[MUSIC] Hi again everybody, and welcome back to the life of the Precambrian. In this lesson, we'll move forward from the to another exciting eon, the Proterozoic. And take a look at some of the highlights in the evolution. We'll go through a period where life are not only restricted to prokaryotes. The eukaryotes appear, but despite that animals evolved during the latest parts of the Proterozoic, the different ecosystems were still very different from what we see today on Earth. The Proterozoic is parted in three era. The oldest, the Paleoproterozoic extended from 2500 million years ago to 1600 million years. The Mesoproterozoic from 1600 to 1000 million years. And the youngest, the Neoproterozoic from 1000 to 541 million years. The archaean-proterozoic boundary is currently placed chronometrically, at 2,500 million years, as can be seen on the charts. But it was suggested, in the geological time scale of 2012, that the base of the Proterozoic instead should be placed at 2,420 million years where substantial glaciations occur for the first time. The magnetism decreases and the oxygen content rise. The aquatic life in both the archaean and most of the Proterozoic was indeed characterized by microscopic organisms. With only very few exceptions, the seabed gave the impression to be calm, pristine, and lifeless without visible organisms except for scattered bacterial mats which sometimes developed into stromatolites in the photic zone. The transition from the Archean to the Proterozoic 2.5 billion years ago resulted in a striking change in Earth's climate and living conditions. Life was dominated by microscopic more or less passive prokaryotes. But significant environmental differences exist between the Archean and Proterozoic. The most obvious is a change from predominantly anoxic conditions in oceans. But both shallow and deep and the atmosphere, so a situation with oxygenated atmosphere and surface waters. This event has been named the great oxidation event. Researchers have estimated that the concentration of oxygen in the atmosphere probably elevated from less than 0.001% of the present atmospheric level in the Archean to about 10-20% of the present level in the Mesoproterozoic. The deep parts of the oceans had to wait for another two billion years to be oxygenated. As we heard about earlier, the oxygenization was caused primarily by the evolution and increase of photosynthetic oxygen-producing Cyanobacteria in the surface waters. But it has been suggested that also other processes such as transitions to more homogenous oceans with lesser degree of stratification in the water column played a role. Another important event, in the beginning of the Proterozoic eon, is appearance of a substantial glacial event. In many places, the glacial event count three succeeding glaciations that occurred from about 2.4 to 2.2 billion years ago. Some scientists call it the First Snowball Earth because it apparently had an almost global distribution and even occurred in low latitudes near the equator. Be careful not to mix it up with the much later glacial event that occurred during the Neoproterozoic Era which is often called the Second Snowball Earth. A widespread and clear indication of the paleoprotozoic glacial event is the precision of glacial diamictites. It's a glacial sedimentary rock type that consists of a wide range of hardened elicified nonsorted to polysorted terrigenous sediment grains, pebbles, and bolders which were suspended in a match matrix during glacial processes. Can you mention a type of glacial sediment that is made in cold areas today? Yes, some of you, are perhaps living in areas which were covered by glaciers during the last ice age. Or perhaps live at latitudes or heights that allow glaciers to develop nearby today. In these areas you may have seen tills, which are loosely packed, unsorted diamicrites deposited by the advancing glaciers. A lithified till is called tillite. Another type of glacial sediment is meltwater sand, gravel, mud which was transported and deposited by rivers formed by meltwater from the glaciers. Moving a little bit forward in the paleoproterozoic era to the time just after the glaciers melted because of rising temperatures. So called red beds were deposited in many places. Red beds are oxidised hematite segmented sandstones and shales which were deposited in continental as well as marine settings. This rock type is very important in paleo-environmental interpretations. Because its occurrence indicates a continuous supply of free oxygen in the atmosphere. The eukaryotes, which include all organism kingdoms apart from bacteria and archaea are distinct from prokaryotes in having a clearly more complex structure such as cell nucleus and organelles. The thought that many eukaryotes evolved