Transcript Lecture 15: The Archean Eon

The Archean Eon
3800 m.y
Subdivisions of the Archean - ERAS
2.5 Ga
2.8 Ga
3.2 Ga
3.6 Ga
3.8 Ga
Origins of Life
Basic minimum requirements for life:
1. Membrane-enclosed capsule to contain bioactive chemicals
2. Energy-capturing chemical reactions (to promote other reactions)
3. Chemical system for replication
RNA (ribonucleic acid) thought to be a very important molecule for
early life formation:
RNA itself is capable of storing genetic information
RNA can also help catalyze chemical reactions and build molecules
Once RNA-based life was established, natural selection works to
quickly evolve more efficient DNA-based system of replication, and
more protein-based biochemical machinery
Did inorganic substances, like clay and pyrite, help organize and
polymerize organic molecules, eventually allowing them to organize to
a point for independent replication?
Experiments of the 50s and 60s:
•Showed that organic molecules (amino acids)
could form from the reaction of atmospheric
gas, electricity, and heat
•Demonstrated that drying and re-wetting of
organic compounds could produce cell-like
membranes and simple proteins
Therefore, was believed that life
arose in shallow pools
However, these compounds would have been
exposed to harsh UV light and oxygen –would
have destroyed any organic compounds
Chemoautotrophs? – Archaebacteria
•Biologists have discovered simple
bacteria in super-heated waters in hot
springs and in deep-ocean volcanic vents
(hyperthermophiles)
•Water is close to boiling point and rich in
sulfur and sulfur compounds
•These bacteria feed directly on the sulfur
for their metabolic energy needs
These bacteria appear to be most primitive life form on Earth, and their mode
of life argues for life arising in the deep ocean at hydrothermal/volcanic
vents dotting sea floors (near rifting zones)
Vents would have provided the chemical and heat energy, chemical
an mineral compounds, and protection from O and UV
Fossil life
Earliest indirect evidence -
BIF of the Isua Supergroup (3.8+ Ga – latest Hadean or
Eoarchean) contains graphite (C)
This graphite has a C12-C13 ratio identical to living organisms
Earliest fossil evidence –
Warrawoona Group rocks in Australia (~3.5 Ga – Paleoarchean)
and slightly younger Onverwacht Group of South Africa…
…sedimentary structures that may be stromatolites
Stromatolites
Finely layered, mound-shaped accumulations of
mud trapped by mats of blue-green algae
(cyanobacteria).
Primitive photosynthetic procaryotes (bacteria)
that grow in marine environments.
Rare in early Archean but become increasingly
common in mid- to later Archean
Recent stromatolites in Australia
Ultra fine-grained Archean cherts in Australia
(3.5 Ga – Paleoarchean) preserve filimentous
structures resembling modern cyanobacteria
Also, cells in apparently different stages of
division occur in rocks of the 3.0 Ga
(Meosarchean) Fig Tree Group of South Africa
Life does not appear to change much for the
next 1.5 Ga (Proterozoic Eon), consisting of
stromatolites and cyanobacteria