the Professor Jeffrey Bada,
himself Miller's student, inherited the original equipment from the
experiment when Miller died in Based on sealed vials from the
original experiment, scientists have been able to show that
although successful, Miller was never able to find out, with the
equipment available to him, the full extent of the experiment's
success. Experimental Design: The origin of life on this
planet
The liquid water was heated to induce evaporation, sparks were
fired between the electrodes to simulatelightningthrough
theatmosphereandwater vapor, and then the atmosphere was cooled
again so that the water could condense and trickle back into the
first flask in a continuous cycle. Point out that 1953 Stanley used
methane (CH4), ammonia (NH3), water (H2O), and hydrogen (H2).This
was a reducing atmosphere which favored the building of
molecules.Since 1953, the evidence indicates that the atmosphere
contained water (H2O), carbon dioxide (CO2), nitrogen (N2). This a
neutral atmosphere.When repeated with these gases, organic
molecules were formed although different from the original results
and in differing amounts, but still demonstrating the plausibility
of this crucial first step in the origin of life. It is assumed
that near active volcanoes, the atmosphere was reducing and could
have contributed to the formation of organic molecules.
Experimental Design: The origin of life on this planet
Within a day, the mixture had turned pink in color,and at the end
of two weeks of continuous operation, Miller and Urey observed that
as much as 1015% of thecarbonwithin the system was now in the form
of organic compounds. Experimental Design: The origin of life on
this planet
Two percent of the carbon had formedamino acidsthat are used to
makeproteinsin living cells, withglycineas the most abundant.
Nucleic acids were not formed within the reaction. But the common
20 amino acids were formed, in various concentrations. 23 amino
acids exist, but only 20 are commonly found in living systems.
Synthesis of Small Organic Monomers
Gnter Wchtershuser proposed the Iron-Sulfur World Theory and
suggested that life might have originated at hydrothermal vents
(underwater geysers). Iron sulfide can donate electrons to
dissolved carbon dioxide to form larger organic compounds. This may
have been the beginnings of metabolic reactions. Wchtershuser
proposed that an early form of metabolism predated genetics. By
metabolism he meant a cycle of chemical reactions that release
energy in a form that can be harnessed by other processes. Also
point out that many cofactors of enzymes require iron sulfide.
Graphic Synthesis of Small Organic Monomers
Mineral rich water is heated by geothermal energy. Simulated
hydrothermal vents using carbon monoxide (CO) and potassium
cyanide(KCN)produced amino acids. This is an attractive hypothesis
because of the abundance of CH4 (methane) and NH3 (ammonia) present
in hydrothermal vent regions, a condition that was not provided by
the Earth's primitive atmosphere. Also point out that many
cofactors of enzymes require iron sulfide. Graphic A major
limitation to this hypothesis is the lack of stability of organic
molecules at high temperatures, but some have suggested that life
would have originated outside of the zones of highest temperature.
There are numerous species of extremophiles and other organisms
currently living immediately around deep-sea vents, suggesting that
this is indeed a possible scenario. Synthesis of Small Organic
Monomers
Meteorites that fall to Earth today will often contain amino acids,
carbohydrates and nucleotide bases. This suggests that organic
molecules could have formed in interstellar clouds and then been
transported to Earth on meteorites. Billions of years ago, there
were enormous amounts of meteorites falling to Earth.
Graphic-http://www.bing.com/images/search?q=Earth+4+Billion+Years+Ago&FORM=IQFRDR
Also when meteors are examined they contain both the L- and
D-isomers of amino acids.Living organisms on earth only make the
L-isomers of amino acids. (More about that in the biochem section
of the course.) Polymer Synthesis It has been shown that a solution
ofamino acids dropped onto a hot clay surface could result in the
formation of polypeptide chains.(There is still much debate about
the appearance of polymers.) Lipids will also form organized
droplets with bilayer much like that of a plasma membrane.
