Origin and Evolution of Life

Origin of Life:

The ability to grow and produce new forms distinguishes living organisms fron non-living objects. The origin of life on Earth is a most complicated problem for thinkers, philosophers and naturalists from ancient time. Different views have been put forward concerning the origin of life by workers of different ages. some of the most important hypothesis and theories regarding the origin of life are as follows:

Origin of Earth and its Atmosphere:

The earth was probably originated 4600 million years ago. The sun and the planets were formed together from a spherical cloud of gases and cosmic dust by gradual condensation. The earth was so hot that everything in it probably existed in gaseous form. It was a molten fire-ball about 5000-6000 centigrade for millions of years. The gases condensed into a molten core and differnt elemnts got stratified according to their density. Heavy metals such as Nickel, Iron formed the solid core of earth whereas lighter ones came towards the surface of earth. Lighter elements like Hydrogen, helium, carbon, oxygen and nitrogen constituted the early atmosphere. The earth had a reducing atmosphere, i.e. the free oxygen was not found in volcanic gases, because the reduced substances immediately combined with oxygen to form oxides.

Special Creation Theory:

This is the oldest view. according to this theory, life was created by some supernatural power of God. This theory was put forward by a Spanish priest, Father Suarez (1548-1671). According to him, thw world was made in six natural days. The earth and heaven were created on the first day and sky on the second day. The ancestors of plants and animals on the third day. The sun, the moon and the stars were created on the fourth day. The birds and fishes appeared on the fifth day. Finally, man was created on the sixth day and then women was created. Adam, the first man was moulded from clay and God infused life into it. This theory has no sound logic and scientific basis. Therefore, it is not accepted by scientists.

Spontaneous Generation of Life (Abiogenesis):

According to this theory, life originated from on-living materials automatically from time to time. It was believed that the frogs, toads, snakes and field mice arose from the mud; parasites, beetles and flies arose from the sweat and manure; aphids and other insects arose under the influence of heat and moisture; and micro-organisms arose spontaneously from air or water.

Greek philosophers of prechristian era like Thales (640-546 BC), Anaximander (611-547 BC), Empedocles (495-435 BC), Plato and Aristotle (348-322 BC) are the followers of this theory. Thales regarded that all living things have originated from water. Ancient Chinese, Babylonian, egyptian and Greeks also believed in spontaneous generaton.

History of Evolutionary Ideas:

Hindu Mythology: According to Hindu Mythology, a God named Brahma created the world from various parts of his own body. Humans and devils from his head, birds from his chest, goats from his mouth and other animals and plants from the hair of his body.

Empedocles (495-435 BC) emphasized that life arose spontaneously but gradually and evolution would result when forces of attraction and repulsion interacted each other. He believed that when reproduction was established, earth felt no need of special creation of organism. The thinking of Empedocles is supposed to be the basic concept of evolution. Hence, Osborn (1896) called him Father of the concept of Evolution.

Aristotle (384-322 BC) was of the opinion that not only insects and worms but also other highly organised creatures could be spontaneously formed. For example: crabs and various molluscas from foul earth and mud; eels and many other fishes from the mud of lakes, from sands and from decaying water, even frogs and salamanders could arise from moist earth. He slightly modified the view of spontaneous generation by stating that once originated spontaneously, an organism could reproduce its own kind to continue the race, but new varieties arose only spontaneously from inert matter. This view was widely accepted till the middle of the 17th century.

Van Helmont (1557-1647) performed an experiment by keeping a dirty sweat-soaked shirt with a few wheat grains in dark cupboard. He claimed the formation of mice in 21 days. In his recipe, the actve principle was thought to be human sweat.

Buffon (1707-1778) postulated change as a result of direct action of the environment. He stated that bigger and complex animals might not form spontaneously, ut micro-organisms in dirty water formed only by spontaneous generation. He suggested that the supreme being had created perfectly designed types emboided in the original species, but the new species aros from them by the process of hybridizaton and degeneration. Thus, the ass was supposed to be a degenerate horse and the ape a degenerate man.

