The First Cells

download report

Transcript The First Cells

The prokaryotes
The first cells
Image Credits Gary Gaugler Bacillus anthracis
Image Credits: Thermophilic archaebacteria
Origins




Evidence for prokaryotic cells is found as
early as 3.9 billion years ago
The prokaryotes had the Earth to themselves
for another 2.4 billion years
Prokaryotes show an extraordinary diversity
of biochemistry
Structurally prokaryotes are quite small and
simple (1-10µm in diameter).
© 2010 Paul Billiet ODWS
Where?





“(a) warm little pond, with all sorts of ammonia and
phosphoric salts, lights, heat, electricity, etc.
present, so that a protein compound was chemically
formed ready to undergo still more complex
changes” Charles Darwin (1871)
organic compounds would accumulate in the Earth's
oceans until they "reached the consistency of hot,
dilute soup." JBS Haldane (1929)
Deep sea hydrothermal vents
Volcanoes
Outer space.
© 2010 Paul Billiet ODWS
Panspermia (aka cosmozoan)



Life came from somewhere else and
seeded Earth
The support for this depends up on
evidence that life exists elsewhere than on
Earth
and the evidence that it may travel through
open space.
© 2010 Paul Billiet ODWS
Exobiology




Mars Viking Probe (1976)
revealed conflicting
evidence of life
Mariner Probe (1997) did
not reveal any more
evidence
Tantalising evidence of
water on Mars
Venus has inhospitable
conditions (surface
temperatures of over
+400°C).
Viking Image Credit NASA
Water and geothermal energy is the key




The moons of Jupiter
and Saturn could
provide the right
conditions
Europa appears to be
covered in ice
Io shows volcanic
activity
Titan has organic
molecules present.
Europa Image Credit NASA
Life can survive in outer space




Bacteria, inadvertently left on
a lunar probe
Collected and cultured
successfully after nearly 2
years in space
Meteorites of Martian origin
show that its early
atmosphere would have been
similar to Earth’s early
atmosphere
They also showed (debatable)
evidence of bacteria
transported by meteorites.
Surveyor 3 Image Credit NASA
Alien Earth



Life has been shown to
exist on Earth in very
inhospitable conditions
that could exist on other
planets
Antarctic dry valleys
Mid-ocean ridges
Image Credit: Antarctica
Image Credit:
Earth Science Australia
Earliest evidence of life



The origin of the Earth itself is estimated as
4.5 billion years
Earliest evidence of life processes 3.9 billion
years ago
This leaves little time for biochemical
evolution.
© 2010 Paul Billiet ODWS
The eternal question
If it can be shown that life came from an
extraterrestrial source the question still
remains…
How did life evolve in the first place?
© 2010 Paul Billiet ODWS
The poisonous gas …Oxygen



The amount of free
oxygen in the
atmosphere increased
after the evolution of
photosynthesis
O2 appeared from about
2.4 billion years ago and
reached about 21% 1
billion years ago
Obligate anaerobes
either became extinct or
found niches where
oxygen is absent.
Image Credit:
Stromatolites, Shark Bay Australia
The Great Oxidation Event
© 2010 Paul Billiet ODWS
Evidence










Iron is soluble in water where there is no
oxygen
Iron rich water is produced by volcanic
activity
Iron rich water entered shallow sunlight
seas
Sunlight permitted cyanobacteria to
photosynthesize and produce O2
O2 combined with the iron and formed rust
(iron III oxide)
Removing the O2 from the water
Methane produced by methanogen
bacteria also reduced the oxygen levels
This carried on until the rate of
photosynthesis was so great, free O2
appeared in the atmosphere (about 2.4 bn
years ago)
This may have been precipitated by a
shortage of nickel (Ni)
Ni is used in the enzymes of methanogens
© 2010 Paul Billiet ODWS
Image Credit: P.Hoffman
The evolution of the eukaryotes




A third possibility was open with the teaming
up of microbes
Endosymbiosis – a large anaerobic cell
teams up with an aerobic cell
The aerobic prokaryote became a
mitochondrion
Eukaryotic cells were formed, bigger and
more complex, eventually forming
multicellular organisms.
© 2010 Paul Billiet ODWS
The evidence for endosymbiosis








Certain eukaryotic organelles have their own DNA
Single naked loop of DNA, like the prokaryotes
The amount of hereditary information is a lot less
than free-living prokaryotes
These organelles have their own ribosomes
Smaller (70S) than those in the cytoplasm (80S)
The ribosomes of mitochondria and chloroplasts are
the same size as those in prokaryotes
The protein synthesis of these organelles is semiindependent of that taking place in the cytoplasm
It is inhibited by the same antibiotic that affects
prokaryotes (chloramphenicol).
© 2010 Paul Billiet ODWS
The structural evidence



These organelles are
found in membrane
envelopes
As though they were
captured in a vacuole
or vesicle by a larger
cell
These organelles are
about the same size as
a prokaryotic cell.
© 2010 Paul Billiet ODWS
Image Credit: Mitochondrion
Mitochondria


These represent an
aerobic prokaryote
that took up residence
in a larger cell
These are found in all
the eukaryotic
kingdoms
(plants, animals, fungi
and protoctista).
© 2010 Paul Billiet ODWS
Image Credit: Mitochondrion
Chloroplasts
These represent a
cyanobacterium type of
prokaryote that was
trapped in ancestral plants
and some protoctista
Image Credit: Cyanobacterium heterocyst
© 2010 Paul Billiet ODWS