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Evolution of Life on Earth
- Earth formed around 4.54 billion years ago, approximately at one-third of the age of universe by accretion from the solar nebula.
- Several theories/postulations have been propounded in reference to the origin of life on Earth, but most accepted theory among them is the ‘Oparin-Haldane Theory’.
Oparin-Haldane Theory of Origin of Life :
- According to this theory life originated on early Earth through physico-chemical processes of atoms combining to form molecules, which in turn reacting to produce inorganic and organic compounds. Organic compounds interacted to produce all types of macromolecules which organized to form the first living system or cells.
- Thus, according to this theory ‘life’ originated upon our Earth spontaneously from non-living matter. First inorganic compounds and then organic compounds were formed in accordance with ever-changing environmental conditions. This is called chemical evolution which cannot occur under present environmental conditions upon Earth. Conditions suitable for origin of life existed only upon primitive Earth.
- Hypothesis given by Oparin about origin of life is also known as Materialistic Theory which was published in his book ‘Origin of Life’.
- Modern views regarding the origin of life include both chemical evolution and biological or organic evolution.
A. Chemical Evolution (Chemogeny) :
1. The Atomic Phase :
- Early Earth had innumerable atoms of all those elements (e.g. hydrogen, oxygen, carbon, nitrogen, sulphur, phosphorus) which are essential for the formation of protoplasm. Atoms were segregated in three concentric masses according to their weight –
(a) The heaviest atoms of iron, nickel, copper, etc. were collected in the centre of the Earth.
(b) Medium weight atoms of sodium, potassium, silicon, magnesium, aluminium, phosphorus, chlorine, fluorine, sulphur etc. were collected in the core of the Earth.
(c) The lighter atoms of nitrogen, hydrogen, oxygen, carbon etc. formed the primitive atmosphere.
2. Formation of Inorganic Molecules :
- Free atoms combined to form inorganic molecules such as H2 (hydrogen), N2 (nitrogen), H2 O (water vapour), CH4 (methane), NH3 (ammonia), CO2 (carbon dioxide), etc. Hydrogen atoms were most numerous and most reactive in primitive atmosphere.
- Hydrogen atoms also combined with nitrogen forming ammonia (NH3 ). So the water and ammonia were probably the first inorganic molecules of primitive Earth.
3. Formation of Simple Organic Molecules (Monomers) :
- The early inorganic molecules interacted and produced simple organic molecules such as simple sugars (e.g. ribose, deoxyribose, glucose etc.), nitrogenous bases (e.g. purines, pyrimidines), amino acids, glycerol, fatty acids etc.
- Terrestrial rains must have fallen. As the water rushed down, it must have dissolved away and carried with it salts and minerals, and ultimately accumulated in the form of oceans. Thus ancient oceanic water contained large amounts of dissolved NH3 , CH4 , HCN, nitrides, carbides as well as various gases and elements.
- Some external sources must have been acting on the mixture for reactions. These external sources might be (i) solar radiations such as ultraviolet rays (UV rays), X-rays etc., (ii) energy from electrical discharges like lightning, (iii) high energy radiations are other sources of energies (probably unstable isotopes on primitive Earth)There was no Ozone layer at that time in the atmosphere.
- A soup like broth of chemicals formed in oceans of the early Earth, from which living cells are believed to have appeared, was termed by Haldane as prebiotic soup, also called hot dilute soup. Thus the stage was set for combination of various chemical elements. Once formed, the organic molecules accumulated in water, because their degradation was extremely slow in absence of any life or enzyme catalysts.
- The formed molecules accumulated in ocean bounded by a covering forming coacervetes which have the capacity to self replication.
Experimental evidence for Abiogenetic Molecular Evolution of Life :
- Stanley Miller in 1953, who was then a graduate student of Harold Urey at the University of Chicago, demonstrated it clearly that ultraviolet radiation or electrical discharges or heat or combination of these can produce complex organic compounds from a mixture of methane, ammonia, water and hydrogen. The ratio of methane, ammonia and hydrogen in Miller’s experiment was 2 : 1 : 2.
- Miller circulated four gases : methane, ammonia, hydrogen and water vapour in an airtight apparatus and passed electrical discharges in it from electrodes at 800o C. Then he passed the mixture through a condenser.
- He circulated the gases continuously in this way for one week and then analyzed the chemical composition of the liquid inside the apparatus. He found a large number of simple organic compounds including some amino acids such as alanine, glycine and aspartic acid.
- Miller conducted the experiment to test the idea that organic molecules could be synthesized in a reducing environment.
- It is considered that the essential building blocks such as nucleotides, amino acids etc. of living organisms could thus have formed on the primitive Earth.
- Early life on Earth formed around 4.0 billion years ago.
- Membrane bound prokaryotes originated around 3.8 billion years ago, which were previously heterotrophs and later on became autotrophs. Autotrophs were just like the blue-green algae of modern period.
- Approximately 2.0 billion years ago eukaryotic cells were formed, from which all the organisms (except bacteria) of our planet evolved
B. Biological or Organic Evolution :
Fundamental presumptions and principle of organic Evolution :
- The fundamental presumption of organic evolution is – ‘the modification of living organisms during their descent, generation by generation from common ancestors’.
- According to the concept of organic evolution, the present day animals and plants have been evolved by a process of gradual change in the earlier simple forms of life, which took place in millions of years.
