1. How cells formed Because of the lack of ozone, UV light could have provided the energy to combine monomers (the basic chemical building blocks) into polymers (larger molecules which are vital for life such as DNA) and polymers into coacervates (protein rich mixtures of polymers which clump together). In the 1950's Urey and Miller tried to reproduce the first steps in a lab with electrical sparks in a gas mixture similar to that thought to have existed in the primitive earth and, using the energy from this simulated lightning, they were able to make amino acids and other monomers needed for life in laboratory condition. On the early earth, rain would have washed the monomers into the oceans The Origins of Life Primitive Earth The earth probably originated 4.5 - 5 x 109 years ago. Assuming life evolved on earth and was not seeded in from elsewhere, the conditions necessary must have existed on earth- so how did this occur? The earth's surface was originally molten and as it cooled the gases escaping from the molten magma were held by gravity as a veil of gas above the surface - a primitive atmosphere rich in ammonia, methane and carbon dioxide. There was no oxygen at this stage and so no ozone layer could form. Once the earth's surface cooled to below 100°C, water vapour would have condensed to form mineral rich oceans. Violent electric storms would have been common. Coacervates How the first polymers formed is less certain, but they may have clumped together on the surface of marine clay particles. As water was absorbed from the monomers into the clay structure (perhaps at the sea's margins as the tide went out) the monomers joined by condensation reactions. The energy required for these reactions may have come from UV light. Several researchers, including Oparin and Fox, have successfully caused protein rich mixtures of polymers to assemble into self-sustaining clumps termed coacervates. Given enough time and enough polymers, some could have formed coacervates with an outer membrane made of lipids (complex organic fatty acids) and DNA on the inside - ie primitive cells. Urey and Miller Experiment

2. Cyanobacteria evolved photosynthetic pigments, capable of trapping light energy and using it to energise the electrons in water. There was little limit to the population growth of these organisms - water, carbon dioxide and sunlight were abundant. Some bacteria formed into large rock-like colonies in shallow warm seas, called stromatolites. Fossils of these have been dated at 3.2 x 109 years old and stromatolite-forming bacteria are still active today in Shark Bay in Western Australia. The oxygen produced by cyanobacteria would have had several effects: This caused the atmosphere to change to that essentially similar to that found today. Life today has evolved its many complex and diverse forms, including man, to survive in this environment. Oxygen, which was toxic to the first bacteria, is now essential. Ultra-violet light which was probably an essential energy source for the early steps in the evolution of life but is now a major risk to living processes. As both photosynthesis and aerobic respiration became established as metabolic reactions, they became the major determining factors in the balance between CO 2 and O 2 in the atmosphere. This balance is necessary for life. Subsequent evolutionary changes, such at the advent of the first true cells and the divergence of Plants, Animals and Fungi have had only minor affects. Human influence is the only terrestrial factor that has the potential to upset this balance Information taken from: http://vle.camsfc.ac.uk/evs/ Most anaerobic bacteria became extinct, oxygen causing breakdown of their enzymes Ozone build-up would shield the earth from UV light Conditions favoured the evolution of aerobic bacteria Evolution of Anaerobic Respiration The difficult step is how these primitive cells mutated to produce the enzymes required to release and trap the energy present in the surrounding monomers - anaerobic respiration. Once it had occurred, however, this cell would have divided rapidly - limited only by the quantities of available monomers. Its ancestors include many types of anaerobic bacteria still alive today. Sulphur bacteria and Nitrifying bacteria evolved the ability to take carbon dioxide from the atmosphere and turn it into the monomers they needed for growth and respiration. Sulphur bacteria would have been much more likely to evolve in the primitive earth conditions than they would today, since there was more volcanic activity and thus more hydrogen sulphide, their energy source molecule. These chemosynthetic bacteria became free from the food shortages affecting other organisms but were themselves limited in distribution by the location of suitable minerals. Cyanobacteria Stromatolites