from their ancestors through endosymbiosis where they absorbed cyanobacteria to develop chloroplasts. The chloroplast originated from chlorphyll-bearing cyanobacteria in the cell. Mitochondria, the eukariotic cells power plants, originated probably in a similar fashion from alphaproteobacteria. Being absorbed in the cell. But it is not definitely proved in which order these events took place. How can you separate a small single celled fossil eukaryote from a large single cell fossil bacteria? Molecular studies may be an obvious solution In this question. But what if you only have a microscope? It can be very difficult to do this under the microscope. But parameters like size, the occurrence of possible operations outside the cell, and their ability to preserve the cell wall are three indicators you can look at. Prokaryotes can be large. They can have operations on the outside of the cell. And they can preserve the cell walls as fossils. But fortunately, as earlier noted by the American scientist Andrew Knoll, there's no known prokaryotes that exhibit all these three characters simultaneously. This means that it often will be possible to distinguish prokaryotes from eukaryotes based on their external morphology. Eukaryotes became more common in the aftermath of the first major Precambrian ice age after about 2.2 billion years ago. Where the oxygen content in the upper parts of the oceans and the atmosphere began to rise considerably. As mentioned earlier the first possible sign on the presence of eukaryotes at the time of recording this video is the so called biomarkers obtained by molecular geochemical measurements of 2.7 billion year old slates from Western Australia. However, moving 600 to 800 million years forward in time, probable microscopic eucharatic fossils start to turn up. In 2010, more than 250 specimens of up to 12 centimeter long individuals with a radial fabric, were described from 2.1 billion years old, black shales of Gabon, in West Africa, by an international research team led by El Albani. Some researchers have questioned the identifications and believe that the structures can be explained by chemical processes related to the pyrite in the shales. Conversely other scientists support the structures as being biologically made. Shortly after, the almost 1.9 billion years old gunflint biota did develop. It is one of the most famous and diverse Precambrian microfossils lagerstätten known today. Described originally by Barghoorn and Tyler in 1965. It provides a key record of the biosphere at the time of changing oceanic chemistry, and gives a detailed insight in the prokaryotes from the Paleo-Proteozoic. Most researchers agree that fossils are predominately fossil bacteria. However, by comparison with the shape and morphology of much younger fungi, it was controversially suggested by the German scientist, Krumbein, in 2010 that many of the forms traditionally interpreted as bacteria, instead may be interpreted as fungi. If this was right, the gunflint specimens would predate all other known occurrences of fungi, with several hundred millions of years. Almost contemporary microscopic fossils have been documented from the Negaunee formation of Michigan, in the United States. The Freeling discovery was made in 1992, it consisted of hundreds of helix shaped, 0,7 to 1.5 millimeter wide, and 30 to 90 millimeter long, carbonaceous fossils, from the 1.9 billion year old Negaunee formation. Due to the fossil structures, complexity, uniform shape and size. It has been suggested that they may belong to an early eukaryote group of an attached algae. Some researchers are skeptical however, and consider them as being giant cyanobacterial coatings. The microscopic carbonaceous body fossil Grypania spiralis is distinctive spirally coiled with flattened tubular structures. Grypania has been described from both the United States, China and India and occur within 1.45 to 1.88 billion years old sediment. The eukaryotic characters include large size and ribbon-sized consistency. A prokaryotic origin may not be fully ruled out. Moving another 400 million years forward in time, an exciting discovery of organic micro fossils found In 1,500,000,000,000 years old deposits appear to be a good candidate for an early eukaryote. More precisely, we're dealing with the possible fungi tappania which is a type of organic-shelled, benthic, multi-cellular microfossil with septate, branching processes. It was described by the British researcher Nick Butterfield in 2005. But already five years earlier Butterfield described very well preserved micro fossil structures with complex multi similarity from 1200 million years old sediments called Bangiamorpha which he interpreted as probably red algae. Both these discoveries were from Canada. [MUSIC]
