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It is thought that all organisms living on the Earth today derived from
a single primordial cell that existed more than 3.7 billion years ago.
But there is quite considerable disagreement about when the oldest of them turned up
and to which group they belonged.
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The reason is that there is only very sporadic and
controversial fossil evidence and it is therefore highly necessary to base
interpretations on the analysis on recent forms of bacteria.
Anoxygenic green non-sulfur bacteria is one.
The second is sulfate-reducing bacteria living in high temperature conditions.
And third the methanogene archaea have all
been related to the first bacteria on Earth.
The earth's oldest fossils, apart from the previously mentioned graphite particles
from Isua, West Greenland have been observed in almost 3.5 billion year old
Archean sediments from Western Australia and South Africa.
From Pilbara, Western Australia, the possible existence of life
has been explained by means of independent types of evidence.
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Firstly, measurements of sulfur isotopes from the rock
samples indicate that the contained organic sulfides are formed by sulphate
reducing bacteria at temperatures of up to, perhaps, 60 degrees celsius.
The second sign of life is microscopic, elongated, carbon containing
structures that have many similarities with bacterial strings or filaments.
These are studied in thin sections by polishing very thin
slices of stone to a thickness of about 0.03 millimeters,
which is mounted on a microscope slide and
can be studied under a microscope with transmitting light.
There are different views concerning the origin of these structures,
as one group of researchers
interprets them as cyanobacteria-like microfossils
that is Schopf and
colleagues, while Brasier and colleagues have interpreted them as
inorganically formed graphite structures formed by hydrothermal influence
for instance, by addition of hot mineralized fluids in the immediate
vicinity of volcanoes.
The third sign of possible living organism,
in these almost 3.5 billion years old rocks, are large and layered structures
that have much in common with today's cyanobacteria formed stromatolites.
Which for instance are known from Australia and the Tonga Islands.
They are found in various levels of the chert layers.
3:47
Subsequently, the minerals, along with grains of sediment precipitating from
the water, are trapped within the sticky,
organic layer that surrounds the bacterial colonies,
which then continues to grow upwards through the sediment to form a new layer.
4:05
Because the oceans and atmosphere were anoxic and not oxic during the Archean Eon,
the stromatolite-producing processes at that time are not fully understood.
But also here,
it probably was phototrophic bacteria that had a leading role.
The presumably oldest microfossils that have been reported so
far were found in the Barberton Greenstone Belt in South Africa.
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The oldest and largest Archaean organic-walled microfossils observed so
far were reported by Emmanuel Javaux and co-workers in 2010.
These microfossils are carbonaceous spheroidal microstructures up to
300 microns in diameter
from 3.2 billion years old shales and
siltstones of the Moodies Group in South Africa.
They may be classified as acritarchs, which is a group of
organic-walled microfossils of mixed origin.
Moving another 500 million years forward in time, new and
very exciting data were published in 1999 and 2003 by Brocks and co-workers.
The Australian research team published a molecular analysis of
organic material, that's hydrocarbons, from the 2.7 billion year old
slates of the Pilbara craton in Western Australia.
The so-called biomarkers which resemble the one shown in the chromatogram
reflect the molecular structure of organic matter in the shale sediments.
Very interestingly, the appearance of molecules called hopane and
sterane documented the existence of cyanobacteria and
eukaryotes 2.7 billion years ago.
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It has been debated, however, whether the results are reliable, or
whether contamination may have occurred.
A similar study of biomarkers of hydrocarbons contained in quartz and
feldspar from the 2.45 billion year old Matinenda Formation in Canada,
showed even more convincing evidence for
the presence of both cyanobacteria and eukaryotes at this time.
A Japanese research group showed in year 2000 that terrestrial
ecosystems existed 2,600 million years ago based on a study
of organic matter in gold and uranium-rich conglomerates and
carbonaceous paleosols. A paleosol is a fossil soil.
This fits well with data from American researchers led by Eva Stüeken
in 2012 who documented the land-based bacteria oxidized continental
iron sulfides between 2.8 and 2.5 billion years ago.
Would the organisms be able to survive on the surface of
the Earth without a protecting ozone layer?
Yes, at least for some time.
