The planet that we live in, our Earth is the only known planet where life exists. But the conditions were not always as friendly as it is right now. In fact, the conditions of the primitive earth were very hostile.
FORMATION OF EARTH AND IMPORTANT EVENTS
TIMELINE
4.5 billion years ago- Formation of the Earth.
3.9 billion years ago- Formation of the Great Oceans.
3.8 billion years ago- Origin of life on Earth
3.2 billion years ago- Evolution of autotrophic life forms.
2.7 billion years ago- Evolution of Cyanobacteria.
2.4 billion years ago- Great Oxidation Event
Our earth was formed approximately 4.5 billion years ago. The young Earth was very hot having temperatures of more than 100°C. There were extensive volcanic eruptions releasing a huge amount of volcanic ash and gases like water vapour, carbon dioxide, sulphur dioxide, hydrogen sulphide etc. The particles of volcanic ash used to collide and resulted in the development of electrical discharge causing volcanic lightning. The gases released from these eruptions formed the early atmosphere which was slightly reducing in nature due to the presence of reducing gases like Ammonia, Methane, Dihydrogen etc. However, the Oxygen was either trapped in the form of oxides of nitrogen, sulphur, carbon or in the form of water vapour. As time passed, the Earth started to cool down and the condensing water vapour poured down in the form of heavy rains. After several hundred million years of rainfall, around 3.9 billion years ago, the Great Oceans were formed. These conditions were perfect for the origin of life on Earth but not for evolution and existence of diverse forms of life that exists today. The origin of life can be best explained with the help of the Oparin-Haldane Theory of Origin of Life which will be covered in a separate article.
The first life evolved around 3.7-3.8 billion years ago, due to the conditions of the primitive earth. The volcanic lightning provided energy for chemical reactions between gases and resulted in the formation of simple organic molecules. These organic molecules reached the oceans along with the rain creating the 'Prebiotic soup'. The continuous chemical reactions between these simple organic molecules led to the development of different types of complex organic compounds like carbohydrates, fats, nucleic acids and proteins. Aggregation of these biomolecules resulted in the development of Protobionts (also known as Coacervates or Microspheres), Protocells and finally the first living organisms, the prokaryotes. As these organisms originated in the prebiotic soup in presence of reducing environment, hence it is assumed that these organisms were Chemoheterotrophs (obtained their energy from chemicals) and Anaerobic in nature.
Around 500 million years later (3.2 b.y.a), evolved the organisms which were able to prepare their own food by performing autotrophic mode of nutrition. First to evolve were Chemoautotrophs (used chemical energy to synthesize organic molecules from inorganic molecules) which were followed by Photoautotrophs.
The Photoautotrophs were the first to harness solar energy for preparing their food but the initial photoautotrophs relied on H₂S to obtain electrons and protons required for photosynthesis. This mode of photosynthesis does not involve the release of O2 and thus called Anoxgenic photosynthesis. These organisms were similar to modern-day Purple and Green Sulphur bacteria. The simplified chemical reaction of anoxygenic photosynthesis can be written as:
CO2 + 2H2S + Light Energy → [CH2O] + 2S + H2O
But things started to change after the evolution of Cyanobacteria (Blue-green algae) as the mode of photosynthesis performed by them was slightly different. Instead of H2S as source of electron and proton, these organisms broke down water molecules. This resulted in the release of free O2 molecules in the atmosphere for the first time. Within a period of a few hundred million years, the Blue-green algae totally transformed the composition of the atmosphere, changing it from a reducing atmosphere to an oxidising atmosphere which is referred to as the Great Oxidation Event (GOE). The simplified chemical reaction of oxygenic photosynthesis can be written as:-
CO2 + 2H2O + Light Energy → [CH2O] + O2 + H2O
The presence of molecular oxygen in the atmosphere also resulted in the development of the ozone layer in the upper atmosphere, preventing the majority of the harmful solar radiations from reaching the Earth, creating a much hospitable condition. However, this change did not produce a favourable outcome for the majority of the existing life forms as they couldn't adapt to the oxidising atmosphere and this led to their extinction (hence Great Oxidation Event is also known as Oxygen Catastrophe). The organisms that survived laid a foundation for the evolution of much more complex life forms like eukaryotes.
Thus, it can be concluded that the Cyanobacteria were the pioneer organisms responsible for modifying the atmospheric conditions into the current oxidising forms and making it much stable as compared to the conditions that were during the origin of life on Earth.
CYANOBACTERIA- ULTRASTRUCTURE
Either unicellular, may form colonies or have a long filamentous body called Trichome.
Lacks flagellation; mobility is provided by the mucilage layer surrounding the body.
The body of filamentous BGA consists of two main types of cells- Vegetative cell and Heterocyst.
The cell wall is multi-layered and made up of Peptidoglycan which is a polymer having long chains of modified sugars like N-acetyl glucosamine and N-acetyl muramic acid linked together with the help of tetrapeptide side branches.
The cytoplasm can be distributed as the pigment-containing peripheral part, Chromoplasm and the colourless central part called Centroplasm.
Chromoplasm contains membranous structures called chromatophores having a wide range of pigments like chlorophylls, carotenoids and phycobilins for the absorption of lights of different wavelengths.
Centroplasm contains structures like ribosomes, gas vacuoles, proteins, enzymes and aggregates of other such molecules (Inclusion bodies).
The genetic material is also present in the cytoplasm itself as being prokaryotes, BGA lacks a well-defined nucleus.
The Heterocysts are specialized thick-walled cells where fixation of atmospheric nitrogen is performed.
Special conditions are required to be maintained in Heterocysts to perform nitrogen fixation:- Anaerobic conditions, production of nitrogenase enzyme and absence of photosynthetic enzymes.
Reproduction- Only performs asexual mode of reproduction either by binary fission (unicellular forms), by fragmentation (filamentous forms) or with the help of specialized large vegetative cells having an abundance of food called Akinetes.
Important examples of Cyanobacteria:-
Unicellular- Microcystis aeruginosa
Filamentous- Nostoc, Anabaena, Oscillatoria (lacks heterocyst)
Watch the animated video here:-
* All the data are obtained from various books and scientific research papers available on NCBI.