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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…
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
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
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)
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”
Protobionts
• Maintain different internal environment
• Microspheres – selectively permeable protein membrane– Display osmotic swelling– Stores energy– Has a membrane potential
• 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
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!
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
• 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!
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
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
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
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
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
“Cambrian Explosion”• Most of the major phyla of animals appear in
the fossil record of the first 20 million years of the Cambrian period
In this format the RNA would be shared by all competing RNA molecules
Shows molecules smaller than RNA can self-replicate!