Conditions for the Origin of Life
For origin of life, atleast three conditions are needed
- There must have been a supply of replicators , i.e. , self - producing molecules
- Copying of these replicators must have been subject to error through mutation
- The system of replicators must have required a continuous supply of free energy and partial isolation from the general environment
The high temperature in early earth would have fulfilled the requirement of mutation.
Protobionts or Protocells. These are atleast two types of fairly simple laboratory produced structures - Oparin's coacervates and Fox's microspheres which possess some of the basic prerequisites of protocells. Although these structures were created artificially, they point to the likelihood that nonbiological membrane enclosures (protocells) could have sustained reactive systems for at least short periods of time and led to research on the experimental production of membrane bound vesicles con taining molecules , i.e. , protocells
Coacervates. The first hypothesis was proposed by Oparin (1920). According to this hypothesis early protocell could have been a coac ervate. Oparin gave the term coacervates. These were nonliving structures that led to the formation of the first living cells from which the more complex cells have today evolved. Oparin speculated that a protocell consisted a carbohydrates, proteins, lipids and nucleic acids that accumulated to form a coacervate. Such a structure could have consisted of a collection of organic macromolecules surrounded by a film of water molecules. This arrangement of water molecules, although not a membrane, could have functioned as a physical barrier between the organic molecules and their surroundings. They could selectively take in materials from their surroundings and incorpo rate them into their structure.
Coacervates have been synthesized in the laboratory. They can selectively absorb chemi cals from the surrounding water and incorporate them into their structure. Some coacervates contain enzymes that direct a specific type of chemical reaction. Because they lack a definite membrane, no one claims coacervates are alive, but they do exhibit some lifelike characters. They have a simple but persistent organization. They can remain in solution for extended periods. They have the ability to increase in size.
Microspheres. An another hypothesis is that early protocell could have been a microsphere. A microsphere is a nonliving collection of organic macromolecules with double layered outer boundary. The term microsphere was given by Sidney Fox ( 1958 1964 ). Sidney Fox demonstrated the ability to build microspheres from proteinoids. Proteinoids are protein like structures consist ing of branched chains of amino acids. Proteinoids are formed by the dehydration synthesis of amino acids at a temperature of 180 ° C. Fox, from the University of Miami, showed that it is feasible to combine single amino acids into polymers of proteinoids. He also demonstrated the ability to build microspheres from these proteinoids. Fox observed small spherical cell - like units that had arisen from aggregations of proteinoids. These molecular aggre gates were called proteinoid microspheres. The first non - cellular forms of life could have originated 3 billion years back. They would have been giant molecules ( RNA, Proteins, Polysac charides etc.).
Microspheres can be formed when proteinoids are placed in boiling water and slowly allowed to cool. Some of the proteinoid material produces a double - boundary structure that encloses the microsphere. Although these walls do not contain lipids , they do exhibit some membrane like characteristics and suggest the structure of a cellular membrane. Microspheres swell or shrink depending on the osmotic potential in the surrounding solution. They also display a type of internal movement (streaming) similar to that exhibited by cells and contain some proteinoids that function as enzymes. Using ATP as a source of energy, microspheres can direct the formation of polypeptides and nucleic acids. They can absorb material from the surrounding medium. They have the ability of motility, growth, binary fission into two particles and a capacity of reproduction by budding and fragmentation. Superficially, their budding resembles with those of bacteria and fungi.
According to some investigators, microspheres can be considered first living cells.
Origin of Prokaryotes. Prokaryotes were originated from protocells about 3.5 billion years ago in the sea. The atmosphere was anaerobic because free oxygen was absent in the atmosphere. Prokaryotes do not have nuclear membrane, cytoskeleton or complex organelles. They divide by binary fission. Some of the oldest known fossil cells appear as parts of stromatolites . Stromatolites are formed today from sediments and photosynthetic prokaryotes ( mainly filamentous cynobacteria- blue green algae ).
Evolution of Modes of Nutrition
Heterotrophs. The earliest prokaryotes presumbly obtained energy by the fermen tation of organic molecules from the sea broth in oxygen free atmosphere ( reducing atmosphere ). They required readymade organic material as food and thus they were heterotrophs.
Autotrophs . Due to rapid increase in the number of heterotrophs the nutrient from sea water began to disappear and gradually exhaused . That led to the evolution of autotrophs . These organisms were capable of producing their own organic molecules by chemosynthesis or photosynthesis.
Chemoautotrophs. Drop in temperature stopped synthesis of organic molecules in the sea water. Some of the early prokaryotes got converted into chemoautotrophs which prepared organic food by using energy released during certain inorganic chemical reactions . These anaerobic chemoautotrophs were like present anaerobic bacteria . They released CO2 in the atmosphere.
Photoautotrophs. Evolution of chlorophyll molecule enabled certain protocells to utilize light energy and synthesize carbohydrates. These were anaerobic photoautotrophs. They did not use water as a hydrogen source. They were similar to present day sulphur bacteria in which hydrogen sulphide split into hydrogen and sulphur. Hydrogen was used in food manufacture and sulphur was released as a waste product.
Aerobic photoautotrophs used water as a source of hydrogen and carbon dioxide as source of carbon to synthesize carbohydrate in the presence of solar energy. The first aerobic photoautotrophs were cyanobacteria (blue green algae) like forms which had chlo rophyll. They released oxygen in the atmosphere as the by product of photosynthesis. The main source of genetic variation was mutation.
Oxygen Revolution. As the number of photoautotrophs increased, oxygen was released in the sea and atmosphere. Free oxygen than reacted with methane and ammonia present in the primitive atmosphere and transformed methane and ammonia into carbon dioxide and free nitrogen
CH4 + 2O2 »»»»»»»» CO2 + 2H20
4NH3 + 3O2 »»»»»»» 2N₂ + 6H20
The oldest fossil belonging to blue green algae, named Archaeospheroides barbertonensis which is 3.2 billion years old. Oxygen releasing prokaryotes first appeared atleast 2.5 billion years ago.
Formation of Ozone Layerl. As oxygen accumulated in the atmosphere, the ultra violet light changed some of oxygen into ozone .
202 + O2 »»»»»»»»2O3
The ozone formed a layer in the atmosphere, blocking the ultraviolet light and leaving the visible light as the main source of energy.
Origin of Eukaryotes.
The eukaryotes developed from primitive prokaryotic cells about 1.5 billion years ago. There are two views regarding the origin of eukaryotes. (i) Symbiotic Origin. According to Margulis (1970-1981) of Boston University, some anaerobic predator host cells engulfed primitive aerobic bacteria but did not digest them.These aerobic bacteria established themselves inside the host cells as symbionts. Such predator host cells became the first eukaryotic cells . The predator host cells that engulfed aerobic bacteria evolved into animal cells while those that captured both aerobic bacteria and blue - green algae became eukaryotic plant cells . The aerobic bacteria established them selves as mitochondria and blue green algae as chloroplasts.
(ii) Origin by Invagination.According to this view cell organelles of eukaryotic cells might have originated by invagination of surface membrane of primitive prokaryotic cells.
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