Prokaryotes are the organisms classified as Bacteria and Archaea, and are the most successful & abundant organisms on Earth. In fact they have been THE dominant group on earth since life appeared and for around 2000 million years were the only life form on earth. Prokaryotes as a group have the largest biomass on the planet e.g. in the oceans, prokaryotes make up 90% or greater than the total weight of living things; there may be 2.5 x 109 prokaryote cells in a gram of fertile soil. Prokaryotes are also the most ancient organisms on Earth: the earliest known fossil cells belong to a prokaryote, and come from rocks in Western Australia that date back 3500 million years. All prokaryotes are small cells that lack the complex internal structures, like mitochondria and chloroplasts, found in eukaryotic cells. Also, although prokaryotes possess DNA on a chromosome, it is not enclosed in a nucleus.
Because prokaryotes are largely invisible to the human eye we tend to forget about them. However, they contributed to the development of an oxygen-rich atmosphere early in Earth's history, and are essential to the processes of decomposition and nutrient cycling, a key role in all ecosystems. They also made a significant contribution to the evolution of the better-known, eukaryote, life forms.
Present-day prokaryotes may resemble early fossils, but they are modern organisms that have successfully adapted to modern environmental conditions. They are found in some of the most extreme environments on Earth, including Antarctica, the depths of the oceans and deep in rocks, round deep-sea vents, and in boiling thermal springs and are ever present in our human environments, including cities, homes and the human body.
The Cyanobacteria (blue-green algae) are a group of prokaryotes that are extremely important both ecologically (especially in global carbon and nitrogen cycles) and evolutionary terms. Stromatolites, which are formed by cyanobacteria, provide living and fossil evidence of cyanobacteria going back 2700 million years. Today stromatolites grow only in shallow, salty pools in hot, dry climates (e.g. Shark Bay in Western Australia), and their abundance in ancient rocks implies similar environmental conditions in those times. Stromatolites and other cyanobacteria were the main contributors to the marked increase in atmospheric oxygen concentrations that began around 2000 million years ago. Today, cyanobacteria are found everywhere - in marine, freshwater and terrestrial environments and as symbionts e.g. lichen - and contribute up to 50% of the atmosphere's oxygen.
DNA evidence suggests that the first eukaryotes (green plants) evolved from prokaryotes (through endosymbiotic events) between 2500 and 1000 million years ago. Fossils of eukaryotes that resemble living brown algae have been found in sedimentary rocks from China that are 1700 million years old, while possibly the oldest photosynthetic eukaryote, Grypania, comes from rocks 2100 million years old. Note that the diversity of modern algal groups, and particularly of their chloroplasts, suggests that these endosymbiotic events were not unusual. Modern algae comprise a range of organisms with very different structures but identical photosynthetic pigments. This suggests that very different host organisms have formed a symbiosis with the same photosynthetic cells. That is, the algal groups must have evolved through separate endosymbiotic events, and the group as a whole is identified on the basis of a similar level of structure, rather than on its evolutionary origins. Such groups, where the members have several different evolutionary origins, are described as polyphyletic.
Cyanobacteria have a close evolutionary relationship with eukaryotes. They have the same photosynthetic pigments as the chloroplasts of algae and land plants. Chloroplasts are the right size to be descended from bacteria, reproduce in the same manner, by binary fission, and have their own genome in the form of a single circular DNA molecule. The enzymes and transport systems found on the folded inner membranes of chloroplasts are similar to those found on the cell membranes of modern cyanobacteria, as are their ribosomes. These similarities between cyanobacteria and chloroplasts suggest an evolutionary link between the two, and can be explained by the theory of endosymbiosis.
For 1500 million years photosynthetic organisms remained in the sea. This is because, in the absence of a protective ozone layer, the land was bathed in lethal levels of UV radiation. Once atmospheric oxygen levels were high enough the ozone layer formed, meaning that it was possible for living things to venture onto the land. The seashore would have been enormously important in the colonisation of land. In this zone algae would have been exposed to fresh water running off the land (and would have colonised the freshwater habitat before making the move to terrestrial existence). They would also be exposed to an alternating wet and desiccating environment. Adaptations to survive drying out would have had strong survival value, and it is important to note that seaweeds are poikilohydric and able to withstand periods of desiccation.
