[MUSIC] Hello, everybody. I will go through the first 4 billion years of history of life on Earth. In other words, the origin of the microbial life. My name is Jan Audun Rasmussen and I am an associate professor here at the Natural History Museum of Denmark, focusing especially on microscopic fossils and fossil cephalopods. I'm a member of the research section named Earth and Planetary System Science. It is very exciting, but also very challenging, to study the earliest forms on Earth. A major difficulty is that only very few rocks have survived the millions and millions of years from just after the formation of the Earth until today. And if you are lucky to find such rocks, they usually have been later metamorphosed by high temperatures and high pressures deep in the crust, meaning that the possible contents of fossils have been destroyed and lost. But luckily, there exist different techniques we can use to test if lifeforms existed, and if they did, to show which sort of lifeforms it was. If we can reconstruct the ancient environments, we'll have a fair chance to make probable which lifeforms we would expect to find. In the coming three lessons, we'll go through some of the proxies that are used to document the earliest life on Earth, chemical, paleontological, geological, biological and astrophysical. Moreover, we will look at some of the organisms that flourished during the 4 billion years from the formation of Earth until the multi-cellular life, especially animals took over only 541 million years ago. And why we had to wait so long for the animals to evolve. A lot of the events we will go through during this course relates to stratigraphy and a timescale. Do you know what stratigraphy is? Well, it's difficult to define stratigraphy in a short way, but it can probably be described as a geological discipline concerned with the description of rock successions and their interpretation in terms of a time scale. Maybe stratigraphy can be visualized this way. If we dig a deep hole through the upper crust of the Earth, each of the different layers we cut through represents a certain time period which is distinguished by its own special properties. For example, minerals or fossils. It is an important discipline in connection with the study of sedimentary rocks and layered crystalline rocks, such as, for instance, lava flows. Common stratigraphic objectives are the subdivision of a sequence of rock strata in the specific units based on the lithology, fossil content or chemical content, and determine the time relationships that are involved. This figure shows the geological time scale. The horizontal scale shows the time from today at zero to the right, to the formation of the solar system at 4,567 million years ago to the left. The geological time scale relates stratigraphy to time and is used by geologists and geophysicists to describe the timing of different events that have occurred during the Earth's history. Every time unit has got a specific name which has been approved by the International Commission on Stratigraphy. The Precambrian is a super eon that is super-divided into the informal Hadean, and the Archean, and the Proterozoic eons. Formally, the Precambrian units are subdivided chronometrically, with their lower and upper boundaries placed at certain chronological ages. But in the future, many of these will be defined by geological events as is seen in the units younger than the Precambrian. The informal Hadean era is a period from the formation of the Solar System to the appearance of the oldest dated rocks found on Earth. The Archean is an era from the appearance of the oldest dated rocks to the first appearance of the glacial rocks. And finally is the Proterozoic, the era covering the period from the first appearance of glacial rocks to the first appearance of the trace fossil Phycodes pedum that defines the base of the Cambrian system 541 million years ago. The latter point is almost coincident with the point where animals radiated substantially, developed hard skeletons and became common throughout our planet. One of our strong tools for understanding ancient life is the study of fossils. Do you remember from earlier what a fossil is? Yes, it is a part, fragment, imprint or trace of an ancient organism. Fossils provide important evidence to the history of life, although fossils are exceedingly rare compared to the number of organisms that actually lived on Earth in times past. Formation of fossils, or fossilization, requires that very special and favorable conditions are present. If not, the remnants and traces of the dead organisms will disappear immediately after their death. The study of fossils is nevertheless a powerful tool for providing insight into the ancient life that has existed right from the time shortly after the formation of Earth to the present. The Solar System and the Earth is almost 4.6 billion years old, but no direct evidence of life has been observed from the first 800 million years of Earth's history. This is not surprising as only very few rocks from this time span exist on Earth today. In addition, the very first part of Earth's history was characterized by exceptionally unpleasant conditions, such as a hot and unfriendly surface, and a period with frequent meteorite impacts causing very poor living conditions with surface temperatures that at times exceeded the boiling point of water. It is uncertain when seawater was first present on the Earth, but researchers suggest that the first water may have existed up to 4.4 billion years ago. This hypothesis is based on a geochemical survey of Earth's oldest known minerals, more precisely, the 4.4-billion-years-old zircon crystals found at Jack Hills in Australia. The oldest generally agreed rocks on Earth is the Acasta Gneiss from northwest Canada, which is 4.03 billion years old. Both these finds were originally igneous, now metamorphic and solidified deep below the surface, and therefore carry no evidence of the conditions that existed on the Earth's surface. This is different with the 3.8-billion-years-old rocks at Isua in southern west Greenland, which have been studied intensely by my colleague, Minik Rosing. Besides basaltic lava rocks, among others, pillow lava and iron ore, black schists do also occur. The schists are metamorphosed mudstones that contain carbonaceous clay particles, which slowly descended through the water column to become embedded in the seabed. They represent