How life originated on earth has been a mystery for mankind. While common man generally believes that God created life, scientists believe that life could have been created by natural means. In the following discussion, we shall be assessing some of these theories.
Theory of special creation
Religions preach that life was created by God whether it is Hinduism, Islam or Christianity. Religions teach that all species of plants and animals were individually created by God for specific purpose and that they do not have any relationships among them. According to Hindu mythology, life was the creation of Brahma, Islam says Allah created living beings and Christianity says that God created all life in 6 days. However, science cannot accept theories that have no concrete evidence.
Cosmozoic theory (=Panspermia)
Micro-organisms from space have been coming on earth along with meteorites and comets for a long time and then perhaps evolved into higher organisms in water. Pieces of Martian rocks have been recovered from Antarctica which contained bacteria-like organisms on them. This can be given as evidence that life could have come to earth from some other planet. Francis Crick and Laslie Orgel found molybdenum in the bodies of plants and animals which in plants is required in nitrogen metabolism. But molybdenum is a rare on earth and could not have been used by organisms.
Another theory postulates that some planet with primitive life must have collided with earth and life got transferred on to earth.. Some workers such as Richter (1865), Helmholtz (1884) and Arrhenius (1908) and explorers like Eric von Daniken and Zecharia Sitchin propose that the universe may have several planets with intelligent life. If such intelligent beings achieved capability of space travel and if they landed on earth in the past, they could have then brought life on earth.
Spontaneous generation theory (=Abiogenesis)
Thales, Anaximander and Aristotle believed that life originated from the non-living things and still continues to evolve by this method. For example, worms and frogs can appear spontaneously from mud and fly larvae can appear in rotten meat. Francisco Redi (1690) was the first scientist to refute this theory by experimentally demonstrating that if you cook fish and meat then no organisms appear in it in a sealed container. But people still believed that microorganisms developed by spontaneous generation.
Another scientist, Spallanzani (1780) boiled and sealed broth and found that no micro-organisms appeared spontaneously. Later, Louis Pasteur (1860) conducted a more sofisticated experiment in which flasks whose neck was drawn like swan’s neck were used to trap micro-organisms and dust. The boiled broth was kept in these flasks did not rot, suggesting that micro-organisms did not appear spontaneously.
Biogenesis
Redi, Richter, Spallanzini and Louis Pasteur rejected the theory of abiogenesis and proposed a new theory called Biogenesis. The theory postulates that life can emerge only from a form of life such as spores, eggs or hibernating animals and not from inorganic substances. But still this theory does not explain how life originated in the beginning.
Biochemical origin of life
This theory is also called biopoiesis. It was proposed by A.I.Oparin (1923) and J.B.S.Haldane (1928). Oparin (1936) published a book titled, The Origin of Life in which the theory was explained in detail. This theory says that life evolved in the primitive atmosphere of earth by reactions in chemicals present at that time. Organic compounds, such as carbohydrates, fats and proteins could have been formed easily by chemical reactions of elements, namely, oxygen, hydrogen, nitrogen and carbon which combined with one another to form larger molecules such as carbon dioxide, carbon monoxide, water, ammonia, methane, hydrogen, acetylene etc.
As the water was formed its vapors along with different gases formed atmosphere as the temperature dropped dow. Condensation of the water vapours and the gases brought down heavy rains. The rain water collecte in low lying areas and created oceans. The water reacted with hot metals on earth’s surface producing acetylene gas that filled the atmosphere of primitive earth. Simple amino acids like Glycine were formed by reaction of ammonia with other compounds. Carbohydrates and fatty acids are also chains of carbon, hydrogen and oxygen and could have been formed by similar method.
Aminoacids possess natural tendency to form polypeptide chains by amide bonding and these chains can easily form proteins in suitable atmosphere that was available in the primitive atmosphere of earth. The larger protein globules formed protein aggregates or Microspheres, Coacervates or Protobionts. These large protein spheres could grow in size by absorbing protein molecules from the surrounding water and then divide by fission, a primitive method of reproduction.
Coacervates got surrounded by a fatty layer that formed a cell membrane like structure. Such organisms were called Eubionts, which could reproduce by fission. Later, enzymes and genetic material also evolved to make reproduction easier. RNAs could have been formed in high energy environment which can also act as enzymes. Earliest genes were probably minerals associated with proteins and capable of replication.
