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AP: CHAPTER 26: ORIGIN OF LIFE


1. Start with the origin of the earth and identify the time frame, conditions, and evidence for each of the following steps leading to current life forms on earth.

a. Origin of the earth

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The earth formed out of a collection of dust and gas around 4.6 billion years ago. The early earth was characterized as an extremely volatile planet with a reducing atmosphere. Kinetic heat and friction prevented any great amount of water from condensing into an “ocean”, although water vapor was present. Because there was hardly any free oxygen, organic molecules could not break down as quickly. The “energy sources” of early earth derived from lighting and the sun’s radiation. Around 3.9 billion years ago, the first oceans began to form and the earth's crust cooled.
Fossilized cholesterol and iron oxide bands are proof of oxygen about 2.7 billion years ago.


b. Prokaryotes

Prokaryotes first appear 3.5 years ago and solely inhabit the earth until about 2 million years ago. Successful protobionts evolved to be able to make at least some of the molecules it needed. This gave way for the first prokaryote. Prokaryotes used molecules from the primitive soup and sunlight to start the process of photosynthesis.
evidence: the oldest known fossils are in the form of stromatolites, (3.5 million years ago) contained mainly many layers of bacteria (the new prokaryotes) and sediment.

bacterias.jpg
--Brian N and Jordan W
c. Oxidizing atmosphere
Most atmospheric oxygen is of biological origin, cyanobacteria, derived from the splitting step of water in photosynthesis. The free oxygen from these organisms probably first dissolved into oceans, and additional oxygen reacted with dissolved iron and precipitated as iron oxide, which accumulated as sediments. The sediments compressed into banded iron formations. Once all the dissolved iron had precipitated the oxygen began to gas out.
Jordan W Brian N and ilian D
http://richard-seaman.com/USA/States/Nevada/ValleyOfFire/Sampler/StripedMountain.jpg
http://richard-seaman.com/USA/States/Nevada/ValleyOfFire/Sampler/StripedMountain.jpg


d. Eukaryotic cells
The first eukaryotic cells appear about 2.1 million years ago. Many scientists believe that these cells were created by a process called endosymbiosis. This is when an endosymbiont lives within a host cell. Mitochondria and plastids are believed to have formed this way. This was beneficial to both parties. The host would get the nutrients released from the endosymbiont and in return it would be protected from the oxygen outside the cell. mitosis and the splitting of cells lead to a new way to reproduce and create genomes.
They formed during the oxygen revolution but researchers also postulate a much earlier eukaryotic origin based on traces of molecules similar to cholesterol found on rocks based dating back 2.7 billion years ago.





e. Multicellular life
The earliest forms of multicellular eukaryotes is 1.5 billion years ago. Most early multicellular eukaryotes are algae. Larger organisms did not come about until 570 million years ago. This is so because of the snowball earth hypothesis: most life would have been constricted to deep sea vents and hot springs due to the severe ice age. Early animals and algae are found in fossils dating back to 570 million years ago. The first multicellular organisms lived in colonies around 1.2 billion years ago such as algea and we have fossil evidance of this finding. Some cells in these colonies became specialized for different functions. Cell differentiation and cell division led to the evolution of tissues, organs and organ systems in fungi, plants and animals.




2. What was significant about the discovery of the iron oxide bands in the sedimentary layers.
MBC.jpg
The significance was that the iron oxide bands proved that oxygen had mixed with water and iron in large enough amounts to create such bands. This shows that at the beginning of these bands there must have been a large amount of oxygen in the atmosphere. This amount of oxygen in the atmosphere comes from the blue-green algae found in oceans today. We know this by comparing fossils from the time of the iron oxide bands.


The iron oxide bands in the sedimentary rock proved that there was a significant amount of oxygen in the atmosphere. This is known because there needs to be enough oxygen to react with all of the iron in the rocks to form the bands. Though it was not the existence of oxygen that made this discovery significant, it was that there must have been something creating the oxygen. This brings about the existence of the first organisms to perform photosynthesis, cyanobacteria. This would eventually lead to the existence of plants and the allowance for herbivores.

