bang. - houston independent school district · origin of life 2 . lo 1.27 the student is able to...

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Macroevolution: Part IV

Origin of Life

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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/580

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• 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 t combine with them and create electrically neutral atoms.

• The first element produced was hydrogen, along with traces of helium and lithium.

http://en.wikipedia.org/wiki/Big_Bang

Macroevolution Part IV: Origin of Life TEACHER NOTES

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• 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.

http://en.wikipedia.org/wiki/Big_Bang

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Creation of the Solar System and the Earth

• Even after Earth formed, it was constantly bombarded by meteori

• An early atmosphere formed that contained water vapor, carbon dioxide, gaseous hydrogen and nitrogen, with little or no oxygen g

• The oldest rocks found containing iron showed no oxidation (rusti

• Oxidation is found in more recent rock formations.

• Our sun formed about 5 billion years ago.

• In our universe there were clouds of dust a asteroids (large space rocks). Collisions between the asteroids formed larger and l asteroids which increased their gravitation pull finally forming planets. Formation of E is estimated at 4.6 billion years ago.

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

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Radioisotopes Can Date Rocks and Fossils

• The half-life of 14C is 5,700 years. All living organisms have the sa 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 h a half-life of 4.5 billion years.

• Radioactive isotopes decay in a predictab pattern over a long period of time.

• A half-life is the amount of time it takes fo exactly one half of a certain amount of radioactive material to undergo radioactiv decay and form a new substance.

• Carbon-14 or 14C decays into 14N.

Emphasize that the atmosphere has a fixed amount of 14C and 12C and that plants fix both 14C and 12C through photosynthesis. We have 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 14N while 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


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Possible Steps in the Origin of Life

• Shown are the steps necessary to create life as 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. http://en.wikipedia.org/wiki/Protobiont

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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 wo not exist.

• When the earth formed, it was extremely hot with many volcanic eruptions.

• Steam and ice from meteorites provided Earth with water.

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. 1953 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)

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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), and hydrogen(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 2007. 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.


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• The liquid water was heated to induce evaporation, sparks were fired between the electrodes to simulate lightning through the atmosphere and water 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.

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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.

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• 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 10–15% of the carbon within the system was now in the form of organic compounds.

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• Two percent of the carbon had formed amino acids that are used to make proteins in living cells, with glycine as the most abundant. Nucleic acids were not formed within the reaction. But the common 20 amino acids were formed, in various concentrations.

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23 amino acids exist, but only 20 are commonly found in living systems.

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Synthesis of Small Organic Monomers

• Günter Wächtershäuser 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.

Wächtershäuser 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. http://en.wikipedia.org/wiki/Hydrothermal_vent Also point out that many cofactors of enzymes require iron sulfide. Graphic http://www.seasky.org/deep-sea/hydrothermal-vents.html


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• 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 http://www.seasky.org/deep-sea/hydrothermal-vents.html 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.

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• 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.)

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Polymer Synthesis

• It has been shown that a solution of amino 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 they incorporate more lipids or pinch off smaller droplets. Some liposomes can perform a simple metabolic reaction.

Graphic http://en.wikipedia.org/wiki/Liposome

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Protocell Synthesis

Some liposomes can reproduce and perform simple metabolic reactions as shown in the figure below.

Graphic


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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: http://polynomial.me.uk/2009/08/ 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 rRNA speeds 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.

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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: http://helicase.pbworks.com/w/page/17605685/Lindsey-Jordan


http://helicase.pbworks.com/w/page/17605685/Lindsey-Jordan

http://helicase.pbworks.com/w/page/17605685/Lindsey-Jordan

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• 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.

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So, When Did Life Begin?

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 are taken directly from the AP Bio Curriculum Framework and constitute part of Essential Knowledge 1.D.2

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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.

Graphic Campbell 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.

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Prokaryotes

Characteristics of the first cells:

1. Prokaryotes

2. Anaerobic as there was no oxygen in the atmosphere

3. First came the common ancestor, then 3.5 billion years ago the prokaryotes evolved into two groups: Bacteria and Archaea

Graphic: http://3dbiology.pbworks.com/w/page/912247/Existing%20Models


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• The next big biochemical pathway that evolved was photosynthesis. When first evolved, photosynthesis did notproduce 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.

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] Consequences of Oxygen Production

Production of oxygen began 2.7 billion years ago. Consequences?

1. Life can no longer arise from nonliving materials.

2. Organisms that can tolerate oxygen are at an advantage. Obligate anaerobes either became extinct or found anaerobic (without oxygen) environments.

3. 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.

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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


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Advantages of Compartmentalizing

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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).

