Origin of Life on Earth

V SEMESTER ZOOLOGY ORIGIN OF LIFE ON EARTH

EARLY THEORIES Many theories were formulated to explain the origin of life before modern times. They are special creation, cosmozoic theory, spontaneous generation and biogenesis.

A. Special Creation theory: It is the oldest theory. Special creation theory states that all living organisms are created by super natural power. It is a religious view. Opening chapter of genesis says that on the 3rd day of creation, God brought forth living creatures, first plants, then fish and birds, land mammals, and finally man. According to Hindu mythology, life was the creation of Brahma, the supernatural being. Special creation theory was discarded as it did not stand in scientific arguments.

B. Cosmozoic theory: According to this theory, life did not arise in our planet. But life was descended from other planet where life existed previously. Helmholtz (1884) said that micro-organisms from space came on earth along with meteorites and comets and then evolved into higher organisms in water. Recently a piece of Martian rock has been recovered from Antarctica. NASA has discovered fossils of bacteria-like organisms on this rock, suggesting that life could have come on earth from outer space. Francis Crick and Laslie Orgel argued that all organisms have molybdenum in their bodies. Plants require it in nitrogen metabolism as the enzymes nitrogenase and nitrogen reductase are catalysed by molybdenum. But molybdenum is a rare element on earth, amounting to only 0.2% of all elements. Cosmozoic theory has recently been extended to include the concept that some planet with primitive life must have collided with earth and seeded it with life. Proponents of this theory are Richter (1865), Helmholtz (1884) and Arrhenius (1908). Explorers like Eric von Daniken and Zecharia Sitchin go a step forward in proposing that the universe has several planets having intelligent life that is capable of space travel and that they have landed on earth in the past and seeded it with life. They provide evidences of extraterrestrial visitations of earth in the past from the writings on clay tablets recovered from Mesopotamia, which carry accounts of extraterrestrial visitations and details of solar system, galaxies etc.

C. Spontaneous generation theory: Ancient Greek philosophers, namely, Thales, Anaximander, Aristotle and scientists of the medieval period, like Harvey, Newton and Needham believed that life regularly originated from the non-living things. They believed that worms and frogs can appear spontaneously from mud and that maggots can appear in meat and mice in refuse spontaneously. The theory was accepted universally for about 2000 years.

Italian scientist, Francisco Redi (1690) was first to refute this theory and experimentally demonstrated that maggots did not appear if cooked fish and meat were placed in covered containers. He kept fish and meat in some open jars and some in jars covered with muslin cloth and showed that maggots appeared in open jars but not in covered jars. Redi published his results in his book, Experiments on the generation of insects. But people still believed that microorganisms developed by spontaneous generation. This concept was refuted by another Italian L. Spallanzani

(1780), who showed that boiled broth if sealed did not develop micro-organisms. However, people thought that by boiling he removed the “vital force” necessary for life. Louis Pasteur (1860) used flasks whose neck was drawn like swan's neck to trap micro-organisms and dust. The boiled broth kept in such flasks did not rot, while broth kept in open flasks started rotting. This experiment conclusively proved that spontaneous generation theory was wrong.

D. Biogenesis theory (Life begets life): It was proposed by Francisco Redi, Richter, and Louis Pasteur. According to this theory life arose from pre existing life. Redi practically proved that life never arose from non living matter. Maggots will arise from decaying organic meat if it contains eggs only. If eggs are absent on such meat, never maggots will arise. Even though biogenesis explains the formation of living organism from pre existing life, it never explains the origin of first living organism. But it laid the foundation for proposing the theories of organic evolution.

MODERN THEORY This theory was simultaneously proposed by a Russian biochemist, A.I.Oparin (1923) and the British biologist, J.B.S.Haldane (1928). Later Oparin published the book, The Origin of Life in 1936. Oparin explained the origin of life from non living materials through the chemical evolution of life. Oparin’s theory is called naturalistic theory or theory of coacervates.

OPARIN’S HYPOTHESIS: The approximate age of earth is 4,600 million years. Whereas the age of first life is 3,500 million years.

FORMAION OF SOLAR SYSTEM The origin of sun, earth and other planets is explained by cosmic evolution. It was proposed by Sir James Jeans. According to him, before 4,600 million years ago, the nebula was a ball like mass. It contained high temperature and everything is in the form of a gas. One star was attracted by the nebula. As the star speedily collided with nebulous mass, the outer atmosphere was broken into nine pieces. These pieces were sprayed in the different distances. As these pieces moved away they gradually lost the temperature. These nine pieces were formed into planets. The remaining nebulous mass is being considered as the sun. As the earth began to cool, the cosmic dust was condensed. Hence on earth solids, liquids and gases are co existing.