the oldest sedimentary rocks deposited on the Earth's surface. The microscopic particles of carbon in the shales contains Earth's oldest evidence of life as both the graphite structure and the ratio between the carbon isotopes, Carbon-12 and Carbon-13, indicate that the carbon must originate from organisms. The graphite particles are not body fossils or trace fossils, but may be classified as chemofossils. So, when did life originate on Earth? Well, we don't know, but we can try to make a qualified estimate. The presence of liquid water is essential for life. It is therefore probable that life on Earth was established somewhere later than 4.4 billion years, which is the age of the Jack Hills zircon crystals carrying isotopic evidence of water, and the more than 3.7 billion years old Isua graphite chemofossils representing the first evidence of life on Earth. "Microbes" is another word for microorganisms and include all organisms that are so small that they can only be studied by use of a microscope. Although most fossil microbes, or microfossils, are unicellular, they constitute a group that has representatives in all the six organism kingdoms that many researchers operate with today. Since Darwin's pioneering work on the origin of species in 1859 and the publication of the German zoologist and philosopher Ernst Haeckel's famous diagram of the organisms' relationships in 1866, which some of you probably are familiar with, there has been intense discussion on how different organisms are related. Until Haeckel's work was published in the 19th century, mainly two kingdoms were operated with, where the plant kingdom was characterized by autotrophic, photosynthetic, passive organisms, while the animal kingdom was characterized by heterotrophic dynamic organisms. Haeckel emphasized, however, the need to introduce a third kingdom comprising all the unicellular organisms, the Protista. As people became more aware of microbes, it became clear that also this division was too simple, which gradually led to the six kingdom system used by many researchers today. However, new data and interpretations on the classifications of organisms do constantly develop. So, what are bacteria? Well, bacteria are single celled microscopic organisms. The cell structure is simpler than that of the other organisms as there is no nucleus or membrane-bound organelles. Instead, their control center containing the genetic information is contained in a single loop of DNA. Bacteria are often classified into five groups according to their basic shapes or morphology, and can exist as single cells, in pairs, chains, or clusters. Bacteria and archaea have generally the same shape, size and appearance. The morphological similarities can make it difficult to visually separate a bacterium from an archaeon, but there are a few differences which can be seen in living specimens. The cell walls in all bacteria except a group called Eobacteria contain so-called peptidoglycan. This is not found in archaea. Another difference is that the bacterial cell membrane has ester bonds while archaea has ether bonds. Archaea are morphologenetically similar to Eukarya than bacteria is to either of them. Mitocondria and chloroplasts found internally in the eukaryotes are almost certainly the descendents of earlier prokaryotic cells that established themselves as internal symbionts of a larger anaerobic cell. The different groups of bacteria and archaea are often restricted to certain environments. Winogradsky was one of the first microbiologists to study bacteria found in biofilm communities. When he made his experiments in the 1880s, he isolated the organisms from nature in small glass containers and made miniature models of a lake cross-section. Today, it is simply called Winogradsky column. It is a simple and low-cost device for constructing a stratified bacterial ecosystem. This stratification develops through days and weeks. It gives an excellent visual demonstration of different modes of metabolism and zonation of the bacterial life in both the water column and the underlying sediment. It is not the intent of this lecture to provide a detailed description of each of the bacterial groups observed, but only give a few words about the most common groups. The sulfate reducing bacteria and certain Archaea may be found in the lowest anaerobic part of the sediment. Photosynthetic green and purple sulfur bacteria that produce sulfur from sulfide occur in the anaerobic parts above the sulfate reducing bacteria. In the upper parts where oxygen gradually becomes more common occurs the sulfur and iron-oxidizing bacteria and in the top the only group of oxygen-producing bacteria, the cyanobacteria, is found. There's unfortunately only very little evidence on surface conditions and climate in the earliest parts of the Archean, often referred to as the Hadean, which lasted from the Earth's formation 4.6 billion years ago to about 4.0 billion years ago, which is the age of the oldest rock on Earth. The Archean covering the periods up to 2.5 billion years was characterized by a predominantal anoxic environment. It was virtually without free oxygen since calculations suggest that atmospheric oxygen levels were lower than 0.1% of current levels, perhaps as low as 0.001%. Instead, it's probable that gases like carbon dioxide, methane and water were common in the atmosphere. The light energy from the sun was up to 30% lower than current levels in the Archean, which would have caused much lower temperatures than today if the atmosphere and surface conditions were similar to the existing. But there are no signs of constant glaciations during the Archean. How do we explain this? Yes, one hypothesis is that the concentration of the greenhouse gases such as methane and carbon dioxide compensated for the lower solar luminosity and caused temperatures well above the freezing point. Another hypothesis was published in 2010 by a Danish-American research team. According to this, the higher temperatures may be explained by a lower albedo on the Earth, which was caused by very small continental areas in lower and middle Archean, and the lack of biologically induced cloud condensation nuclei. Living conditions in both oceans and atmosphere were very different from those found on Earth today. And the Earth's atmosphere in the Archean has been compared to one which is now found on Saturn's moon Titan, which is dominated by nitrogen and methane. [MUSIC]