- The theories of organic evolution explains convincingly the origin of life. It also explains how the wide variety of plants and animals came into existence in the world.
- In organic evolution, there are different theories of evolution, but following theories are accepted universally.
They are :
(i) Theory of Inheritance of Acquired Character (Lamarckism)
(ii) Theory of Natural Selection (Darwinism)
(iii) Mutation Theory (Hugo de Vries)
I. Theory of Inheritance of Acquired Character (Lamarckism) :
- Lamarckism is the first theory of organic evolution, named after Jean-Baptiste Lamarck (1744-1829), a French biologist.
- Lamarck’s famous book is ‘Philosophie Zoologique’ (1809).
- He announced in 1801, a theory of organic evolution which has been known as ‘Theory of Inheritance of Acquired Character’.
- His evolutionary ideas are²
- Internal forces of life tend to increase the size of the organism and because of an ‘inner want’ new structures appear.
- The direct environmental effect over living organisms.
- Use or disuse of organs.
- Inheritance of acquired character.
- Long neck of giraffe due to lack of surface vegetation gradually.
- Aquatic birds – they had to go to water due to lack of food etc. Some structures ‘web’ between their toes developed and wings for fly gradually reduced.
- Flat fishes (deep sea fishes).
- Whales lost their hind limbs.
- The wading birds (e.g. Jacana) developed its long legs through generation of sustained stretching to keep the body above the water level.
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II. The Theory of Natural Selection (Darwinism) :
- Charles Darwin explained natural selection in his book ‘On the Origin of Species’ (1859 ; Full Title – On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life). It is considered to be the foundation of evolutionary biology.
- It includes the following elements –
- The universal occurrence of variation
- Over production (rapid multiplication)
- The struggle for existence : Intra-specific struggle between same species. Inter-specific struggle between different species. Environmental struggle due to earthquake, tidal waves, the burning of valcanoes are all causes for killing large populations.
- Survival of the fittest (Natural Selection): Organisms struggle for existence and organisms with advantageous characters survive, while those which lack such variations perish. Thus, individuals having favorable variations have better chances of living long enough to reproduce.
- Inheritance of useful variations : The organisms after getting fitted to the surroundings transmit their useful variations to the next generation, while non-useful variations are eliminated.
- Neo-Darwinism : Neo-Darwinism is a modified form of Darwinism. Neo-Darwinism term generally used for describing any integration of Darwin’s theory of evolution by natural selection with Gregor Mendel’s theory of genetics. George Romanes first used this word in 1895.
- Modern synthetic theory is the result of work of a number of scientists namely T-Dobzhansky, R.A. Fisher, J.B.S. Haldan, Small Wright, Ernst Mayr and G.I. Stebbins.
- Stebbins in his book ‘Process of organic evolution’ discussed the ‘synthetic theory’. This is one of the proven theories of organic evolution. It includes the following factors–
- Variation or Recombination
- Natural Selection
III. Mutation Theory :
- Dutch botanist Hugo de Vries, in his book ‘The Mutation Theory’ (1901) proposed this theory.
- He observed on ‘Evening Primrose’, Oenothera lamarckiana. He studied this plant in wild forms for many years continuously and observed certain spontaneous changes in some of these wild plants.
- Mutation Theory states that evolution is a jerky process where new varieties and species are formed by mutations (discontinuous variations) that function as raw material of evolution.
Salient features of mutation theory are :
- Mutation appear all of a sudden. They become operational immediately.
- The same type of mutation can appear in a number of individuals of a species.
- All mutations are inheritable.
- Useful mutations are selected by nature. Lethal mutations are eliminated. However, useless and less harmful ones can persist in the progeny.
- Accumulation of variations produce new species, sometimes a new species is produced from a single mutation.
Important Facts :
- Archaeopterix : It looks like a bird. It has wings and beak like birds. However its teeth and tail are like those of reptiles.
- Archaeopterix is, therefore, considered as a connecting link between reptiles and birds, thereby suggesting that birds have evolved from reptiles.
- The fossils of Archaeopterix was first discovered from limestone deposits near Solnhafen, Germany.
- Archaeopterix lived in the late jurassic period of Mesozoic era around 150 millions years ago.
- Dinosaurs : During the Mesozoic or ‘Middle Life Era’, life diversified rapidly and giant reptiles dinosaurs and monstrous beasts roamed the earth. The period, which spans from 252 million years ago to about 66 millions years ago was also known as the age of reptiles or the age of dinosaurs.
- Mesozoic era has been divided into three periods viz– the Triassic (251-199.6 millions years ago), the Jurassic (199.6 to 145.5 million years ago) and the Cretaceous (145.5 to 65.5 millions years ago).
- Cro-Magnon : The earliest known Cro-Magnon remains are between 35000 and 45000 years old based on radiometric dating.
- Cro-Magnon had powerful bodies, which were usually heavy and solid with strong muscles with height about 180 cm.
- The Cro-Magnon had straight forehead like modern humans. Their faces were short and wide with a large chin. Their brains were slightly larger than the average human’s of today. The brain capacity of Cro-Magnon was 1600 cc.
- The name ‘Cro-Magnon’ was coined by Louis Lartet, who discovered the first Cro-Magnon skull in Southwestern France in 1868.
- The Cro-Magnon are considered the last direct ancestor of modern man and it was a sub-species of modern human named Homo sapiens.