Our Earth
The earth was supposed to have been formed about 4.5 billion years ago which was later differentiated into three main parts
Crust. It is the outer - most solid, rocky surface of the earth.
Mantle. It is the middle part of earth which is solid and consists of iron and magnesium silicates.
Core. It is the central part of earth, which is differentiated into semisolid outer core and solid inner core. The heavy metals such as iron and nickel , sank into the central part of the earth.
Earth originally had only two components, solid mass called lithosphere surrounded by a gaseous envelope termed atmosphere. The liquid component, known as hydrosphere, appeared later when the earth cooled down to a temperature below 100 ° C.
Theories of the Origin of Life
Many theories have been put forward to explain the origin of life. Some important theories about origin of life are described below.
Ancient Theories of Origin of Life
Following ancient theories are important to mention.
Theory of Special Creation
The greatest supporter of this theory was Father Suarez. According to this theory life was created by supernatural power. According to Bible the first man, Adam and the first woman, Eve were created by God. According to Hindu mythology the world was created by God Brahma. The first man was Manu and the first woman was Shradha. While muslims believe Allah as creator of life. Special creation theory lacks scientific evidences, on account of which it is not accepted.
Theory of Spontaneous Generation (Abiogenesis or Autogenesis)
This theory states that life originated from nonliving things in a spontaneous manner. This concept was held by early Greek philosophers like Thales, Anaximander, Xanophanes, Empedocles, Plato, Aristotle, etc. In ancient Egypt, it was believed that the mud of the Nile could give rise to frogs, toads, snakes, mice and even crocodiles when warmed by the sun. Van Helmont (1577-1644) held that human sweat and wheat grains could give rise to organisms. He placed a dirty shirt in a receptacle containing wheat bran and found that after 21 days the gases from the shirt and wheat had formed living mice. These beliefs have no scientific grounds and hence are discarded.
Evidences against the Theory of Spontane ous Generation. The theory of spontaneous generation was disproved by many scientists of 17th, 18th and 19th centuries. They proved that new organisms can be formed from pre - existing ones, i.e. , omnis vivum ex ovo or vivo (" Biogenesis of Harvey - 1651 and T. H. Huxley - 1870). Noted scientists who experimentally challenged the theory were Francesco Redi (1626-1697), Lazzaro Spallanzani ( 1729-1799 ) and Louis Pasteur (1822-1895)
Redi's Experiment. Francesco Redi, an Italian physician, took the flesh and cooked it so that no organisms were left alive. Then he placed flesh in three jars, of which, one was uncovered, the second was covered with parchment and the third one was covered with fine muslin. He kept these jars for a few days and ob served that maggots developed only in the uncovered jar though the flies also visited other jars.
Spallanzani's Experiment. Spallanzani (1765), an Italian scientist disproved the spontaneous generation of microorganisms. He experimented that animal and vegetable broths boiled for several hours and soon after sealed, were never infested with microorganisms. From this experiment he concluded that high temperature had killed all living organisms in the broths and without them life could not appear. When the broths were left exposed to air, were soon invaded by microorganisms.
Pasteur's Experiment. Louis Pasteur, a French scientist took broths in a long necked flask and then he bent the neck of the flask. He boiled the broths in the flask to kill any microorganisms that might be present in them. The curved neck acted as a filter. If the flask with ' swan neck' (curved neck) is kept for months together, no life appeared, as the germ laden dust particles in the air were trapped by the curved neck which serves as filter. If the swan neck was broken off, the broths developed colonies of moulds and bacteria. Thus, he showed that the source of the micro - organisms for fermentation or putrefaction such as for milk, sugar and wine, etc., was the air and the organisms did not arise from the nutrient media.
Thus Louis Pasteur (famous for " Germ Theory of Disease and Immunology ") finally disapproved abiogenesis and proved biogenesis.
But according to biogenesis , life originated from pre - existing life which does not explain the origin of life. So biogenesis is also disapproved.
Theory of Panspermia or Cosmozoic Theory or Spore Theory
This theory was proposed by Richter (1865). According to this theory, 'protoplasm' reached the earth in the form of spores or germs or other simple particles from some unknown part of the universe with the cosmic dust , and subsequently evolved into various forms of life. Helmholz (1884) speculated that 'protoplasm' in some form reached the earth with falling meteorites. Arrhenius (1908. Nobel Prize Winner of 1903 in Chemistry) postulated the (= Panspermia Theory) and stated that organisms existed throughout the uni verse and their spores etc. , could freely travel through space from one star to the others. In fact , panspermia theory is the alternative name of cosmozoic theory.
Evidences against Cosmozoic Theory. Living matter cannot survive the extreme cold , dryness and ultra - violet radiation from the sun required to be crossed for reaching the earth.
Theory of Eternity of Life
This theory was proposed by Preyer in 1880. According to this theory, different types of living beings have always existed on earth and shall continue to exist forever, changing only in form.
Evidence Against Theory of Eternity of Life. It is accepted that earth had not always existed. If life is eternal, where did it exist before this planet was formed.
Theory of Catastrophism.
Georges Cuvier ( 1769-1832 ) , Father of "Modern Palaeontology" and Orbigney (1802 1837) were the chief advocates of this theory. According to this theory cataclysms (great destruction) or catastrophic (concerning disastrous event) revolution occurs upon earth from time to time which completely destroys all organisms (living beings). New organisms, then, suddenly form from inorganic matter. Each creation consists of life quite different from that of the previous one. In fact, this theory is merely a modification of theory of special creation. This theory is also not accepted.
Please Do Not enter any sparm link in comment box