Origin of microbes
How did microbes take control of earth?
Tree of life organized in three
• Bacteria (exclusively microbial)
• Archaea (exclusively microbial)
• Eukarya (mainly microbial)
Carl Woese tree of life based on a comparison
of ribosomal RNA gene sequences
Origin of life and microbes
Microfossils from the Apex Chert, Australia
1 billion years old. They resemble modern
Evidence of presence of microbes in ancient earth
Evidence for microbial life can be found in rocks ~ 3.5 billion years old (Earth formed 4.5 billion
SEM of the South African Greenstone Belt rock showing
3.45 billion years old
This is thought to be a bacteria-like organism obtained from
Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001
To find out if this is a real microbe or just a mineral
• Cannot cultivate a microbe
• Cannot get a DNA
Bacteria came to earth through a meteorite??? How can
bacteria survive a meteorite?
- presence of endospores
• Fossilized microbial mats consisting of layers of filamentous prokaryotes trapped in
Fossilized stromatolites Modern Stromatolites
Formed by filamentous bacteria Formed by cyanobacteria
Scientists believe that oxygen was produced first
to change the composition of the atmosphere
Stromatolites in Thunder Bay
Stromatolitic limestone from the Eagle head Lake area north
of Thunder Bay. Age>1billion years old
Theories on the origin of life
o Alexander Oparin (Soviet chemist) and John BS Haldane (British biochemist and
geneticist) are the precursors of the modern hypothesis on the origin of life.
Planetary science, clues for organic
• Strongly reducing conditions in the atmosphere CH4, NH3, H2 and water vapour
• UV radiation and electric discharges
• Inorganic carbon
Reducing conditions produced hydrocarbons from CO2, giving origin to organic compounds.
Reaction of this compounds with nitrogen formed amino acids, leading to synthesis of proteins
that accumulated in hot ponds.
The hypothesis of the primordial soup
• In a primordial soup, proteins and other organic compounds formed colloidal systems:
coacervates. The first microbial forms of life evolved from coacervates.
o They are rich in organic molecules and they have the possibility of colliding and
Harold Urey and Stanley Miller 1950’s
• Harold Urey:
o Primitive Earth had a reducing atmosphere.
o Planets in the solar system have this kind of atmosphere. Their atmosphere
contained methane, ammonia, hydrogen and water.
• Stanley L. Miller
o First experiments designed to replicate chemical reactions in the primitive earth.