Text Readings
Chapter #17 in Audesirk, Audesirk and Byers: “The History of Life” Pg. #332-145.
Geologic Time......
1 Geological Sources - 4.5 Billion Years
Atmospheric Hydrogen
Gases: Gas (H2) likely escaped early atmosphere Nitrogen (N ) 2 Carbon Monoxide (CO) Water Vapor Carbon Dioxide (CO2) (H2O) Methane
(CH4) Ammonia
(NH4)
Primitive Earth Life on Earth?
• Earth formed about 4.5 billion years ago • Life arose 3.9 to 3.5 billion years ago during the Precambrian Era – Oldest fossil organisms found to date are estimated to be about 3.5 billion years old
Organic Molecules
• Stanley Miller and Harold Urey (1953) – Noted that the atmosphere of early Earth probably contained methane, ammonia, hydrogen, and water vapor, but no oxygen
2 Organic Molecules
• Miller and Urey (1953) – Simulated early Earth’s atmosphere by mixing the above gases in a flask and adding an electrical discharge to simulate lightning • Simple organic molecules appeared after a few days
Organic Molecules
• Similar experiments by Miller and others have produced amino acids, short proteins, nucleotides, and ATP
Organic Molecules
• Exact composition of “atmosphere”was unimportant – Must contain carbon, hydrogen, and nitrogen, and exclude oxygen • Type of energy source was unimportant – Electrical discharge, UV light, and heat were equally effective
3 Organic Molecules
• The lack of both life and oxygen gas on early Earth allowed large quantities of these organic molecules to accumulate in areas protected from UV radiation (beneath rock ledges, in oceans)
Organic Molecules
• UV radiation bombarded early Earth’s surface because there was no ozone to block it. • UV radiation can break apart organic molecules. • Accumulated simple organic molecules combined to form complex organic molecules
RNA (Ribo-nucleic Acid)
• May have been the first self-reproducing molecule. • Thomas Cech and Sidney Altman (1980s) discovered an RNA molecule (ribozyme) that could catalyze a chemical reaction, a role that was thought to be performed only by protein enzymes.
4 RNA (Ribo-nucleic Acid)
• Over time, DNA (Deoxyribose Nucleic Acid) replaced RNA as the information-carrying genetic molecule and RNA took on its present role as an intermediary between DNA and protein.
Membrane-Like Vesicles
• Vesicles are small, hollow spheres formed from proteins or proteins complexed with other compounds – Have been formed artificially by agitating water- containing proteins and lipids – Have a well-defined outer boundary that separates internal and external environments – Depending on composition, membrane may be remarkably similar to that of a real cell http://cse.stanford.edu/class/sophomore-college/projects-01/cellular-automata/beginning/beginning2.html
Membrane-Like Vesicles
• Vesicles resemble living cells – Under certain conditions, may absorb material from the external solution, grow, and divide.
http://cse.stanford.edu/class/sophomore-college/projects-01/cellular-automata/beginning/beginning2.html
5 Membrane-Like Vesicles
• Certain vesicles (protocells) may have been the precursors of living cells.
http://cse.stanford.edu/class/sophomore-college/projects-01/cellular-automata/beginning/beginning2.html
But Did All This Happen?
• The experiments of Miller, Urey and others showed/demonstrated that organic (carbon- bearing) molecules, along with simple membrane-like structures, would have formed on Earth.
But Did All This Happen?
• Given enough time and a sufficiently large pool of reactant molecules, even extremely rare, seemingly unlikely events can occur many times.
Time Randomness
6 The First Organisms
– The First Cells in Earth’s Oceans Were Prokaryotes, Cells that Lack a Membrane-bound Nucleus. – Some Organisms Evolved the Ability to Capture the Sun’s Energy. – Photosynthesis Increased the Amount of Free Oxygen in the Atmosphere.
The First Organisms
– Aerobic Metabolism Arose in Response. – Some Organisms Acquired Membrane-Enclosed Organelles in a process known as endosymbiosis.
Prokaryotic Cells
• Two cell types seen among all living things – Prokaryotic (“before nucleus” in Greek) • Only 1-2 micrometers in diameter • Lacking organelles or a nucleus
7 Prokaryotic and Eukaryotic Cells
– Eukaryotic (“true nucleus” in Greek) • Larger than prokaryotic cells • Contain a variety of organelles, including a nucleus
Prokaryotic and Eukaryotic Cells
• Cell types named after presence or absence of a nucleus –The nucleus is a membrane-enclosed sac containing the cell’s genetic material
Capturing the Sun’s Energy
• Photosynthesis requires sunlight, CO2, and hydrogen. – Earliest source of hydrogen believed to be hydrogen sulfide. – Eventually, water replaced hydrogen sulfide as the source of hydrogen and photosynthesis became water-based.
8 Stromatolites – Ancient Bacteria (“Stroma” – bed, “lithos”–rock)
- Unicellular
- 3.5 Billion years old
- Formerly known as “Blue-green Algae”
- Photosynthesis by the bacteria depleted O2 in the surrounding water, leaving Calcium - (O2 given off as a waste product in the atmosphere) Carbonate (CaCo3 ) as a precipitate (layer)
The Great “Oxidation” Event
• Water-based photosynthesis resulted in the release of oxygen gas as a by-product. • Initially, oxygen combined with iron in the Earth’s crust to form iron oxide. • Chemical analysis of rocks suggests that significant levels of atmospheric oxygen first appeared about 2.2 billion years ago.
Aerobic Metabolism
• The accumulation of oxygen in Earth’s atmosphere probably: – Exterminated many anaerobic organisms. – Provided the environmental pressure for the evolution of aerobic metabolism.
9 Aerobic Metabolism
• The evolution of aerobic metabolism was significant because aerobic organisms can harvest more energy per food molecule than anaerobic organisms.
Membrane-Enclosed Organelles
• The first eukaryotes (cells that possess membrane-bound organelles) appeared about 1.7 billion years ago • Several organelles (mitochondria, chloroplasts, centrioles) may have arisen when primitive cells engulfed certain types of bacteria (the endosymbiont hypothesis)
Wednesday ………
Categorizing Life (Taxonomy)
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