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Early Earth and the Origins of Life Why do we
care?
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Early Earth and the Origins of Life August 25,
2009

• Lecture overview
• formation of the solar system and Earth
• conditions on early Earth
• theories concerning the origin of life on Earth
• geologic evidence for earliest life on Earth

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The Geologic Timescale
LIFE
Life!?!
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Planetary disk
with thanks to L. T. Elkins-Tanton, JPL
NASA/JPL
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Ca-Al inclusions in carbonaceous chondrites
4567.20.6 million years old
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Linked solidification and cooling processes
magma oceans at first
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The Archean Earth System

• Archean 4.03 to 2.5 Ga
• Late Heavy Bombardment (4.1 to 3.8 Ga)
• Atmosphere hardly any molecular oxygen (O2)
  appears at 2Ga
• UV radiation (no ozone shield)
• Sun less luminous (20-30)
• Length of day 15 hours (584 days per year)
• CO2 and CH4 -rich atmosphere
• Likely warm surface conditions (80o to 45oC)
• High rate of heat transfer from core to surface,
  possible thin crust, plate tectonics just being
  established
• Magnetic field by 3.2 Ga

w/ thanks to Stanley M. Awramik, UCSB
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Steps towards Life

1. synthesize biomolecules (organic C, amino acids)
2. organize them into macromolecular systems
3. emergence of self-replicating codes of molecules
   (RNA)
4. emergence of molecular evolution via natural
   selection

after Hazen, 2006
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The Miller/Urey Experiment

• Stanley Miller and Harold Urey (1953)
• ran experiment for one week
• observed 10 of C in organic form
• 2 of C in amino acid form
• but
• too high-energy environment
• (lightning not continuous)
• -early Earth contained appreciable
• CO2 (in addition to CH4)
• on the other hand
• -meteorites contain appreciable
• amino acids

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A modern-day Miller/Urey experiment
Jeff Bada, Scripps Inst. of Oceanography

• redid the Miller/Urey experiment
• found 22 amino acids (more than M/U!)
• also analyzed the original M/U vials
• using modern-day analytical techniques

Johnson et al., Science, 2008
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Hot volcanoe and cold soup models regarding
the origin of Life on Earth
-organic molecules and amino acids can be
organized by mineral lattices as templates
Hazen, 2006
What about combining the two? hydrothermal vents?
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A schematic of evolution of life on Earth
Modified from Awramik and McNamara, in press
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Stromatolites biofilms of cyanobacteria capture
and bind sediment
Pre-Cambrian?
Today, at Sharks Bay, Australia
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An Opinion

• true consensus for lifes existence seems to be
  reached only with the bacterial fossils of the
  1.9-billion-year-old Gunflint Formation
  Moorbath, S., 2005. Nature, 434, p. 155)

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Tyler Barghoorn, Science, 1954
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Candidates for Early Archean Fossils
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3.85Ga rock
apatite (Ca-PO4) grain in rock
Nature, 1996
carbonaceous remnant material after
apatite dissolved with acid
And the controversy continues.
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A primer for Schopf et al., 2002

• ion microprobe
• -instrument that focuses a beam of charged ions
  onto the surface of a sample
• charged ions generate ions for mass spectrometric
  analysis (separation by mass)

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Raman spectroscopy -instrument detects extremely
small changes in energy of light/laser/IR
radiation -in Schopf 2002, a laser is used to
excite the sample molecules into a higher energy
state they report Raman shifts that denote
changes in laser wavelength -shifts denote
presence of specific chemical bonds and their
relative orientations
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kerogen -concentrated, high-molecular-weight
organic compounds occuring in sedimentary
rocks -precursor to fossil fuels (formed upon
heating of kerogen)
an oil shale
carbonate (X-CO3) -most commonly CaCO3, the
mineral in limestone -many marine organisms form
carbonate shells (corals, foraminifera)
limestone cliffs
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C isotopes -C has two stable isotopes (12
protons, different neutrons) 12C (99) and
13C (1) -for the most part, they behave
identically -however, there is a slight
difference in their reactivity -photosynthesis
causes a very large shift in the relative
abundance of 13C and 12C such that 12C is
preferentially incorporated into biological
materials
cross-section of a leaf
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trichome -any small outgrowth or appendage on
certain plants and algae
trichomes on a cannabis plant
chert -finely-grained, silica-rich,
microcrystalline rock -is a product of high-T
chemical alteration of rocks
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greenschist -metamorphic rocks resulting from
low-T, moderate pressure environments -typically
somewhat green in color