There are many views on how life originated on earth. The earth is said to have come into existence 5 billion years ago and life on earth came into existence only a billion years after that. There were many scientists who studied the fossils and performed experiments to postulate their versions of the theory of evolution.
Theories of Origin of Life
In olden days, people believed that the universe and life on it was an act and result of God’s creation. There was, however, no evidence to support these claims.The early scientists and Greeks believed that life on earth never began on earth itself. They believed that it came from somewhere else in outer space and they named this phenomenon ‘panspermia’.
Then came the theory of spontaneous generation of life during the times of Aristotle who believed that life was born from decaying and rotting inorganic matter such as hay, straw, and other non- living material. This theory was rubbished by Louis Pasteur.
Next came the ‘chemical evolution of life’ theory by two scientists- Oparin from Russia and Haldane from England. They believed that the generation of life on earth was a slow chemical process which occurred from pre-existing non-living materials such as amino acids, proteins and nuclear material such as RNA. They postulated that these organic materials came together under conditions of high temperature, reducing atmosphere(without oxygen) and gases released from volcanos all of which were favourable to produce simple living forms. So they called this mixture of organic materials as ‘organic soup’.
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Miller Urey Experiment
Miller and Urey were two scientists who believed Oparin and Haldane and wanted to prove their theory. So, they performed the famous ‘Miller- Urey experiment’ in the lab.
- They replicated the environmental and atmospheric conditions that may have existed in the primitive earth in their lab by taking gases like methane, ammonia, hydrogen and water vapour in a closed vessel.
- When they came back to check the results of their experiment after a week, they detected the presence of amino acids, nitrogen bases, fats, and sugar.
- With these results, the theory of gradual chemical evolution was accepted.
- This experiment gave the other scientists to piece the remaining pieces together.
It was believed that monomeric units that formed due to chemical evolution polymerized to form polymeric units and that gave rise to the unicellular microorganisms and eventually gave rise to multicellular more complex species. This is called biogenesis.
Similar to the above-discussed theories of the origin of life, there were many theories of the evolution of life.
Theory of Evolution
There were many theories of evolution that were postulated based on the evidence but the most accepted important ones are as follows:
Lamarck’s Theory of inheritance of Acquired Characters
This theory of evolution was published in the book ‘Philosphic Zoologique’ by French scientist Jean Lamarck. His theory was based on the study of fossils of the organisms of the past with their present relatives including the homologous, analogous and vestigial organs. He believed that an organ or structure got modified in the current animal from its ancestor due to the change in the environment and the utility of that particular organ. His theory also said that the organs or structures modified themselves to suit the new needs of the organism. His theory contained three postulates:
- New needs
- Acquisition of new characters, organs or structures
- Inheritance of acquired characters
Example: The giraffe originally did not have its long neck. But, the trees were very tall and it could not feed on the minimal number of short trees. So, as an adaptive measure to be able to eat leaves on the tall trees, the giraffe’s neck started growing longer. This was a way of adaptive mechanism due to new needs.
Lamarck’s theory of evolution was disproved by scientists like Weismann and Mendel by carrying out their own experiments which were not consistent with Lamarck’s postulates.
Darwin’s Theory of Natural Selection
Charles Darwin along with Wallace postulated this theory of evolution. Darwin believed that life thrives and multiplies by the principle of natural selection. The theory of natural selection had the following postulates:
- Increased reproduction- He said that each and every organism or species tend to reproduce to the maximum to let its species live and multiply. He said that populations of all organisms multiply geometrically.
- The arithmetic growth of food and availability of space- Darwin says that though the organism and their species multiply geometrically, food multiplies only arithmetically and the space available remains a constant. With both these restrictions to the growing population of different species puts a tab on the population growth.
- Struggle- with a rapidly multiplying population, there is an increased demand for both space and food. But, due to the restricted space and slowly growing food, there arises a competition between the members of a species and between species for the food and space. The organism or species that is able to fight its own species or the other species or the environment itself lives.
- Variations- Due to this struggle for food and space, each organism or species tend to get an advantage over the other. In order to do so, they tend to develop certain features or structures that give them that added advantage to succeed in the struggle of life. So, this struggle brings about variations.
- Survival of the fittest- Darwin’s theory of natural selection thus believes that those who can survive till the end are the ones that nature selects.
Solved Example for You
Q: Who postulated the theory of chemical evolution?
- Miller and Urey
- Oparin and Haldane
- Darwin and Wallace
Sol: The correct answer is (c) Oparin and Haldane
Oparin and Haldane believed that the life on earth came into existence from a pre-existing inorganic material such as mud, clay, hay etc. They believed a gradual conversion of these materials in the primitive environment of earth gave rise to small molecules which later on polymerized to give rise to complex molecules and organisms.