(By Dr. Girish Chandra)
Since time immemorial, man has been putting forward different theories to explain the origin of life and man himself on earth and how different kinds of life originated on earth and changed from time to time. Since ancient Greek’s time written records have been left about the views held by the people of that period and subsequent changes in the evolutionary theories. Three distinct periods can be identified based on the transformation in the evolutionary thoughts, namely, Ancient Greek theories, Pre-modern theories and Modern theories as given below.
Thales (639-544 BC): He lived in the Ionian colonies on the coast of Asia Minor. He left no writings but was a profound thinker and travelling and studying in Egypt, educated himself. He was a merchant, engineer, mathematician, astronomer and philosopher. He even coined the word “philosopher”. He believed that the whole universe was governed by natural laws and observed that water was the most abundant material on earth and that all life originated directly from water. According to him earth was a solid disc floating on seawater.
Anaximander (611-547 BC): He was a pupil of Thales and had broad views on the origin of nature. He believed that intermixing of earth, water, air and fire produced life. Every life has an ethereal substance, “apeiron” that is endless, unlimited and does not age or decay. He thought that stars were holes in the sky through which fire flowed. A total eclipse was due to closing of such a hole. He was the first one to describe earth as a sphere but believed that it was the centre of the universe. He designed the first map of the world and published the results of his researches in a poem, On Nature. He believed that life originated from primordial fluid spontaneously and that the first animals were fish, their descendants later reached land. Later they modified their mode of living according to the environment. Man was supposed to have come from lower species, perhaps an aquatic one. Man burst out from this fish-like animal as a butterfly comes out of the pupa.
Xenophanes (576-490 BC): He was a pupil of the mathematician Pythagoras. He identified fossils of aquatic animals on mountain lands and declared that mountains were once covered with water. He correctly interpreted fossils as remains of past animals but later workers; including Aristotle did not understand him.
Empedocles (504-433 BC): According to his theory, the four basic elements, namely, fire, water, earth and air originated from a combination of four fundamental qualities, viz. hot, dry, cold and wet and then acted upon by love and hate. All animals and plants originated from different combinations of these elements. Man had these elements in more refined and evenly mixed state. He boasted of his supernatural powers to cure and heal, bring rain, change the direction of wind etc. He is known to have rid a town of malaria by arranging drainage of swampy districts. His writings bear an impression of his belief in the survival of fittest. The suggestion that earth once had greater creation power than existed during his era also reflected his evolutionary belief.
Democritus (470-380 BC): He was deeply interested in travelling and gaining knowledge and extensively travelled in Egypt. He wrote about 70 books. Greatest contribution is his atomic theory, according to which universe is made of atoms, which move in space and all physical changes are due to union and separation of atoms. Spread of diseases was due to particles coming from atoms from other planets. He dissected animals, including human beings and described complexity of organs and relationship of these among lower and higher organisms. He considered brain to be the organ of thought and centre of all activity. He was more accurate than Aristotle who thought heart to be the centre of vital activity. He was the first Greek to attempt a classification of animals, on the basis of presence or absence of red blood. He claimed that spider’s web was produced inside the spider’s body, whereas Aristotle thought that it was cast off skin. He also explained sterility due to presumed contraction of uterus. Correct explanation followed much later after chromosomal study with microscope.
Aristotle (384-322 BC): He was one of the greatest a biologist ever lived and was interested in all knowledge available in his day. He wrote 146 books, in which he included everything known at that time. Many books have since been lost. He learned medicine from his father and went to Plato’s school in Athens at the age of 17 and lived there for 20 years. He studied marine biology in the island of Lesbos. He demonstrated scientific method of observation of things in nature, but did not undertake experimentation. His observations and interpretations on marine animals were remarkably accurate. Without any instruments he made observations on small objects like eggs and embryos of fish and molluscs. He traced the development of Octopus and Sepia from egg to adult stage. He also studied adaptations in sea animals and migration in fishes. He classified animals into Vertebrates (red blooded) and Invertebrates (without having red blood). He classified dolphins and whales in mammals contrary to the existing belief. At the age of 42, he was called by King Philip to teach his son Alexander, who later became more interested in military pursuits. When Alexander died in 323 BC, revolutionary forces came to power and Aristotle being close to him, he had to flee Athens to live in exile. He died a year later at the age of 62. His thoughts dominated for over 1000 years. Essence of his theory was that the force of intelligence, which is not found in non-living things, guides living things. Imperfect forms are gradually transformed into perfect forms. His classification gives a chain from lower to higher animals that he published as Scala Naturae (Ladder of Nature). He also believed in spontaneous generation theory of origin of life.