Theory of Biogenesis:

[Life arises from pre-existing life]

According to this theory, living beings arise from living beings only. This theory was experimentally proved by following scientists:

A) Contribution during post Renaissance Period: Francesco Redi (1626- 1698) had experimentally proved that life could not originate from non-living things. he conducted experiments in three sets of jar containing dead tissues: here meat. One set of jar was kept open, the second one was covered with a fine net and third one was covered witha piece of paper. He observed little white worms and larvae (maggots) on decayed flesh only in the opened flasks. In other two sets of sealed jars, worms and larvae developed outside the net and the paper, but not inside the jar. Thus, he proved that maggots were not born spontaneously but due to contamination of meat by flies. From, the above experiment, he concluded that maggots came only from pre-exsiting flies and were not spontaneously generated by any other form of material.

B) Lazaaro Spallanzani (1729-1799): In 1767 Spallanzani, an Italian scientist performes an experiment to conclude that even primitive, unicellular organisms cannot arise from non-living matter. He prepared flasks of meat broth which were boiled for several hours. Some of the flasks were sealed properly while others kept open or loosely sealed. Micro-organisms appeared in those flasks which were open or loosely-corked while the sealed flsks remained sterile. He concluded that air carried micro-organisms which germinate when they get moisture and food. They were not formed abiogenetically.

C) Louis Pasteur (1822-1895): In 1860-1862 Louis Pasteur, a French microbiologist disproved the concept of spontaneous generation of microbes. He experimentally proved and gave the scientific explanation that life originates only from pre-existing life. This theory was known as"the theory of biogenesis". He used a swan-neck flask in his experiment. These flasks have long neck bent into S-shaped curve. He prepares a meat broth in these flasks and boiled for several hours. The various steps oh his experiments are as follows:

1) He poured the nutrient solution into the flsak.

2) Bent the neck of flask in the shape of 'S' using heat.

3)Liquid was boiled for several hours. Bioling killed all micro-organisms.

4) Then the flask was left unsealed on a laboratory bench.

5) Still the solution remained free of microbial particles contamination for months, because their necks were shaped to trap microbial particles and to allow only the movement of air.

6) But when he broke the neck of one of these flasks, microbes appeared in the flask within a few days.

Thus, with the help of this simple experiment, it was demonstrated that self-generation of organisms is impossible.

D) Eramus Darwin (1731-1807): Charles Darwin's grandfather, Eramus Darwin wrote about the process of evolution in his book Zoonomia (1794) and poem Temple of Nature. He clearly expresses that millions of year have been required for the process of organic evolution. He further stated that all life arose from one primordial protoplasmic mass.

Scientific Hypothesis- Chemical Evolution of Life:

The most modern and convincing theory of the origin of life is the theory of shemical evolution. Russian Biochemist Alexander Ivanovich Oparin (1923) and an Englishman J.B.S. Haldane (1928) independently gave the "biochemical theory of origin of life'. They proposed that the origin of life occured along the origin and evolution of earth and its atmosphere. Therefore, this theory is called "Oparin- Haldane Theory".

According to this theory, life on the earth was originated by non-living inorganic molecules through a series of chemical reactions. The inorganic molecules form simple organic compounds, simple compounds form more complex compounds, which finally produced first livingbeings, the cells.

Oparin provided a biochemical explanation of origin of life in his book, "The origin of life on earth" which was published in 1936. This theory can be explained under two seperate headings:

1) Chemical Evolution (Chemogeny): It involves the following steps:

A) Formation of Simple Compounds: About 4.2 billion years ago, the conditions on primitive earth were favourable for the chemical evolution. Under such conditons, hydrogen combined with oxygen , nitrogen and carbon to form water, ammonia, methane and cynamide. These existed in gaseous state due to high temperature and formed the atmosphere of the primitive earth. Water existed as vapour. The water vapour condensed to form rain. The water again returned to atmosphere because of superheated earth. this cycle continued for millions of years and resulted in the cooling of earth and water bodies like rivers, lakes and oceans came intothe existence.

