GEOL 102 Historical Geology
Spring Semester 2012
The Archean Eon II: The Origin of Life
Abiogenesis: origin of life:
- Based on our inferences of modern life forms and of the environments that existed in the Archean, the transition from non-living matter
into living things occurred sometime late in the Hadean or early in the Archean
- Laboratory experiments show that most of the building blocks of life will form out of inorganic chemicals in a variety of situations:
near clays; near deep-sea vents; inside the spaces of rocks near deep-sea vents; even near ice
- Was likely a step-wise transformation: Origin of simple building blocks into Origin of biological monomers (simple organic materials like amino acids) into
Origin of biological polymers (more complex molecules formed of chains or other arrangements of monomers strung together) into Competition
among varied biological polymers for utilization of space, materials, and energy into Winners of this competition (the ones that more
effectively take up space, materials, and resource) produce more duplicates
- From this point on, we might consider these things "alive". They are certainly operating under the Darwinian rules of evolution:
duplication (with occasional errors) and competition
- Over time, the particular combination of features that characterize life on Earth (e.g., lipid bilayer membranes, RNA ribozymes,
proteins) develop & DNA replaces RNA as the store of genetic information
- Oldest (controversial) chemical traces of life at 3.8 Ga (Isua Fm., Greenland)
- Oldest (controversial) body fossils of prokaryote-grade organisms at 3.5 Ga
- More convincing body fossils of prokaryote-grade organisms (including contraversial cyanobacteria: blue-green algae) by 3.4 Ga, so
had definitely been around by 3.4 Ga or earlier (in western Australia & South Africa)
- Stromatolites at 3.3-3.5 Ga (Warrawoona Group, Australia)
Archean life would have very short food chains, but contained a great diversity of chemical pathways. Life would have been limited to the
water. Most of the shallow seafloors, shores, etc. would be covered in algal/bacterial slime. As photosynthesizers spread, the oceans and
atmosphere began to fill with oxygen.
Earliest life was probably exceedingly simple: far simpler than any modern form.
Would have been:
- Anaerobic: evolved outside the presence of free oxygen (free oxygen would
probably poison it!)
- Heterotrophic (a consumer, absorbing organic molecules from water) and/or Lithotrophic (both autotrophs and consumers,
using nonorganic material [aka rocks] for energy and biological synthesis)
- Prokaryotic: no nucleus nor other complex organelles
Once thought to have appeared in "quite little pond" ("primordial soup").
More likely: formed near the (then more active) oceanic vents along mid-ocean ridges:
- Abundant energy supply and mineral supply
- Far more common in Archean than now (due to higher geothermal activity)
- Ocean water would protect early life from UV rays
- Even today many exceedingly simple prokaryotes live near vents, black smokers, etc.
As fuel supply decreased, various new forms appeared from among the proto-organisms:
- Photosynthesizers first to use sunlight directly.
- 1rst order consumers: absorb other cells.
With photosynthesizers come free oxygen: begin to change global atmosphere. Has most
direct effect as the Proterozoic begins.
LUCA: the "Last Universal Common Ancestor" (i.e., the ancestor of Bacteria, Archaea, and Eukaryota) would not have been the
first living thing. Instead, there would have been a history and diversity of living forms that included the ancestors of LUCA as well as
many diverse side branches that have no living survivor. Based on genetics divergence data and biomarker traces, LUCA would have
to have been present by around 3.8-3.5 Ga (and thus around in the Paleoarchean, if not earlier).
NOTE: for many decades the earliest photosynthesizers were thought to be cyanobacteria
(aka "blue-green algae"). However, the fossils of supposed Archean cyanobacteria do not show definite cyanobacterial traits, but could
instead be any form of prokaryote. Biomarkers (chemical traces) do not show any clear presence of cyanobacteria until
the Proterozoic. Additionally, cyanobacteria are aerobic, and thus unlikely to have done well in the anaerobic conditions of
the Archean. Instead, the photosynthesizers of the Archean were anaerobes, possibly including the purple sulfur bacteria and
green sulfur bacteria (which do not release oxygen as a waste product) and the green bacteria and heliobacteria (which do).
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Last modified: 19 January 2012