Epicurus (341-270 BC): He tried to combat superstitious beliefs. He thought the world as a natural phenomenon, governed by natural causes. He opposed Aristotelian argument of the grand design and purposefulness of events. He agreed with the atomic theory of Democritus but still believed in the spontaneous generation theory.
DECLINE OF SCIENCE
In the time to follow, Aristotle’s thoughts overshadowed every other thinking. Lucretius (99-55 BC) rejected much of Aristotle’s work and published his thoughts in his book, De Rerum Natura (On the nature of things). Pliny (23-79 AD) compiled a store of information in his “Natural History”, which was a good source of information. Galen (130-200 AD) was a physician who made investigations in anatomy and physiology.
Revival of classical learning in Romans and Greeks took place in 14-16th centuries. Leonardo da Vinci (1452-1519) realised that fossil shells on Apennine Mountains indicated that they must have been covered by sea in the past. He did not develop the idea further. Harvey (1578-1657) discovered circulation of blood. Little was accomplished in biology between Aristotle’s times to about 1500 AD. Aristotle’s thoughts prevailed during this period. By the end of second century AD, Greek science was virtually dead.
Francis Bacon (1561-1639): He was a philosopher who called upon men to seek knowledge by experiment and reasoning. He visualised a great plan of the origin and governing of earth and its inhabitants. He was an effective writer and a popular lecturer and gathered many facts but did not organise and coordinate them. He started a movement for free discussion and established academies for it. He only revived Aristotle’s ideas and said that variations caused evolution. He recognised that transitional forms are present connecting two groups but gave wrong examples that flying fish is transitional form between fish and birds and that bats connect birds and mammals.
Francesco Redi (1621-1697): He refuted spontaneous generation theory by experimentation with cooked fish ad dead snakes and showed that flies did not appear in closed jars. He presented his findings in a book, “Experiments on the generation of insects.”
De Maupertius (1698-1759): He developed a theory of evolution based on mutation, selection and geographical isolation but was not understood as he was far ahead of his time in scientific thinking. He developed a theory of heredity based on animal breeding and human heredity.
De Maillet (1656-1738): He studied fossils and pointed similarities between the aquatic and terrestrial forms and proposed that terrestrial animals evolved from aquatic ones but gave wrong examples of mermaids and flying fish. He entangled facts with myths and was afraid of church. He therefore attributed his unorthodox views to an Indian philosopher, “Telliamed”, which was his own name spelled backwards.
Carolus Linnaeus (1707-1778): A Swedish naturalist, he made extensive field trips (about 4600 miles) and collected lots of specimens of plants and animals. While in Holland he became a physician but continued to study nature and wrote 7 books from there. Later he became Professor of Botany at Uppsala. He promoted binominal nomenclature in which he skilfully used his predecessors’ works and published Systema Naturae in 1751. His classification reflected evolutionary relationship, although he believed in special creation theory.
Buffon (1707-1788): His actual name was George Loius Leclerc. He was a French naturalist, politician and writer and discussed evolution at length and published 44 volumes on natural history. He believed in inheritance of acquired characters and proposed that the factors that influence evolutionary changes are: direct effect of the environment, migration, geographical isolation, overcrowding and struggle for existence. He emphasised that new forms of life gradually develop from the old ones. But he compromised with the special creation theory and gave wild speculations. For example, pig was described as a compound of other animals, the ass as a degenerated horse and ape degenerated man. He was a prolific writer and interpreter but not an original investigator and his writings showed contradictions.
Erasmus Darwin (1731-1802): A British philosopher and grandfather of Charles Darwin, he gave clear statement on inheritance of acquired characters than Buffon. He wrote a book, Zoonomia in 1794, in which he proposed that life originated from a primordial protoplasmic mass. Age of earth was determined in million years. He concluded that species descended from common ancestors and that struggle for existence causes evolution. Charles Darwin wrote more from his grand father than originally supposed. For every volume written by Charles, there is corresponding chapter in Erasmus’ book. However, Charles found Zoonomia more speculative than scientific.