Liposomes can reproduce as theyincorporate more lipids or pinch off
smaller droplets.Some liposomes can perform a simple metabolic
reaction. Graphic Protocell Synthesis Some liposomes can reproduce
and perform simple metabolic reactions as shown in the figure
below. Graphic Protocells in a RNA World
Cells consist of a lipid membrane, a genome of DNA that is
transcribed into RNA and ribosomes that translate the RNA into
proteins. The lipid membrane rather easily assembles into
liposomes. Some metabolic reactions could have started in the
presence of iron-sulfide at the thermal vents with products that
eventually moved into the liposomes. Graphic: Discuss The RNA world
hypothesis for the origin of genetic information, with the
importance of natural selection with RNA in the RNA world.
Currently, DNA codes for RNA which in turn codes for protein, with
some of those proteins being the very catalysts/enzymes that allow
each of the previous steps to occur. So, the question becomes how
could this system have evolved? If proteins are needed for the DNA
to both replicate and for it to be transcribed and translated, how
could life have the needed catalytic proteins present before the
DNA had coded for them? Most commonly accepted solution to this
dilemma is that RNA was the first genetic material since we now
know that RNA molecules can serve as both templates (for
replication and translation) AND as catalysts (ribozymes). RNA can
self-replicate as seen in certain RNA viruses. RNA will form 3-D
molecules as there is certain base pairing occurring RNA can act
like an enzyme (riboszyme) .The rRNAspeeds up the peptide bond
formation on a ribosome and other ribozymes on a spliceozome
removes introns in a RNA transcript. Mutations can occur when RNA
is replicating itself so natural selection would RNA molecules that
could replicate better and function better. Protocells are also
called protobionts. Protocells in a RNA World
The first genetic material is believed to be RNA. RNA can
self-replicate and RNA can have exhibit catalytic properties like
enzymes. A ribozyme is an RNA molecule that can be catalytic, but
self-cleaving ribozymes are consumed by their reactions. The RNA
world hypothesis describes an early Earth with self-replicating and
catalytic RNA but no DNA or proteins. Graphic: So, When Did Life
Begin? The Earth formed approximately 4.6 billion years ago, but
the environment was too hostile for life until about 3.9 billion
years ago. The earliest fossil evidence for life dates to 3.5
billion years ago. Taken together, this evidence provides a
plausible range of dates when the origin of life could have
occurred. It should be obvious from the slide, but ask students to
determine the plausible range of dates for the origin of life.
(Life arose sometime between 3.9 and 3.5 billion years ago). The
dates in the slide are taken directly from the AP Bio Curriculum
Framework and constitute part of Essential Knowledge 1.D.2
Prokaryotes As genomes increased in size, it became more
advantageous for DNA rather than RNA to become the primary molecule
of the genome. Why? DNA is physically more stable than RNA and a
less likely to mutate during replication. GraphicCampbell Point out
that anaerobic respiration is glycolysis, thus it is a very ancient
biochemical or metabolic pathway.Almost all cells use this pathway
which serves as strong evidence for there being a common ancestor
for all forms of life. Glycolysis is a conserved core process, in
the words of the AP Bio Curriculum Framework. It is used by
virtually all life forms to generate ATP. Prokaryotes
Characteristics of the first cells: Prokaryotes
Anaerobic as there was no oxygen in the atmosphere First came the
common ancestor,then 3.5 billion years ago the prokaryotesevolved
into two groups: Bacteria and Archaea Graphic: Photosynthesis The
next big biochemical pathway that evolved was photosynthesis.When
first evolved, photosynthesis did not produce oxygen. (3.5 billion
years ago) About 3 billion years ago modern day photosynthesis took
place in cells resembling cyanobacteria and evolved to produce
oxygen, thus converting the atmosphere from a reducing atmosphere
into an oxidizing atmosphere. This conversion caused major changes
on Earth. Graphic -Campbell Emphasize that cyanobacteria (also
called blue-green algae) formed large mats that collected and
precipitated minerals as sediments. There are fossilize
stromatolites found in Australia containing cells that resemble
cyanobacteria. Consequences of Oxygen Production
Productionof oxygen began 2.7 billion years ago.Consequences? Life
can no longer arise from nonliving materials. Organisms that can
tolerate oxygen are at an advantage. Obligate anaerobes either
became extinct or found anaerobic (without oxygen) environments.