Nucleic acids are more efficient coding materials and probably evolved later. RNA was perhaps the first genetic material evolved as proposed by Crick, 1968 and Watson et al., 1986. RNA molecules have also been known to act as enzymes. DNA is a complex molecule and might have been formed much later may be by RNA by some of kind of reaction as created by reverse transcriptase.
This kind of primitive organisms can be called Progenotes, which could be the ancestors of all life. They must have fed on the organic molecules present in water and could synthesize organic substances required by the cell. Prokaryotes, such as bacteria must have evolved in the next step. We have fossils of cyanobacteria in 3,500 million year old rocks suggesting that they were the first living organisms to evolve on primitive earth. Prokaryotes gave rise to Eukaryotes, when DNA got enclosed in a nuclear membrane about 1600 million years ago.
Evidences supporting biochemical origin of Life
Many scientists believed that if life could evolve by chemical reactions naturally in oceans then it should be possible to produce it in laboratory. S.L. Miller (1953) designed an experiment in which conditions of primitive earth could be recreated. He boiled water in a flask to produce steam which was directed into a large chamber in which methane, ammonia, hydrogen and water vapors could be subjected to electric sparks. The resultant material was cooled to obtain liquid material.
Amino acids like glycine, alanine and aspartic acid were formed in cooled liquid. Several other workers repeated this experiment and obtained almost all amino acids. Bahadur (1954) in his experiments obtained all amino acids by mixing paraformaldehyde, ammonia and ferric chloride together and exposing it to sunlight. Lowe, Rees & Markham (1963) heated liquid hydrogen cyanide and aqueous ammonia and obtained amino acids. Abelson (1966) mixed carbon dioxide, nitrogen and hydrogen together and exposed the mixture to ultraviolet radiation. Amino acids glycine, alanine, serine, aspartic acid and glutamic acid were obtained in this experiment.
Protein-first hypothesis
It is now generally accepted that amino acids were formed in the oceans but could these amino acids also polymerise to form microspheres. This could have happened in three possible conditions: 1) If amino acids are dehydrated and heated. 2) If amino acids are absorbed in clay. 3) If amino acids reacted with cyanide or phosphate compounds. Fox (1957) after heating a mixture of 20 amino acids could produce polypeptides.
The spheres of polypeptides were similar to coacervates, which were called protenoids. Microspheres exhibit some properties of cells. They can divide by fission. They are covered by a double layer of non-fatty membrane and they can produce ATP to obtain energy. Steinman & Moser (1967) conducted experiments and found that formation of peptides could be stimulated by other peptides that acted as enzymes.
Gene-first hypothesis
Nucleotides could not have been formed in the natural conditions as existed on primitive earth as nucleotides require a series of enzymes to bind together the nucleoproteins, sugars and phosphates. But several scientists have opined that this could have happened on young earth. Ponnamperuma (1965) produced ribose and deoxyribose by sujecting a mixture of methane, ammonia and water to radiation. Even formaldehyde solution under specialized conditions formed sugars.
Experiments conducted by Schramm (1965) revealed that polynucleotides could be formed in the presence of phosphorus-containing compounds. Horowitz (1959) gave the theory that life began in the form of “naked genes” without a cell and cell membrane and that these had the capabilities of self-duplication. They also had the ability to influence environment to ensure supply of materials for the cell. Although sufficient evidence is not available to prove this theory but Muller (1965) and Sagan (1965) believed that some polynucleotide chains could act as enzymes.
Possibility of Life in outer space
Life is a complex of chemicals that has achieved extreme specialization and capacity to produce its own kinds. The universe has countless number of galaxies scattered all over. Our own galaxy called Milky Way contains about 100 billion stars, each one them having solar systems of their own. The number of earth-like planets capable of supporting life is estimated to be more than 100,000 within our own galaxy. Millions of galaxies lie within our range of telescope. So earth-like planets is not a rarity in the universe.
In spite of strong possibility of life in the universe, the distances are astronomical. Even communication to such long distances can take years and travel to such planets will not be possible within the lifetime of an individual. Although life may exist in several places in the universe, it is scattered, separated by long distances.