3.2 Origin of the microbial world: Part 2 - Jan Audun Rasmussen

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來自 University of Copenhagen 的課程

起源：宇宙、太阳系、地球和生命的形成

372 個評分

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University of Copenhagen

起源：宇宙、太阳系、地球和生命的形成

372 個評分

The Origins course tracks the origin of all things – from the Big Bang to the origin of the Solar System and the Earth. The course follows the evolution of life on our planet through deep geological time to present life forms.

從本節課中

Origin of the microbial world / The Cambrian Explosion and Exceptional Preservation

In this module Jan Audun Rasmussen and Danny Eibye-Jacobsen will show you how life evolved during the first 4 billion years since the creation of the Earth. As you will see, it is very challenging to study the oldest life forms of our planet. During this enormous time span – which covers about 80% of the Earth’s history – microbial life slowly evolved to form a crucial component of the biosphere. Toward the end of the period the deepest foundations of the different groups of animals evolved. All of the life forms surrounding us today can be traced back to this time.

3.1 Origin of the microbial world: Part 1 - Jan Audun Rasmussen 16:55

3.2 Origin of the microbial world: Part 2 - Jan Audun Rasmussen12:53

3.3 Origin of the microbial world: Part 3 - Jan Audun Rasmussen13:01

3.4 Origin of the microbial world: Part 4 - Jan Audun Rasmussen 9:52

3.5 Origin of the Metazoans and evolution of life at small scale - Danny Eibye-Jacobsen15:37

3.6 The Cambrian Explosion and exceptional preservation: Part 1 - Danny Eibye-Jacobsen12:58

3.7 The Cambrian Explosion and exceptional preservation: Part 2 - Danny Eibye-Jacobsen12:29

3.8 The Cambrian Explosion and exceptional preservation: Part 3 - Danny Eibye-Jacobsen6:35

與講師見面

Henning Haack
Associate Professor
Natural History Museum
James Connelly
Professor
Natural History Museum of Denmark
Vivi Vajda
Professor
Department of Geology, Lund University
Jon Fjeldså
Professor, Curator
Thomas Gilbert
Professor
Michael Houmark-Nielsen
Associate Professor
Natural History Museum of Denmark
Bent Erik Kramer Lindow
Curator
Natural History Museum of Denmark
Gilles Guy Roger Cuny
Associate Professor
Natural History Museum of Denmark
Ole Seberg
Associate Professor
Natural History Museum of Denmark
Gitte Petersen
Associate Professor
Natural History Museum of Denmark
Tais Wittchen Dahl
Assistant Professor of Geobiology
Arne Thorshøj Nielsen
Associate Professor
Natural History Museum of Copenhagen
Martin Vinther Sørensen
Associate Professor, Curator
Natural History Museum of Denmark
Svend Stouge
Associate Professor
Natural History Museum of Denmark
Danny Eibye Jacobsen
Associate Professor, Curator
Natural History Museum of Denmark
Jan Audun Rasmussen
Associate Professor
Natural History Museum of Denmark
Emily Catherine Pope
Assistant professor
Natural History Museum of Denmark

[MUSIC]

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.

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.

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.

Especially this Strelly Pool Chert is important.

As William Schopf of Harvard University has stated: The cone-shaped structures

are difficult to explain if you do not accept them as being biologically formed.

Today as seen in the picture, stromatolites are lithified

sedimentary growth structures formed by the activity of cyanobacteria.

The layers are produced as calcium carbonate precipitates over the growing

mat of bacterial filaments.

Photosynthesis in the bacteria depletes carbon dioxide in the surrounding water,

initiating the precipitation.

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.

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.

They consist of microscopic, tubular

structures that are on average 50 microns long and 4 microns in diameter.

But let's move further 300 million years up and

reach the Mesoarchean and Neoarchean Eras.

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.

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

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.

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.

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?

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.

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]

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