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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 chemical or metabolic pathways with a common function.

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EndosymbiosisAs 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 notdestroyed.

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. http://en.wikipedia.org/wiki/Endosymbiotic_theory

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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 mother’s 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 (1938-2011). 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!

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Same Song, Second Verse• A symbiotic relationship develo

between an ancestral autotrophbacterial prokaryote (not an Archaea) and a eukaryotic cell.

• The autotrophic prokaryote eventually became the chloropl

• Eventually some of the genes fr the chloroplast were relocated t the nucleus but other genes remained in the chloroplast.

• Occurred after the endosymbio event of the mitochondrion as a photosynthetic eukaryotic cells have mitochondria!

The chloroplast has its own DNA, which codes for redox proteins involved in electron transport in photosynthesis.


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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 http://en.wikipedia.org/wiki/Chloroplast

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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 as opposed to a phylogenetic tree as shown in the previous slides.

Graphic Campbell Mention horizontal or lateral gene transfer. Many believe that the gene sequencing indicates 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.


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Evolution of Singled Celled Eukaryotes

Earliest evidence of eukaryotic cells is 2.7 billion years old

Graphic Campbell Lipids are biomarkers for eukaryotic cells. A biomarker is a compound made only by a particular type of cell.

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] Eukarya and Sexual Reproduction

• The next evolutionary milestone is the advent of sexual reproduct 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.


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Eukarya and Multicellular Organisms

• Following the emergence of sexual reproduction, the next milestone is the evolution of multicellular eukaryotes. T oldest multicellular species known is a small algae appearing 1.2 billion years a

• By 900 mya representatives of all the major lineages, including algae, fungi, animals had evolved in the seas.

• Larger and more diverse multicellular organisms do not appear until 565 milli years ago due to the fact that landmass as well as the seas were largely covered with ice often referred to as the “Snow Earth”.

Multicellularity gave way to cell specialization and specialized body parts.

Graphic Campbell Multicellular organisms require more coordination between cells. Specialization becomes important and increases both efficiency and chances of survival.

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Cambrian Explosion

• Many phyla of living animals first appear in the fossil reco during the Cambrian Explos of 535-525 million years ago

• Appearance of animals with exoskeletons, spines, and bo armor.

• Some animals became carnivores with large claws a 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.

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Colonization of Land

• Larger more complex forms of move onto land about 500 mil years ago.

• Successful plant adaptations s as a vascular system for transporting materials, supportive tissue, waxy cuticle to prevent dehydration, and th ability to reproduce on land occurred 420 million years ago

• By 50 million years ago there w a great diversification of plant

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.

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• Fungi moved onto land about the same time as plants. They structurally simpler than animals or plants.

• Needed animal adaptations include: prevention of dehydratio support, and reproductiv success on land.

Graphic Campbell


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• 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

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Changes on Earth Influence Evolution

• The earth’s crust consist of several solid plates as shown. The pl (40 km thick) are sitting on a layer of molten magma. These plat can move by sliding under one another, or gliding past one anot

Graphic Campbell

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Continental Drift

• Over the course of time the continents have been together as o large land mass (Pangaea) and at other times they have been apart a they are now.

• These movements cause changes i climate, ocean currents, the forma of glaciers and other geological phenomenon.

• These changes occur slowly, but ca 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.

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] Changes on Earth Influence EvolutionThe 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 multitude of volcanic eruptions.

• At this time all the continen were one (Pangea).

• The eruption is thought to have produced a tremendous amount ash, blocking the sunlight, thus affecting glacial formations and th oceans.

• The Permian extinction or Great Dying wiped out 96% of all marin species and 70% of the terrestrial species.

Graphic -http://skywalker.cochise.edu/wellerr/students/pinatubo2/project.htm Mt. Pinatubo cooled the global temperature 0.5oC in 1992. 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.


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Cretaceous Mass Extinction

• This mass extinction occurred 65 million years ago. Fifty percent o 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 in 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 http://www.hhmi.org/biointeractive/shortfilms/index.html. It can take 5-100 million years for biodiversity to recover. Mass Extinctions also allow for adaptive radiation to occur.

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Great Oxygen Catastrophe

• Since the beginning of photosynthesis, 2.4 billion years ago, oxygen has been produced as a byproduct of metabolic process 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: http://bio230fall2010.wordpress.com/2011/08/

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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. http://news.stanford.edu/news/2011/march/permian-mass-extinction-032411.html


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] 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 up and oxygen production decreased to levels even lower than today.

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Tempo of Evolution

Graphic- Campbell

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Tempo of Evolution

Graphic- Campbell

49 Created by:

Carol LeiblScience Content DirectorNational Math and Science


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