Some oxygen formed ozone, O3 which filtered out UV radiation like
the oceans do. Graphic Campbell Emphasize the reactivity of oxygen.
It has a very high electronegativity, thus it is an electron hog
and very reactive.The halogens are also very reactive. Consequences
of Oxygen
Organisms that tolerated oxygen survived. The next major
biochemical pathway that evolved was aerobic respiration. Aerobic
respiration is more efficient at making ATP than anaerobic
respiration. Graphic Campbell Advantages of
Compartmentalizing
The phylogenetic tree shown indicates that Eukarya are more closely
related to Archaea than Bacteria. At one time, these two domains
had a common ancestor before splitting into two groups.The
evolution of the eukaryotic cell involved two processes:
Compartmentalizing& Endosymbiosis Emphasize that Archaea was
thought to be very ancient but many are beginning to believe that
is not the case because of rRNA evidence.It is important to also
note that all life had a common ancestor. The graphic indicates
that Eukarya and Archaea had a more recent common ancestor than
Bacteria and Eukarya.Also emphasize that Archaea can live in very
extreme environments.Some species can tolerate very high
temperatures (thermophiles) , other high salinity (halophiles) and
other low pH (acidophiles). Advantage of Compartmentalizing
Excess membrane folded inward to make the endoplasmic reticulum,
nuclear envelope, Golgi apparatus, and lysosomes. The advantage of
having compartments (or organelles) is to localize chemicalor
metabolic pathways with a common function. Endosymbiosis As
eukaryotes were evolving, there were two separate events that
resulted in additional organelles for eukaryotes. A symbiotic
relationship developed between an ancestral aerobic heterotrophic
bacterial prokaryote (not an Archaea) and a eukaryotic cell. For
whatever reason, this energy-producing aerobic prokaryote took up
residence inside the engulfing eukaryotic cell and was not
destroyed. Graphic Campbell The endosymbiotic theory argues that
mitochondria, plastids (e.g. chloroplasts), and possibly other
organelles of eukaryotic cells, originate through symbiosis between
multiple micro-organisms. According to this theory, certain
organelles originated as free-living bacteria that were taken
inside another cell as endosymbionts. Mitochondria developed from
proteobacteria and chloroplasts from cyanobacteria. Endosymbiosis
The energy-producing aerobic prokaryote eventually became the
mitochondrion. Eventually some of the genes from the mitochondrion
were relocated to the nucleus but other genes
remained.(mitochondrial DNA) Humans inherit their mothers
mitochondrial DNA since only the nucleus of a sperm fertilizes a
human egg. Graphic Campbell- Emphasize that the endosymbiosis
hypothesis is proposed by Lynn Margulis ( ).It was not considered a
viable hypothesis for many years.It was rejected by 15 journals and
finally accepted by Journal of Theoretical Biology. 1966 Kwang Jeon
was studying Amoeba proteus when one of his culture became infected
by a bacterium.Some amoebas died right away but others kept
growing.Kwang continued to culture the amoebas for five additional
years.The descendant amoebas were host to many bacterial cells and
remained healthy.When treated with antibiotics both the bacteria
and amoebas died. Normally, the antibiotic does not kill amoebas!