C + 2H2 --------> CH4

N2 + 3H2 -------> 2NH3

H2 + 1/2 O2 -------> H2O

B) Formation of Organic Compounds: As the temperature cooled down, molecules and minerals undergone condensation, polymerisation, oxidation and reduction to form simple organic compounds. The organic compounds include alcohols, aldehydes, amino acids, glycerol, fatty acids, simple sugars, purines and pyrimidines. These reactions were operated under the influence of ultraviolet rays, radiations, electricity of thunderstorms and volcanic activities. The organic compounds formed in the hot water ocean had been described by Haldane as 'the broths" or "primordial soup" or "the diluted soup".

C) Formation of Complex Organic Compounds: Simple organic molecules combined to form complex organic compounds like polysaccharides, fats, proteins, nucleotides, nucleosides, nuclei acids etc. Due to absence of oxygen, the organic compounds on the primitive earth could remain unspoiled and could react to produce new compounds. The formation of protein molecules can be considered landmark in the event of the origin of life. The formation of nucleic acids was the another important event in the transformation of non-living to living.

2) Biological Evolution (Biogeny):

A) Formation of Coacervates: The complex organic compounds of oceanic soup were accumulated due to intermolecular attraction and form large and highly organized colloidal systems. These were named Coacervates. Formation of Coacervates was called coacervation. The coacervates acquired the ability to exchange energy and could grow. They grow in size as a result of absorbing substances dissolved in the surrounding water. After attaining the maximum size, they multiplied by breaking down into smaller droplets. Coacervates are considered to be living molecules which gave rise to the life or in other words, the coarcervates are described as intermediates between the molecules and the organisms.

B) Formation of Primitive Life: The coacervates contains biologically important macromolecules such as proteins, lipids, nucleic acids, nucleoproteins and other organic and inorganic substances. Later, a thin limiting membrane was developed around the coacervates. Some of the proteins of coacervates started working like enzymes for both destructive and synthetic reactions. Therefore, the coacervateses started absorbing organic substances from oceanic soup and became anaerobic heterotrophs or protocells or eobionts. The coacervates got associated with lipoprotein membrane and nuclei acids to produce eobionts. Eobionts developed into prokaryotes. The first "cell like" structures with division power were called "eobionts" or "pre-cell". Oparin called them "protobionts". Eobionts originated about 3800-4200 million years ago. The prokaryotes evolved about 3.8 billion years ago. They were very simple in structure and without a definite nucleus. These early prokaryotes evolved about 3.8 billion years ago. They were very simple in structure and without a definite nucleus. These early prokaryotes were chemoheterotrophs and were anaerobes.

C) Origin of Autotrophs: Primitive autotrophs developed from heterotrophs. The primitive autotrophs used chemical energy for synthesis of carbohydrates. they are called chaemoautotrophs. Later some autotrophic bacteria synthesized bacteriochlorophyll. they trapped solar energy but did not produce oygen, because they did not use water. The first aerobic photosynthetic organisms probably were some cyanobacteria like prganisms evolved 3300-3500 million years ago. They were the earliest oxygen producing photoautotrophs. With the evolution of chlorophyll molecule, oxygen was released as a bi-product of photosynthesis and this changed the earth's atmosphere reducing to oxidising one. therefore, all possibilities of further chemical evolution were finished. In 1968, a 3.2 billion years old fossil of such a blue-green algae named Archaeo-spheroides babertonesis has been reported from Africa.

D) Origin of Eukaryotic Cell (True Nuclear Cell): With the evolution of free oxygen in the environment, conditions suitable for aerobic respiration established upon the earth. This is probably about 2.7 billion years ago. The aerobic respiration yield about 20 times more energy in a biological system. The prokaryotes, therefore, gradually modified to adapt the aerobic mode of respiration. They developed a true nucleus, mitochondria and other cell organelles. Thus, free living eukaryotes originated in the ocean probably about 1.6 billion years ago. These eukaryotes were like unicellular organisms of today. Cellular organisms were then gathered to form colonies and finally gave rise to diverse forms of life.