Louis Pasteur (1822-1895): He was a biochemist than a biologist and did some work on fermentation. He rejected spontaneous generation theory and proved by experimentation. He took 18 flasks that were left open outdoor and found that 16 of them developed organisms, while in 19 flasks that were kept closed in a lecture hall, only 4 developed organisms. Different results were shown when air was introduced from different sources, which made him aware of the presence of spores in air. He developed the theory of Biogenesis.
Spallanzani, Lazaro (1729-1799): An Italian physiologist, he also disproved the theory of spontaneous generation by experiments. He used different media and boiled them for one hour and sealed when hot, thus excluding all organisms. He was criticised by Needham for over-boiling the medium and destroying the vegetative force which was necessary for the growth of life. He was a proponent of Biogenesis, which means Life begets life.
Lamarck, Jean Baptist (1744-1829): A French biologist who joined military service initially but had to retire due to poor health. In 1778 he became Botanist in Jardin due Roi in Paris, where he prepared his Dictionary of Botany and later wrote Flora of France. With the help of Buffon he obtained a job in the Museum of Natural History and changed his field of study from Botany to Zoology and wrote 7 volumes of his best known work, Philosophie Zoologique. He studied fossils in detail and introduced several new terms, viz. ‘species’, ‘biology’, ‘invertebrates’ and ‘vertebrates’. He described animal kingdom in a series that graded from simpler to complex forms and gave the first diagram of evolutionary tree. He made use of inheritance of acquired characters to explain the theory of evolution, which had the following salient features:
He gave the example of the long neck of giraffe to explain his theory of use and disuse and appetency. The theory still remains to be demonstrated experimentally.
He married 4 times, had many children, was always poor and in later life became blind and died alone at the age of 85 in a pathetic state.
Cuvier, George (1769-1832): He is considered to be the father of comparative anatomy and palaeontology. He prepared the geological time table and believed that fossils are the result of catastrophe whence new creations are made. He believed in fixity of species and in special creation theory and opposed Lamarck’s theory. He was a scientific dictator of France, wielding unparallel scientific and political influence.
Later, Carl Gegnbaur (1826-1903) of Germany carried out extensive studies on comparative anatomy of all vertebrate classes and provided data in support of Darwinian Theory.
Saint-Hilaire, Geoffroy (1772-1844): A colleague of Curvier, he opposed his views and supported Lamarck and Buffon. He explained small variations and recognised effects of isolation, including physiological and geographical isolation. He had a public debate with Cuvier on the origin of squids and although correct in principle, lost owing to poor presentation and arguments. After French revolution he divided the Chair in Zoology with Lamarck.
Charles Darwin (1809-1882): After 5-year Voyage of the Beagle and studies on Galapagos Islands, he gave the theory of Natural Selection to explain evolution (see chapter on Darwinism for details).
Weismann, August (1834-1914): A German biologist he differentiated between somatoplasm and germplasm, which was the hereditary material carried from generation to generation. He believed in the continuity of germplasm, which he thought occasionally underwent change. He was of the view that struggle of existence was between the vital and invisible particles in the germplasm. A staunch supporter of Darwin, he was proficient in microscopic study of cells.
T.H. Huxley (1825-1895): He was foremost of Darwin’s defenders. By 12 years of age he had read all books in his father’s library. Later joined medical services on the ship Victory, during whose voyage he took extensive notes. After returning to England he wrote such large volumes that no publisher would publish them. Published Oceanic Hydrozoa and wrote on all branches of biology. He was not convinced of Lamarck’s work and supported Darwin so vigorously that he was nicknamed as “Darwin’s bulldog”.
deVries, Hugo (1848-1935): A Dutch botanist, he was interested in variations among organisms. He wrote a book in 1901, The Mutation Theory, which provided much needed explanation for the occurrence of variations. He studied variations in the evening primrose, Oenothera lamarckiana, which was introduced in Europe and USA, and in which several dwarf and giant types developed by mutation. He firmly believed that mutation controls all evolutionary events. Original mutation theory was gradually modified and refined, and is now accepted as a change in genes that bring about variations.
