Origin of Life

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Course: B.SC. (MICRO)Subject : ELEMENTARY MICROBIOLOGY

Unit: 1.2

• Universe formed 15 billion years ago (Big Bang)

• Galaxies formed from stars, dust and gas

• Earth formed 4.6 billion years ago

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• Suns energy stripped away 1st atmosphere

• 2nd atmosphere formed from volcanic outgassing

• Primitive atmosphere: CO2, water vapor, lesser amts

of CO, N2, H2, HCl, and traces of NH3 and CH4 (3.5 bya)

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• O2 came in 1.5 bya

• Autotrophic Organisms: photosynthesis

• Another environmental change

• Result in evolution

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• 0.5 billion years ago

• Atmosphere O2 to 1% current

• Compare to present: 78% N2, 21% O2, 0.04% CO2, + trace gasses

• Relatively small, most single cell

• Start of multicellularity

• Increase in cell complexity5

Life began~ 3.5 bya

Organic molecules (C H O N P S) swimming in shallow seas

Stage 1: Abiotic synthesis of organic molecules such as proteins, amino acids and nucleotides

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Stage 2: joining of small molecules (monomers) into large molecules

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Stage 3: origin of self-replicating molecules that eventually made inheritance possible

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Stage 4: packaging these molecules into pre-cells, droplets of molecules with membranes that maintained an internal chemistry

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MAJOR EPISODES IN THE HISTORY OF LIFE

– Earth was formed about 4.6 billion years ago.

– Prokaryotes

• Evolved by 3.5 billion years ago

• Began oxygen production about 2.7 billion years ago

• Lived alone for almost 2 billion years

• Continue in great abundance today

– Single-celled eukaryotes first evolved about 2.1 billion years ago.

– Multicellular eukaryotes first evolved at least 1.2 billion years ago.

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Resolving the Biogenesis Paradox

– All life today arises by the reproduction of preexisting life, or biogenesis.

– If this is true, how could the first organisms arise?

– From the time of the ancient Greeks until well into the 19th century, it was commonly believed that life regularly arises from nonliving matter, an idea called spontaneous generation.

– Today, most biologists think it is possible that life on early Earth produced simple cells by chemical and physical processes.

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Figure 15.312

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A Four-Stage Hypothesis for the Origin of Life

– According to one hypothesis, the first organisms were products of chemical evolution in four stages.

– Stage 1: Abiotic Synthesis of Organic Monomers

• The first stage in the origin of life has been the most extensively studied by scientists in the laboratory.

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The Process of Science: Can BiologicalMonomers Form Spontaneously?

– Observation: Modern biological macromolecules are all composed of elements that were present in abundance on the early Earth.

– Question: Could biological molecules arise spontaneously under conditions like those on the early Earth?

– Hypothesis: A closed system designed in the laboratory to simulate early Earth conditions could produce biologically important organic molecules from inorganic ingredients.

– Prediction: Organic molecules would form and accumulate.

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The Process of Science: Can BiologicalMonomers Form Spontaneously?

– Experiment: An apparatus was built to mimic the early Earth atmosphere and included

• Hydrogen gas (H2), methane (CH4), ammonia (NH3), and water vapor (H2O)

• Sparks were discharged into the chamber to mimic the prevalent lightning of the early Earth

• A condenser to cool the atmosphere, causing water and dissolved compounds to “rain” into the miniature “sea”

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The Process of Science: Can BiologicalMonomers Form Spontaneously?

– Results: After the apparatus had run for a week, an abundance of organic molecules essential for life had collected in the “sea,” including amino acids, the monomers of proteins.

– Since Miller and Urey’s experiments, laboratory analogues of the primeval Earth have produced

• All 20 amino acids

• Several sugars

• Lipids

• Purine and pyrimidine bases (found in DNA, RNA & ATP)

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Stage 2: Abiotic Synthesis of Polymers

– Researchers have brought about the polymerization of monomers to form polymers, such as proteins and nucleic acids, by dripping solutions of organic monomers onto

• Hot sand

• Clay

• Rock

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Stage 3: Formation of Pre-Cells

– A key step in the origin of life was the isolation of a collection of abiotically created molecules within a membrane.

– Laboratory experiments demonstrate that pre-cells could have formed spontaneously from abioticallyproduced organic compounds.

– Such pre-cells produced in the laboratory display some lifelike properties. They:• Have a selectively permeable surface

• Can grow by absorbing molecules from their surroundings

• Swell or shrink when placed in solutions of different salt concentrations

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Inorganic compounds

Abiotic synthesisof organic monomers

Abiotic synthesisof polymers

Formationof pre-cells

Self-replicatingmolecules

Membrane-enclosed compartment

Complementarychain

Polymer

Organic monomers

Figure 15.UN0420

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Stage 4: Origin of Self-Replicating Molecules

– Life is defined partly by the process of inheritance, which is based on self-replicating molecules.

– One hypothesis is that the first genes were short strands of RNA that replicated themselves without the assistance of proteins, perhaps using RNAs that can act as enzymes, called ribozymes.

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Original “gene”

ComplementaryRNA chain

Figure 15.522

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From Chemical Evolution to Darwinian Evolution

– Over millions of years• Natural selection favored the most efficient pre-cells

• The first prokaryotic cells evolved

– Prokaryotes lived and evolved all alone on Earth for 2 billion years before eukaryotes evolved.• Are found wherever there is life

• Far outnumber eukaryotes

• Can cause disease

• Can be beneficial

– Prokaryotes live deep within the Earth and in habitats too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote to survive.

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References:

• Images (1 to 4) : http://www.tutorvista.com/content/biology/biology-iii/origin-life/origin-life-steps.php

• Image 5: http://www.bio.utexas.edu/faculty/sjasper/Bio213/earthhist.html

• Image 6: http://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment#mediaviewer/File:Miller-Urey_experiment-en.svg

• Image ( 7 & 8) : http://www.slideshare.net/gdot204/evolutionary-theory-11965379

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