Same Song, Second Verse A symbiotic relationship developed between
an ancestral autotrophic bacterial prokaryote (not an Archaea) and
a eukaryotic cell. The autotrophic prokaryote eventually became the
chloroplast. Eventually some of the genes from the chloroplast were
relocated to the nucleus but other genes remained in the
chloroplast. Occurred after the endosymbiotic event of the
mitochondrion as all photosynthetic eukaryotic cells have
mitochondria! The chloroplast has its own DNA,which codes for redox
proteins involved in electron transport in photosynthesis. Evidence
for Endosymbiosis
Both mitochondria and chloroplasts have their own ribosomes but the
ribosomes more closely resemble bacterial ribosomes. Both
mitochondria and chloroplasts carry out protein synthesis like a
bacterial cell. Both have their own DNA but it more similar to
bacterial DNA than nuclear DNA. Graphic Evidence for
Endosymbiosis
Since eukaryotic cells have genes from a common ancestral cell that
gave rise to both Archaea and Eukarya, and have genes from the
Bacterial domain due to endosymbiosis, scientists often refer to
The Ring of Life asopposed to a phylogenetic tree as shown in the
previous slides. Graphic Campbell Mention horizontal or lateral
gene transfer. Many believe that the gene sequencingindicates that
eukaryotes have a composite ancestry.Some nuclear genes in
eukaryotes are similar to archaeal genes, while others are more
similar to bacterial genes.Archaea-like nuclear genes govern
genetic processes (DNA replication, transcription and
translation).Bacteria-like nuclear genes tend to govern metabolism
and membrane formation. Genes (not all) from the chloroplasts and
mitochondria have been relocated to the nucleus. Evolution of
Singled Celled Eukaryotes
Graphic Campbell Lipids are biomarkers for eukaryotic cells.A
biomarker is a compound made only by a particular type of cell.
Earliest evidence of eukaryotic cells is 2.7 billion years old
Eukarya and Sexual Reproduction
The next evolutionary milestone is the advent of sexual
reproduction in eukaryotic cells.Red algae is pictured on the right
and it is the oldest species known to reproduce sexually. Sexual
reproduction ensures genetic variability and exchange of genetic
material.As a result, evolution occurs more rapidly. Most
eukaryotes are protists (singled-celled eukaryotes) and not
multicellular organisms. Graphic Starr and Taggart Sexual
reproduction is an important part of evolution on Earth.It is a way
to have new recombination of genes and new phenotypes.Some of these
new phenotypes may have a better chance of survival and more
importantly a better chance of reproducing resulting in the passing
of advantageous genes to offspring. While mutations are the only
source of new genes, sexual recombination provides many new
combinations of existing genes upon which natural selection can
act. Eukarya and Multicellular Organisms
Following the emergence of sexual reproduction, the next milestone
is the evolution of multicellular eukaryotes. The oldest
multicellular species known is a small algae appearing 1.2 billion
years ago. By 900 mya representatives of all the major lineages,
includingalgae, fungi, & animals had evolved in the seas.
Larger and more diverse multicellular organisms do not appear until
565 million years ago due to the fact that landmasses as well as
the seas were largely covered with ice often referred to as the
Snowball Earth. Graphic Campbell Multicellular organisms require
more coordination between cells.Specialization becomes important
and increases both efficiency and chances of survival.
Multicellularity gave way to cell specialization and specialized
body parts. CambrianExplosion Many phyla of living animals first
appear in the fossil record during the Cambrian Explosion of
million years ago. Appearance of animals with exoskeletons, spines,
and body armor. Some animals became carnivores with large claws and
other features for capturing prey. Graphic Campbell Point out
previous to this time animals had soft bodies, and were herbivores,
scavengers or filter feeders. Also point out that this explosion
was only somewhat rapid in a geological sense with it actually
taking over 10 million years to occur. Evolutionary biologists
point out that this explosion actually began prior to the Cambrian
period. Colonization of Land Larger more complex forms of life move
onto land about 500 million years ago. Successful plant adaptations
such as a vascular system for transporting materials, supportive
tissue, waxy cuticles to prevent dehydration, and the ability to
reproduce on land occurred 420 million years ago. By 50 million
years ago there was a great diversification of plants. Graphic
Campbell While some cyanobacteria had colonized the land over a
billion years ago, they were small and very simple compared to the
more complex forms of life that colonized land. Colonization of
Land Fungi moved onto land about the same time as plants.They
structurally simpler than animals or plants. Needed animal
adaptations include: prevention of dehydration, support, and
reproductive success on land. Graphic Campbell Colonization of Land
Arthropods were the most successful.
