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Early Earth and the Origins of Life
Why do we care?
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
The Geologic Timescale
LIFE
Life!?!
Planetary disk
with thanks to L. T. Elkins-Tanton, JPL
NASA/JPL
Ca-Al inclusions in carbonaceous chondrites
4567.2±0.6 million years old
Linked solidification and cooling
processes
Elkins-Tanton
magma oceans at first…
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
Steps towards Life
1.
2.
3.
4.
synthesize biomolecules (organic C, amino acids)
organize them into macromolecular systems
emergence of self-replicating codes of molecules (RNA)
emergence of molecular evolution via natural selection
after Hazen, 2006
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
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
“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
A schematic of
evolution of life on Earth
Modified from
Awramik and
McNamara, in
press
Stromatolites:
biofilms of cyanobacteria
capture and bind sediment
Pre-Cambrian?
Today, at Shark’s Bay,
Australia
An Opinion
• “…true consensus for life’s 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):
.
Tyler
& Barghoorn,
Science,
1954
Candidates for Early Archean Fossils
3.85Ga rock
apatite (Ca-PO4) grain in rock
Nature, 1996
“carbonaceous” remnant
material after apatite
dissolved with acid
And the controversy continues….
A primer for
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)
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
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
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
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
greenschist
-metamorphic rocks resulting from low-T,
moderate pressure environments
-typically somewhat green in color