[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]