Several organisms have developed different shields
and mechanisms that can protect them against ultraviolet radiation.
The characteristic red and
black colored quartz banded iron ore, it's also called BIF or
Banded Iron Formation, already existed 3.8 billion years ago in West Greenland,
but they were most prevalent during the period from
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approximately 2.6 billion years to 2.4 billion years.
These rocks are unlike anything we see forming today and obviously tell us
something about the prevailing conditions at the time where they formed.
However, we still debate exactly what they can tell us.
The mechanisms of the formation of BIF are debated.
In one model the banded iron deposits precipitated under anoxic conditions where
iron-rich waters, perhaps originating from hydrothermal vents, came into contact
with local pockets of oxygenated water generated by photosynthetic cyanobacteria.
This allowed that the iron was oxidized and
subsequently precipitated as iron oxides as a thin cover, alternating with
the anoxic, muddy seabed which was later preserved as chert and slates.
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An alternative model was published by an American-Canadian research team in 2005.
They suggested that also anoxygenic photoautotrophic bacteria were
able to oxidize iron 2 under anoxic conditions at a few hundred meter depth,
and surprisingly maybe played a larger role in the formation of
BIF than cyanobacteria.
The quartz banded iron ore disappeared about 1.8 million years ago,
probably because the oxygen content of the ocean bottom water became so
high that iron was readily oxidized and thus removed from the system.
9:42
An exception to this general trend can be seen in the deep sea where the environment
probably did not become aerobic before latest Proterozoic between 1,000 and
541 million years ago.
This type of bacterial oxidation of ferrous iron to ferric iron occur in lake
Matano in Indonesia today at depths below where the cyanobacteria have been found.
Lake Matano is presently being investigated in great detail.
It's a very unique place where life is living under conditions
very similar to what we think
were the conditions during certain parts of the Precambrian.
What do you think caused the appearance of oxygen?
Well, yes,
evolution of oxygenic phototrophic cyanobacteria is one of the answers.
And also a probable increase in the number of cyanobacterial specimens occurred.
But why did this occur?
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Well, many scientist believe that the rise in
atmospheric oxygen also had a literally deeper explanation.
And was connected with processes in the mantle and
the crust of the Earth, deep below the surface.
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After the formation of the Earth almost 4.6 billion years ago,
the Earth went through a long phase of cooling and differentiation.
But, this changed about 2.8 billion years ago, when the mantle was heated and
caused the most substantial formation of new crust in Earth history.
The crust was formed rapidly and was followed by widespread
hydrothermal activity and continental emergence.
This events, what has been named the Late Archean Superevent,
peaked probably around 2.7 billion yeas ago.
Scientists have suggested that the emergence of continents caused
the development of large shelf areas around the continents, which
received nutrients through weathering of the terrestrial areas.
As a result, it is believed that the amount of microscopic organisms including
cyanobacteria did increase significantly during the latest Archean.
So is there any visual evidence of this event?
Yes, luckily there is.
For instance did the first continental flood basalts appear during this event for
just 2.7 billion years ago
in Pilbara, Australia together with shallow marine and lake carbonates.
We have now seen different types of evidence for the development of
prokaryotic life during the Archean. In the next lesson we'll look at how the Proterozoic rise
in oxygen content was linked with the rise in eukaryote evolution and diversity,
and also have a closer look at some of the prokaryotes and
eukaryotes that typified the Proterozoic.
[MUSIC]
Henning HaackAssociate Professor
James ConnellyProfessor
Vivi VajdaProfessor
Jon FjeldsåProfessor, Curator
Thomas GilbertProfessor
Michael Houmark-NielsenAssociate Professor
Bent Erik Kramer LindowCurator
Gilles Guy Roger CunyAssociate Professor
Ole SebergAssociate Professor
Gitte PetersenAssociate Professor
Tais Wittchen DahlAssistant Professor of Geobiology
Arne Thorshøj NielsenAssociate Professor
Martin Vinther SørensenAssociate Professor, Curator
Svend StougeAssociate Professor
Danny Eibye JacobsenAssociate Professor, Curator
Jan Audun RasmussenAssociate Professor
Emily Catherine PopeAssistant professor
