origin of life earth formed 4.6 billion years ago life ~ 3.5 billion years ago what was the...
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Origin Of Life• Earth formed 4.6 billion years ago
• Life ~ 3.5 billion years ago
• What was the primitive environment of Earth like?– Reducing (electron adding) atmosphere– No electron hogging oxygen gas was present– “Primordial Soup” just waiting for lightning…
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Life….
• Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages:1. Abiotic synthesis of small organic molecules
2. Joining of these small molecules into polymers
3. Packaging of molecules into “protobionts”
4. Origin of self-replicating molecules
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Step One: Chemical Evolution• Oparin and Haldane:
– Separate theories about early Earth– Reducing atmosphere + high energy levels
contributed to spontaneous chemical evolution
• Miller & Urey (1953, U of C) tested hypothesis
• First organic compounds may have been synthesized near submerged volcanoes and deep-sea vents
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Other theories
• Some organic compounds may have come from space– Carbon compounds have been found in some
meteorites that landed on Earth
• Small organic molecules polymerize when concentrated on hot sand, clay, or rock (Sidney Fox – Protenoids, microspheres)
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Protobionts
• Aggregates of abiotically produced molecules surrounded by membrane/membrane-like structure
• Experiments demonstrate that protobionts could have formed spontaneously
• Liposomes– small membrane-bounded droplets that can
form when lipids/other org molec added to water– Grow and appear to split “give birth”
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Protobionts
• Maintain different internal environment
• Microspheres – selectively permeable protein membrane– Display osmotic swelling– Stores energy– Has a membrane potential
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• Polypeptide, nucleic acid, and polysaccharide aggregates– Hydrophobic macromolecules surrounded by
shell of water molecules– Can absorb substrates– Release products– May have enzymes associated with them
Coacervates
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What’s Missing?
• In chemical evolution the environment selects for best suited molecules to survive
• What ingredient necessary for life is missing in all protobionts?
• No hereditary material that would ensure continuity!
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Which Came First RNA or DNA?
• RNA!
• RNA has the following properties:– Less complex than DNA– Less stable than DNA– Short strands of RNA can replicate– Ribozymes (RNA molecules) - show
enzymes can be non-protein molecules
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• Experiments have proven that RNA can evolve
• The RNA world was slowly able to direct protein synthesis and preserve and copy the genetic information
• As evolution continued different early cells showed varying success with life
• One trend resulted in DNA as the ultimate hereditary material
• RNA world DNA world!
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Early Life
• Prokaryotes – only life (3.5 ~ 2 bya)
• Electron transport systems essential to early life
• Earliest types of photosynthesis did NOT produce O2
• Oxygenic photosynthesis ~ 3.5 bya in cyanobacteria
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Land plants
Animals
Paleozoic
Meso-
zoic
Ceno-zoic
Origin of solarsystem andEarth
Multicellulareukaryotes
Single-celledeukaryotes
Prokaryotes
Atmosphericoxygen
ProterozoicEon
ArchaeanEon
Humans
Billions of years ago
41
2 3
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Endosymbiotic Theory • How did eukaryotes evolve from prokaryotes?• Oldest eukaryotic cell fossils ~ 2.1 bya• Endosymbiosis theory
– mitochondria & plastids were formerly small prokaryotes living within larger host cells
• Prok ancestors of mitochondria/plastids gained entry to host cell as undigested prey/parasites
• In the process of becoming more interdependent, the host and endosymbionts would have become a single organism
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Plasmamembrane
Cytoplasm
DNA
Ancestralprokaryote
Endoplasmic reticulum
Nuclear envelope
Infolding ofplasma membrane
Engulfing of aerobicheterotrophicprokaryote
Nucleus
Cell with nucleusand endomembranesystem
Engulfing ofphotosyntheticprokaryote insome cells
Mitochondrion
Ancestralheterotrophiceukaryote
Ancestralphotosynthetic eukaryote
Mitochondrion
Plastid
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Earliest Multicellular Eukaryotes• Molec clocks – earliest common ancestor of multi
euk ~ 1.5 bya (oldest fossils – 1.2 bya)• The first multicellular organisms were colonies,
collections of autonomously replicating cells
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“Cambrian Explosion”• Most of the major phyla of animals appear in
the fossil record of the first 20 million years of the Cambrian period
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In this format the RNA would be shared by all competing RNA molecules
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Shows molecules smaller than RNA can self-replicate!
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