Ernst Haeckel (1834-1919): He made a connection between development of individual and evolution of race, which he called The Biogenetic Law, published in his book, Generalle Morphologie, in 1866. The idea was first proposed by von Baer (1792-1876) of Germany but later popularised by Haeckel by the famous phrase, “Ontogeny recapitulates phylogeny”. He argued that gill slits develop in the embryos of all vertebrates even when they are terrestrial lung breathers. Similarly heart, central nervous system, cartilaginous skeleton, notochord and even tail in the early development show remarkable similarity with one another in all vertebrates. All animals start their embryonic development from zygote, suggesting that all life evolved from a single cell. He believed that paleogenetic characters are ancestral traits that are retained in the embryos and coenogenetic characters are secondary, adaptive and non-ancestral that appear later in the development. However, Ernst Haeckel’s theory was criticised on three counts as follows: 1) Embryos of higher animals resemble only the embryos of lower animals and not the adults. 2) Ontogeny only shows affinity and not evolution. 3) Retaining ancestral embryonic stage has no selection value and therefore would be wasteful and time consuming. Modern gene theory explains the sequence of events and the subsequent end product of ontogeny in the right perspective and, therefore, Haeckel’s original theory has now been rejected.
The theorem of natural selection: R.A. Fisher (1932) gave the theorem of natural selection in his book, The cause of evolution, according to which the rate of increase in fitness of any organism is equal to its genetic variance at that time. More a species develops genetic heterozygosity, more fitness it would demonstrate during natural selection. Fisher studied replacement of grey form of industrial melanic moth in England by carbonaria form and thought that this was due to genetic variance in the population.
Concept of punctuated equilibrium: The concept was given by two American palaeontologists, S.J. Gould and N. Eldredge (1972), who thought that fossil record does not support Darwin’s concept of Gradualism, which suggests that species evolve by gradual changes over millions of years. They proposed that species evolved by sudden bursts of rapid change, punctuated by long periods of static equilibrium. Fossil records indicate that species have evolved rapidly within a few thousand years and once evolved they remained unchanged for millions of years. Origin of birds from reptiles and evolution of whales, ungulates, carnivores, Chelonia, grasses and flowering plants appears to have taken place suddenly.
Significance of isolating mechanism: Importance of isolating mechanism was first proposed by M. Wagner. Later Ernst Mayr (1971) and G.L. Stebbins (1976) recognised that isolation was the chief component in the production of new species. Isolation is any mechanism which prevents interbreeding between two populations. The concept has been dealt with in detail in a separate chapter.
Quantum evolution theory: The principle of quantum evolution was formulated by G.G. Simpson (1944) and Grant (1971). Ayala (1975) called it Saltation and Goldschimdt (1948) Macrogenesis. They proposed that macromutations give rise to entirely new kinds of organisms, sometimes monsters, which survive and perhaps move to new environment and give rise to new species or groups. However, Mayr (1970) did no support the idea.
Adaptive Radiation: H.F. Osborn (1902) coined the term Adaptive Radiation to explain emergence of several species from a single ancestor. G.G. Simpson (1953) said that adaptive radiation is rapid proliferation of new species from of single ancestor by occupation of varied ecological niches. Savage (1969) and Volpe (1985) thought that adaptive radiation was produced by macroevolution caused by major genetic changes. The phenomenon was first pointed out by Charles Darwin in finches after his visit to Galapagos Islands. Later, it was analysed by David Lack (1969). Several distinct species emerged from seemingly single population that was blown from South American coast to the Galapagos Islands.
Law of irreversibility: Also known as Dollo’s Rule, it was first proposed by the Belgian palaeontologist, Dollo (1895), who proposed that body structures once lost during the course of evolution could not be regained even if needed by the animal later. Terrestrial animals that returned back to water and became fish-like in appearance could not grow gills again for respiration. For example, extinct reptiles Ichthyosaurus and Plesiosaurus, dolphins and whales, although completely fish-like could not grow gills for respiration but continued breathing with lungs.
Principle of preadaptation: George Mivart (1871) believed that certain preadaptations are necessary for the animal to venture into new environment. Although these preadaptations may be non-adaptive in the existing environmental conditions, they are maintained in the population if not disadvantageous and become useful when the animal moves to a new environment. Evolution of horse is a good example of preadaptations in dentition and limb bones developing in Eocene and Oligocene horses although they were still living in forests.
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