The significance is huge for the fact that they prove that oxygen was plentiful in the atmosphere. The bands are iron oxide that can only occur if the iron comes in contact with oxygen. This is also evidence of the existence of organisms capable of photosynthesis very early on in the history of life.
~Ilian D











3. Describe the theory of endosymbiosis.

The theory of endosymbiosis says that mitochondria and plastids used to be small prokaryotes living within larger cells. The prokaryotic ancestors of mitochondria and plastids were bacteria engulfed by a larger cell. Because they both benefited from this situation, the bacteria living inside the cell was passed down from generation to generation. The evidence is that mitochondria reproduce and move independently within the cell.

external image endosymbiosis.gif




4. Why did evolution seem to slow 750 to 570 million years ago?
snowball_eart.gif
Evolution seemed to slow due to a global ice age, called the snowball earth hypothesis. This ice age caused the freezing of the entire earth killing almost all of the living life forms. The only organisms that seemed to survive were in deep sea vents. The killing of these organisms made the surviving organisms have to restart evolution, slowing the process




5. What was special about the Cambrian Explosion?

The Cambrian explosion was the sudden appearance of most of the major phyla of animals in the fossil record about 530 million years ago. It was accompanied by a major diversification of other organisms. The Cambrian explosion causes much debate because it was so abrupt and seemingly out of nowhere. Charles Darwin even saw it as one of the major objections to natural selection.


cambrian.jpgCambrian_Explosion.jpg
Most of the major animal phyla suddenly show up the fossil record during the first 20 million years of the Cambrian period. Many large animals with hard shells and exoskeletons appear. Martin A.



6. Describe a few adaptations essential for the invasion of plants onto land.

One adaptation that was essential for the invasion of plants onto land was that they developed a water resistant coating on their leaves so that they didn’t lose water as quickly to the air. This allowed the plants to be able to soak up water and store it in there leaves and release it when necessary. Another adaptation that was essential for the invasion of plants onto land was pigment change. Plants had to change the pigment on their leaves to help protect them from the sun. – Jackie H.
water-moving.gifplant.jpg
Another adaptation that plants endured was the opening of the pores on their leaves. These open pores allow for water movement and gas exchange.

First, new plants colonizing on earth developed a waterproof coating of wax on their leaves to slow the loss of water. This helped prevent dehydration in plants, making it possible to reproduce on land. Also, plants colonized in the company of fungi. Roots of plants associated with fungi help absorb water and minerals from the soil. This symbiotic relationship between plants and fungi help both obtain nutrients form each other.

Plants needed a way to prevent dehydration so that they would be able to produce on land. Plants that evolved from green algae developed a wax coat on their leaves to mitigate water loss to the air. Plants also developed a thick cell for structure and support. They also evolved a vascular root system to get water deep in the soil. Stomata developed in the leaves for gas and water flow in and out of the plant. Martin A.


Plant_Cell.jpgStomata.gif



7. Scientific Hypothesis for the origin of life (briefly elaborate or explain each of these)
a. The first cells may have originated by chemical evolution on a young Earth: external image mRNA-colored.gif
Nucleotides used phosphate molecules to bond to other nucleotides, forming RNA, which could interact with amino acids and make them produce proteins. Fat molecules aexternal image mu_apparatus1234406029.jpgssembled into bubbles, trapping RNA inside; once a bubble was full it divided into two.
b. Abiotic synthesis of organic monomers is a testable hypothesis:
Laboratory experiments demonstrate that protobionts could have formed spontaneously from abiotically-produced organic compounds. Monomers linked together to form polymers.
c. Laboratory simulations of early-Earth conditions have produced organic polymers: The Miller-Uley experiment proved that organic molecules can form in a reducing atmosphere.
d. RNA may have been the first genetic material: RNA can synthesize proteins and be the catalysts in a number of enzyme-like functions. Ribozymes make copies of RNA with nucleotide building blocks, adenine, guanine, cytosine, and uracil, and these blocks create a single-stranded chain. Once RNA sequences began to carry genetic information in protobionts, further adaptations were possible.
e. Protobionts can form by self-assembly: Polymers were found to synthesize abiotically, and protobionts are the collection of abiotically produced molecules.
f. Natural selection could refine protobionts containing hereditary information:
external image protobiont.jpg
Protobionts form spontaneously from mixtures of organic molecules. They contain RNA that codes for metabolic proteins. These protobionts absorb food and the proteins catalyze it to make energy which can be used for growth and division to daughter cells. Natural selection would favor protobionts that grow and replicate. When the organic molecules in the earth’s water bodies were gone, the protobionts would rely on natural selection to either obtain energy by photosynthesis or predation. It would only take the creation and evolution of one protobiont to give rise to the all the different organisms we see today.
g. Debate about the origin of life abounds:
Scientist attempt to achieve causality in research, therefore they need to control the inaccuracy. As a result, scientists try to define specific conditions leading up to the effect they seek to replicate, like life. Trying to identify the specific conditions necessary for life, a problem arises. Life can have many forms, not just our organic understanding of it. Lamarck, Darwin, and many other scientists have debated the origin of life.

