Evolutionary thoughts

Evolutionary Theories

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.

Ancient Greek theories

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.


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.

Premodern theories

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.