The different steps in the formation of first living organism are explained below.

A. Environment of early earth: According to this theory, life developed in the primitive atmosphere of earth by simple chemical reactions that took place as the earth started cooling down from the hot ball that it was when it got detached from the sun. Organic compounds, namely, carbohydrates, fats and proteins, consist of mainly Carbon, Oxygen, Hydrogen and Nitrogen. These elements were present in the primitive atmosphere of earth, when the atmosphere cooled down to 5000-6000 degrees about 4600 million years ago. As the earth’s temperature came down to about 1000oC, the elements started combining with one another to form larger molecules such as carbon dioxide, carbon monoxide, water, ammonia, methane, hydrogen, acetylene etc.

There was no free oxygen at this time and the atmosphere was highly reducing. As the water vapors were formed along with different gases in a highly turbulent atmosphere and as the temperature dropped down further, condensation of the vapors and the gases brought down heavy rains, which created oceans. Hot metal carbides on earth’s surface reacted with rain water producing acetylene gas that filled the atmosphere.

B. Production of polymers: Ammonia donated NH2 to acetic acid to produce the simplest amino acids like Glycine. As all amino acids have basically the same structure, they must have been formed by the simple chemical reactions taking place in the primordial broth, and since carbohydrates and fatty acids are also chains of carbon, hydrogen and oxygen, these organic compounds also could have been easily formed by simple chemical reactions.

Aminoacids must have naturally formed polypeptide chains by amide bonding to form complex proteins in the high energy and high temperature oceans of the primitive earth. Such protein globules must have formed protein aggregates or poteinoid Microspheres, which were called Coacervates or Protobionts by Oparin. Whether such protein globules can be called livings things is debatable, even though they could grow in size by absorbing protein molecules from the substrate and then divide by binary fission and probably had enzymatic activity too.

Lipids must have been naturally formed in water and coated the surface of coacervates, transforming them into Eubionts, which could maintain their shape, grow and multiply by fission due to this new primitive cell membrane around them. If eubionts could grow and reproduce, they must have developed primitive enzymes and some sort of replicating genetic material to store information for proteins.

C. Origin of nucleic acids: Nucleoproteins are difficult to form in natural conditions but various kinds of protein chains inside the eubionts must have made more and more complex combinations to give rise to nucleoproteins and eventually to simple RNA. This is likely to have happened owing to energy filled atmosphere and high energy available for chemical reactions at that time. Cairns & Smith (1971), however, believe that the earliest genes were minerals associated with proteins, capable of self-replicating and coding for useful proteins. Nucleic acids as coding material came later and being more efficient were selected out against minerals. It is now believed that RNA was the first genetic material evolved (Crick, 1968, Watson et al., 1986). RNAs have also been found to act as enzymes that catalyze their own replication. DNA might have been formed by RNA with the help of some kind of reverse transcriptase enzyme. Such organisms appeared about 4000 m.y.a and were much like today's PPLOs (Pleuropneumonia-like organisms). We can call them Progenotes, the ancestors of all life that developed later. They were slow growing and slowly replicating heterotrophs that contained only a few genes and few proteins and a limiting plasma membrane. They fed on the readymade organic molecules abundantly present in the primordial broth. Ability to synthesize organic substances required by the cell using enzymes perhaps evolved later.

Progenotes then gave rise to more complex Prokaryotes, such as bacteria which contained free DNA and a cell-wall. Fossils of cyanobacteria from 3500 million year old rocks have been recovered.

Prokaryotes apparently gave rise to Eukaryotes, when DNA got enclosed in a nuclear membrane about 1600 m.y.a. Thus the single-celled organisms evolved, which gradually gave rise to all life on earth.

Evidences supporting biochemical origin of Life Publication of Oparin’s book created a kind of stir in scientific community and many thought that if life could originate naturally in oceans by simple biochemical reactions, it should be possible to produce it in laboratory by recreating the primitive atmosphere of earth. Biochemical origin theory appeared to be very sound and hence several scientists tried to test the theory by experimentation. S.L. Miller, a research student of the Nobel laureate Prof. Harold C. Urey was assigned the project in 1953 to design an experiment in which conditions of primitive earth could b recreated.