-Amanda P.



external image 297_EarlyEarth.jpg
8. Describe the hypothesized conditions on earth when life arose.

Life began when the air cooled enough for molecules to condense and form bodies of water. The atmosphere was still thick with different compounds, like nitrogen, carbon dioxide, methane, hydrogen, and ammonia, released from volcanoes. The atmosphere was an electron-adding, or reducing, environment in which organic compounds could have formed from simple molecules. The oceans, once created, had many hot springs and deep-sea vents, which is where the first life is thought to have developed.




9. What did Louis Pasteur demonstrate with his experiment?

He demonstrated that spontaneous generation does not occur in micro organisms.
Pasteur showed that bacteria are not able to spawn from nothing. Pasteur revealed that the microorganisms present were actually from an outside source, and that they had reproduced to form a colony.external image moz-screenshot-3.jpg
Louis_Pasteur.jpgbacterias.jpg



In Louis Pasteur’s experiment he used a flask open to air, on the left, and an s shaped flask, on the right. The s shaped flask was meant to stop dust particles from getting into the broth which filled the flasks. After boiling the two broths, to kill any living matter in the liquid, he let them sit. The broth susceptible to open air, became cloudy, with living matter, and the broth in the “S” shaped flask stayed the same. He concluded that if spontaneous generation could occur the broth in the “S” shaped flask would have also became cloudy, but since it did not, it was evident that germs only come from other germs.




10. List the four stages for the formation of life.
a. The abiotic synthesis of small organic molecules such as amino acids and nucleotides.

b. The joining of small molecules (monomers) into polymers, including proteins and nucleic acids.

c. The origin of self-replicating molecules that eventually make inheritance possible.

d. The packaging of all these molecules into "protobionts," droplets with mem
branes that maintain an internal chemistry different from the surroundings.




11. What metabolic processes would you expect to see in protobionts?
Primitive metabolic enzymes, simple reproduction, and the maintenance of an internal chemical environment different from that of their surroundings.
. external image moz-screenshot-1.jpgexternal image c26x12protobionts.jpg


external image moz-screenshot.jpg
-Amanda P.



12. Why is RNA now thought to be the first genetic code?
RNA is thought of to be the first genetic code because it is a lot less sophisticated than DNA and we know that DNA evolved from RNA. RNA plays a central role in the making of proteins. The ribosomes use the RNA to make the proteins. RNA when it was first around was very short and as time went on, they became more adaptive, longer and eventually developed DNA and the double-helix structure. RNA became DNA through many years of mutation and natural selection to make the genetic material better.




13. What did Oparin, Haldane, Miller and Urey accomplish?
Oparin and Haldane independently hypothesized that Earth's atmosphere had been a reducing environment, in which organic compounds could have formed from simple molecules. The energy for this to happen could have come from lightning and UV radiation. Haldane suggested that the early oceans were a solution of organic molecules from which life arose. Miller and Urey tested their hypotheses by creating conditions in a lab similar to how conditions were on Earth when life first arose. Their test yielded a variety of amino acids found in organisms today, proving the hypothesis was correct.
millerurey.jpg





14. What are some of the possible locations for the first life forms?

deep-sea_vent.jpeg prebiotic.jpg

Submerged volcanoes, deep-sea vents, and weak points in Earth’s crust where hot water and minerals gush into the ocean. These regions are rich in inorganic sulfur and iron compounds, which are important in ATP synthesis in modern-day organisms, thus proving to be a potentially perfect area for the Earth's first life forms.
-Amanda P