Download Chapter 14: History of Life

Chapter 14: History of Life
(on Earth)
Modern Biology © 2009. Holt,
Rinehart, & Winston, pp 278-290.
Biogenesis vs. Spontaneous Generation
• Biogenesis
– Current view
– All life comes from preexisting life
• Precept of the Cell Theory
• Spontaneous Generation
– Popular before 1600s
– Life could arise from nonliving things
• Maggots from rotting meat
• Fish from dried ponds
Francesco Redi (1626-1697)
• Redi noted that maggots arose where adult
flies had previously landed
• Maggots turned into sturdy oval cases
• Flies emerged from those oval cases
• Did 1st experiment in 1668 showing that
maggots only arose from uncovered meat.
– Concluded flies gave rise to maggots, not the meat
Lazzaro Spallanzani (1729-1799)
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Tested if microorganisms arose spontaneously
Had 2 separate groups of broth-filled flasks
Boiled broth in all flasks
Left 1 group open to the air but sealed others
Only broth in the open flasks became cloudy
Opposition claimed air’s “vital force” was
absent from closed container & this prevented
microorganisms’ growth
Louis Pasteur (1822-1895)
• Addressed Spallanzani’s experimental design
– Air’s “vital force” must have access to broth
• Used broth in curved-necked flasks
– Curve allowed air exchange with the room
– Curve prevented solid particles’ entry to flask
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Boiled broth to kill microorganisms in flasks
Left flasks undisturbed for a year  clear
Broke off curved neck allowing access to air
After 1 day  broth was cloudy
Life in a New Solar System
• Our Solar System’s History
– Initially part of swirling mass of stellar dust & gas
– Gravity pulled most together  our sun
– Occurred around 5 x 109 years ago
• Our Planet’s History
– Dust & gas continued revolving around sun’s mass
– Debris continually collided with itself  planets
• Earth formed over ~ 4 x 108 yr span of collisions
Estimating Age
• Geological history = stratification of surface
• Radiometric dating
• Different isotopes have different half-lives
– Radioactive isotopes have constant half-lives
• C-14 = 5.7 X 103 yrs
• U-235 = 7 x 108 yrs
U-238 = 4.5 x 109 yr
Th-232 = 1.4 x 1010 yrs
• Calculate age of sample by comparing
quantities of stable element with its isotope
Earth’s History
• Oldest rocks & crystals = ~ 4 x 109 yrs
– Collisions with space debris ceased
• Proposed early atmosphere
– NH3 or N2, H2, H2O, & CH4 or CO2
• Synthesis of Organic Compounds
– Miller-Urey Apparatus tested Oparin’s hypothesis
– Treated compounds with heat & electricity
– Produced amino acids; later ATP & nucleotides too
Molecules to 1st Cell-like Structures
• Fox, et al, mixed simple organic compounds
• Spontaneous formation of cell-like structures
– Microspheres
• Protein molecules
• Membrane organization
– Coacervates
• Collections of droplets
• Composed of lipids, amino acids, & sugars
• No response to natural selection
– No hereditary material
Cyanobacteria structure
First Life Forms
• RNA may have been critical macromolecule
– Ribozyme; self-replication; basic biochemistry
• 1st cells? anaerobic, heterotrophic prokaryotes
– Ate up spontaneously formed organic molecules
• 1st autotrophs likely did chemosynthesis
– CO2 = C source; energy from oxidizing inorganics
– Archaea/ archaebacteria: hostile environments
• Photosynthetic prokaryotes @ 3.5 x 109 yrs
– Fossils look like our current cyanobacteria
– Produced O2  109 yrs  our atmosphere today
Earliest Evidence for life on Earth
Cyanobacteria
First Eukaryotes
• Differences
– Larger cells; DNA in chromosomes inside nucleus
– Membrane-bound organelles
• Theory of Endosymbiosis
– Aerobic prokaryotes engulfed by eukaryote
– Modern Chloroplasts & Mitochondria
• Evidence
– Independent replication
– Contain some unique DNA which is circular