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Origins of Life-2
Unit 1 Part 1
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Origins of Life -2
Objectives• Describe the chemical and physical conditions of the
pre-biotic Earth• Discuss several theories of the origins of life on Earth.• Describe how the success of prokaryotic life has
changed the chemical conditions of this planet.• Discuss how our understanding of the origins of life on
Earth is helping us look for life (or the evidence of past life) elsewhere in the universe (discuss examples Mars, Titan, etc.).
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Earth• The earth is 4.5 to 4.7 Billion Years Old
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Major landmarks in biological evolution
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No Life 0.5-1 billion years Life?
How did we get here?
4.5 billion years ago ~3.5 billion years ago
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Inorganic Organic Cellular organization
ProgenoteUniversal Ancestor
Pre-life Earth was a mixture of chemicals Elements of life H, O, C, N, S, P, (and a little P, Fe, Na, and K)
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Theories of the Origins of Life
1. Spontaneous formation of organic molecules
2. “RNA” world
3. Solid-phase “Iron-Sulfur” world
•
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Theories about the origin of life on Earth
• Spontaneous Formation of Organic Molecules (Oparin and Haldane 1920s) – the combination of reduced gasses and
electricity (or UV radiation or nuclear radiation) caused the formation of more complicated organic compounds
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• Spontaneous formation evidence– Urey & Miller experiments (1950s)
• Created amino acids, sugars, fatty acids, thioesters, and nucleotide bases in the lab
– Amino acids and other organic molecules are found in meteorites (shows that it can happen without human intervention)
– Detected in space (spectral analysis)
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Theories
• “RNA world”– RNA has two properties that make it an
attractive first polymer of life1. Contains information (nucleotide bases)
2. Catalytic (RNA can do some chemical reactions)
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The RNA World
The current “most accepted “theory of life evolving hypothesizes an RNA world
RNA in the early world would have functioned as a self replicating molecule, eventually developing a number of minimal catalytic properties
self-replicates Proteins take over catalysis
DNA becomeslong term storage
and major coding molecule
Packaging evolves - RNA codes and catalyses
RNA
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Rybozymes
The first ribozyme discovered was a piece of mRNA that could self-edit – cut out the introns.
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Order of the Information Macromolecules
DNAProteinsRNA
RNA required for protein synthesis (mRNA, tRNA, rRNA)DNA is a modification of RNA
Where did the first RNA come from?
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Ribosymes
• The first RNA polymers may have formed on clay templates (clay molecules have very regular structures)
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Origin of Life
RNA World-Most accepted theory-In early world as self-replicating molecule-Later developing other catalytic properties
Stages of RNA World:1-Formation of nucleotides (spontaneous formation?)2-Formation of RNA molecules (on clay template?)3-RNA replication
RNA molecules that evolve catalytic activities (protein synthesis) had a selective advantage
4-The link between sequence of RNA and sequence of proteins (translation)
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The RNA World
self-replicates Proteins take over catalysis
DNA becomeslong term storage
and major coding molecule
Packaging evolves - RNA codes and catalyses
RNA
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Theories
• Solid-phase “Iron-Sulfur” world – life formed on solid surfaces
The reaction between Fe+2 and HS- (or H2S) can be coupled to CO2 fixation
This type of reaction can occur in high temperature, high pressure environments
- hydrothermal vents
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Iron-sulfur world Evidence
• Iron and sulfur are important catalytic elements in biology (oxidation/reduction reactions)
• Laboratory demonstrations of the production of pyruvate under “prebiotic” conditions
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Abiotic reactions leading to organic compounds – occur under high temp/high pressure conditions
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Cells
Inorganic Organic Cellular organization
ProgenoteUniversal Ancestor
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Origin of Cells
Organic molecules have tendency to aggregatemembrane-like structures form easily and even happen in laboratory experiments
Microspheres- " coacervates“- form spontenously and can replicate by pinching off new spheres
Proteinous microspheres with catalytic and self- replicating properties could be formed in the lab (Fox 1965, Protobionts, Progenotes)
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Coacervate growth and division
Collection of aggregated polymers
Grow by adding new polymers
Form a semi-permeable membrane
When they get too big they divide
Demonstrates how polymers aggregate and act like membranes
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Progenote or Protobiont
• Proteinaceous microspheres – contain proteins and lipids but no nucleic acids– Self replicating– Some ability to catalyze reactions– “pre-cells”
• Progenotes would eventually pick up RNA and DNA, develop enzymatic capabilities and membrane organization = primitive cell
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Origin of Life
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Where did life start?
• Hydrothermal vents– RNA lineage puts hyperthermophiles nearest
to the universal ancestor (see fig 2.3 where are the hyperthermophiles?)
– Rich in reduced compounds Fe+2, H2S, H2 etc.
– Protected from sterilizing cosmic radiation
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Where did life start?
• “Panspermia” – extraterrestrial – life landed here on a meteorite – Life started earlier than previously thought
(when the earth was less than a billion years old)
– Organic material is found in meteorites– Life exists on earth in habitats similar to those
found (or once existed) on Mars, Europa, Titan
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Magnetite crystals
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The first organism must have employed a simple strategy to obtain energy. Primitive metabolism was anaerobic, and likely chemolithotrophic, exploiting the abundant sources of FeS and H2S
present.
Brock pg 428
Fermentations, and anaerobic respiration probably appeared later along with anoxygenic photosynthesis followed by oxygenic photosynthesis. The latter led to development of an oxic environment, and to great bursts of biological evolution.
One possibility is FeS + H2S ==> FeS2 + H2
The resulting H2 could have been used to drive a primitive ATPase with S0 as a potential electron acceptor
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OxygenEarliest Evidences: oldest fossils Oldest photosynthetic microbes 3.5-3.2 B.Y.
- Bacterium-like- Unicellular- Evidence for breakdown products of photosynthesis
Cyanobacteria, 3.5 B.Y.
Stomatolites, 3.5- 0.7 B.Y.
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OxygenEvidence for O2 production:
Banded Iron Formations (BIF) BIF found in ocean sediments red
bands are high in Fe2O3 and Fe3O4 (red bands)- forms when reduced iron reacts with O2
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Oxygen
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Oxygen• Only known source of O2 in the atmosphere
is oxygen producing photosynthesis
• Photosynthesis produced O2 in the oceans that combined with the Fe producing iron oxides-sinking down to ocean floor producing BIF
• BIFs occur in geological rock formations dating back to 3.2-2 B.Y.
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Life on Earth
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Oxygen• BIFs occur 3.2-2 B.Y. ago, suddenly disappear.• Red beds, terrestrial formations similar to BIF,
but much lower in iron concentrations • Red beds, indication of presence of O2 in the
atmosphere• Red beds are present in terrestrial sediments in
last 2 B.Y.Formation of red beds beginning 2B.Y. ago,
after all reduced Fe in the oceans had been oxidized
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Life on Earth
Oxygen sinks: Volcanic gases scavenge O2 Aerobic respiration uses O2 Weathering of rocks containing reduced elements
such as carbon, sulfur, and iron Above sinks has not changed since Archean time But BIFs sink finally became saturated
2 B.Y. ago Massive iron-rich ores that are used today to
make steel are legacy of photosynthetic microbes Summary: massive change in chemistry of the
Earth, mediated entirely by bacteria
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• Why did the availability of oxygen drive evolution (or why are eukaryotes aerobic)?– Hint – see the Nealson 1999 paper.
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Major landmarks in biological evolution