UREY AND MILLER’S EXPERIMENT

Miller and Urey experimentally proved the formation of complex molecules in the laboratory. They supported Oparin’s hypothesis.

Miller made an apparatus to synthesize complex materials, amino acids. Miller’s apparatus contains two rounded glass flasks and glass tubes. The rounded flasks are connected by glass tubes. In one flask, Miller took fresh water. He took C, H, N and phosphorus in one connected tube. In the another flask, Miller took marine water. He boiled the fresh water. Hence steam was produced. The steam was passed over the elements and finally mixture was sent into marine water flask. Miller and Urey observed ammonia, methane and phosphates in the marine water flask. These chemicals sent into a glass tube, which was connected to electrodes. The electrodes produced sparks. Then the chemical mixture was cooled and sent into the ‘u’ shaped tube. After one week, Urey and Miller observed amino acids in the ‘u’ tube. Miller succeeded in obtaining amino acids like glycine, alanine and aspartic acid in the trapped liquid. The results were published on May15, 1953 in Science, Volume 117, page 528, under the title, “A production of aminoacids under possible primitive earth conditions.”

Several other workers repeated this experiment and obtained almost all amino acids. Bahadur (1954) obtained almost all aminoacids by subjecting a mixture of paraformaldehyde, ammonia and ferric chloride to strong sunlight. Pavlovskaya and Pasynskii (1959) exposed a mixture of gases to ultraviolet radiation and obtained alanine, glycine, aspartic acid and glutamic acid. Lowe, Rees & Markham (1963) synthesized many organic compounds by heating liquid hydrogen cyanide and aqueous ammonia to 90oC for 18 hours. Abelson (1966) found that a mixture of CO, N2, and H2 would react to form hydrogen cyanide and when subjected to ultraviolet radiation formed aminoacids glycine, alanine, serine, aspartic acid and glutamic acid.

That aminoacids formed naturally in the primordial oceans was supported by William Schopf of Harvard University, when in 1967 he detected 22 aminoacids in the rock formations that were 3000 million years old.

Protein-first hypothesis If aminoacids were formed naturally in the oceans, could they also polymerise naturally to form protein microspheres. Calvin (1965) thought that this could happen under three possible conditions: first, if they are dehydrated and subjected to heat. Second, if aminoacids are absorbed in clay or minerals and third, if they reacted with cyanide or phosphate compounds. Sydney Fox (1957) heated a mixture of 20 amino acids and obtained long chains of polypeptides, which immediately formed microspheres in water. They were equivalent to coacervates of Oparin and were called protenoids. Later in 1965, she placed a mixture of dry aminoacids in a block of hot lava and obtained an amber-colored liquid, which when diluted with hot salt solution formed microspheres. Sydney Fox believed that microspheres exhibited some properties of cells, such as they divided by fission, were covered by a double layer of non-fatty membrane and in the presence of zinc they split ATP to obtain energy.

If peptides were formed spontaneously in the primitive oceans, could they duplicate themselves or synthesize new ones in the absence of genetic material. Steinman & Moser (1967) experimentally proved that peptide production could be stimulated by peptides already present in the substrate, without participation of nucleic acids. Perhaps some of them acted as enzymes.

Gene-first hypothesis Formation of nucleotides is perhaps the most unlikely event to envisage in the natural conditions of primitive earth as it requires catalytic action of several enzymes to bind together nucleoproteins, sugars and phosphates. But several workers have indicated that it could have happened naturally on the young earth. Ponnamperuma (1965) produced ribose and deoxyribose sugars by irradiating a mixture of methane, ammonia and water. He also indicated that formaldehyde in aqueous solution under suitable condition condensed to form simple sugars. Schramm (1965) experimentally demonstrated that polynucleotides were formed when phosphorus-containing compounds were present under water-free conditions. Oro (1965) and Ponnamperuma (1965) indicated that adenine is readily formed from hydrogen cyanide in special conditions and that purines and pyrimidines could have been formed under non- biological conditions due to strong ultraviolet radiation.

Horowitz 1959) proposed that life began in the form of “naked genes” which had the capability of self-duplication and mutation and ability to influence environment to ensure supply of materials for the cell. There is no concrete evidence to prove it but Muller (1965) and Sagan (1965) suggested that probably there were some polynucleotide chains that could act as enzymes.