Because prokaryotes are largely invisible to the human eye we tend to forget about them. However, they contributed to the development of an oxygen-rich atmosphere early in Earth's history, and are essential to the processes of decomposition and nutrient cycling, a key role in all ecosystems. They also made a significant contribution to the evolution of the better-known, eukaryote, life forms.
Present-day prokaryotes may resemble early fossils, but they are modern organisms that have successfully adapted to modern environmental conditions. They are found in some of the most extreme environments on Earth, including Antarctica, the depths of the oceans and deep in rocks, round deep-sea vents, and in boiling thermal springs and are ever present in our human environments, including cities, homes and the human body.
The Cyanobacteria (blue-green algae) are a group of prokaryotes that are extremely important both ecologically (especially in global carbon and nitrogen cycles) and evolutionary terms. Stromatolites, which are formed by cyanobacteria, provide living and fossil evidence of cyanobacteria going back 2700 million years. Today stromatolites grow only in shallow, salty pools in hot, dry climates (e.g. Shark Bay in Western Australia), and their abundance in ancient rocks implies similar environmental conditions in those times. Stromatolites and other cyanobacteria were the main contributors to the marked increase in atmospheric oxygen concentrations that began around 2000 million years ago. Today, cyanobacteria are found everywhere - in marine, freshwater and terrestrial environments and as symbionts e.g. lichen - and contribute up to 50% of the atmosphere's oxygen.
DNA evidence suggests that the first eukaryotes (green plants) evolved from prokaryotes (through endosymbiotic events) between 2500 and 1000 million years ago. Fossils of eukaryotes that resemble living brown algae have been found in sedimentary rocks from China that are 1700 million years old, while possibly the oldest photosynthetic eukaryote, Grypania, comes from rocks 2100 million years old. Note that the diversity of modern algal groups, and particularly of their chloroplasts, suggests that these endosymbiotic events were not unusual. Modern algae comprise a range of organisms with very different structures but identical photosynthetic pigments. This suggests that very different host organisms have formed a symbiosis with the same photosynthetic cells. That is, the algal groups must have evolved through separate endosymbiotic events, and the group as a whole is identified on the basis of a similar level of structure, rather than on its evolutionary origins. Such groups, where the members have several different evolutionary origins, are described as polyphyletic.
Cyanobacteria have a close evolutionary relationship with eukaryotes. They have the same photosynthetic pigments as the chloroplasts of algae and land plants. Chloroplasts are the right size to be descended from bacteria, reproduce in the same manner, by binary fission, and have their own genome in the form of a single circular DNA molecule. The enzymes and transport systems found on the folded inner membranes of chloroplasts are similar to those found on the cell membranes of modern cyanobacteria, as are their ribosomes. These similarities between cyanobacteria and chloroplasts suggest an evolutionary link between the two, and can be explained by the theory of endosymbiosis.
For 1500 million years photosynthetic organisms remained in the sea. This is because, in the absence of a protective ozone layer, the land was bathed in lethal levels of UV radiation. Once atmospheric oxygen levels were high enough the ozone layer formed, meaning that it was possible for living things to venture onto the land. The seashore would have been enormously important in the colonisation of land. In this zone algae would have been exposed to fresh water running off the land (and would have colonised the freshwater habitat before making the move to terrestrial existence). They would also be exposed to an alternating wet and desiccating environment. Adaptations to survive drying out would have had strong survival value, and it is important to note that seaweeds are poikilohydric and able to withstand periods of desiccation.
The earliest evidence for the appearance of land plants, in the form of fossilised spores, comes from the Ordovician period (510 - 439 million years ago), a time when the global climate was mild and extensive shallow seas surrounded the low-lying continental masses. (These spores were probably produced by submerged plants that raised their sporangia above the water - wind dispersal would offer a means of colonising other bodies of water.) However, DNA-derived dates suggest an even earlier colonisation of the land, around 700 million years ago.