The tetrapods inhabited the terrestrial environment 365 million
years ago. The common ancestor for man and chimpanzee existed 6-7
million years ago. Homo sapiens arrived 195,000 years ago. Graphic
Campbell Changes on Earth Influence Evolution
Graphic Campbell The earths crust consist of several solid plates
as shown.The plates (40 km thick) are sitting on a layer of molten
magma.These plates can move by sliding under one another, or
gliding past one another. Continental Drift Over the course of time
the continents have been together as one large land mass (Pangaea)
and at other times they have been apart as they are now. These
movements cause changes in climate, ocean currents, the formation
of glaciers and other geological phenomenon. These changes occur
slowly, but can result in mass extinctions. Graphic Campbell
Continental drift has allowed allopatric speciation to occur on a
very large scale.Climates changed dramatically as a result.Example:
The tip of Labrador, Canada was once on the equator.The change
between the two latitudes is about 40o.This took 200 million years
to complete. Changes on Earth Influence Evolution
The Permian Extinction or the Great Dying A single volcano can cool
global temperatures. 251 million years ago in Serbia there was a
collision of continents which caused a multitude of volcanic
eruptions. At this time all the continents were one (Pangea).
Graphic
-http://skywalker.cochise.edu/wellerr/students/pinatubo2/project.htm
Mt. Pinatubo cooled the global temperature 0.5oCin Karakatua in
1883 cooled global temperatures 1.2oC. There are five major
extinctions in which 50% or more of all the species were
eliminated.Students do not have to know the 5 extinctions but do
need to know that changes on Earth can cause mass extinctions. The
eruption is thought to have produced a tremendous amount of ash,
blocking the sunlight, thus affecting glacial formations and the
oceans. The Permian extinction or Great Dying wiped out 96% of all
marine species and 70% of the terrestrial species. Cretaceous Mass
Extinction
This mass extinction occurred 65 million years ago. Fifty percent
of all marine species went extinct and many families of terrestrial
animals also went extinct. This extinction was caused by a
meteorite colliding with Earth.It caused great tsunamis and a
massive plume of debris that rose into the atmosphere and spread
around Earth. It heated the atmosphere several hundred degrees and
ignited massive fires.It also blocked the sun which greatly
affected plant life. Graphic Campbell There are five major
extinctions in which 50% or more of all the species died
out.Students do not have to know the 5 extinctions but do need to
know that changes on Earth can cause mass extinctions.The two most
famous ones are mentioned in this and the previous slide.The
Cretaceous Mass Extinction is famous for bringing about the end of
the dinosaurs. Great Resource: Students might enjoy knowing the
evidence for this event and how it became generally accepted by the
scientific community. See The Day The Mesozoic Died short film at
It can take million years for biodiversity to recover.Mass
Extinctions also allow for adaptive radiation to occur. Great
Oxygen Catastrophe
Since the beginning of photosynthesis, 2.4 billion years ago,oxygen
has been produced as a byproduct of metabolic processes.For the
first billion or so years after the onset of photosynthesis, oxygen
levels remained low because the oxygen reacted with minerals like
iron that could be oxidized. Graphic: Great Oxygen
Catastrophe
Increases in oxygen levels led to a mass extinction of obligate
anaerobes that could not tolerate oxygen. Maximum oxygen levels
were reached 250 million years ago. The greatest mass extinction in
Earth's history occurred 250 million years ago, when 90 percent of
all marine animal species were wiped out, along with a huge portion
of plant, animal and insect species on land.A massive amount of
volcanism in Siberia is widely credited with driving the disaster,
but even after the immense outpourings of lava and toxic gases
tapered off, oxygen levels in the oceans, which had been depleted,
remained low for about 5 million years, slowing life's recovery
there to an unusual degree. Great Oxygen Catastrophe
The large plants lived in low, swamp land.As the plants died they
were buried in the swamp. The buried plants did not decompose and
eventually became deposits of coal. The swamp lands dried upand
oxygen production decreased to levels even lower than today. Tempo
of Evolution Graphic- Campbell Tempo of Evolution Graphic- Campbell
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