Theories of Origin of Life and Evolution
Theories of Origin of Life
(i) In previous days, people believed that the universe and life on it was an act and result of God’s creation.
(ii) There was, however, no evidence to support these claims.
(iii) The early scientists and Greeks believed that life on Earth never began on Earth itself. (iv) They believed that it came from elsewhere in space and that they named this phenomenon ‘panspermia’.
(v) 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 discarded by Louis Pasteur.
(vi) Next came the ‘chemical evolution of life’ theory by two scientists – Oparin from Russia and Haldane from England.
(vii) 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.
(viii) They postulated that these organic materials came together under conditions of high temperature, reducing atmosphere (without oxygen) and gases released from volcanoes all of which were favourable to produce simple living forms.
(ix) So they called this mixture of organic materials ‘organic soup’.
Miller-Urey Experiment
Miller and Urey were two scientists who trusted Oparin and Haldane and wanted to prove their theory. So, they performed the famous ‘Miller- Urey experiment’.
(i) They replicated the environmental and atmospheric conditions that may have existed on the primitive Earth in their lab by taking gases like methane, ammonia, hydrogen and water vapour in a closed vessel.
(ii) 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.
(iii) With these results, the theory of gradual chemical evolution was accepted.
(iv) This experiment allowed the other scientists to piece the remaining pieces together.
It was believed that monomeric units that formed thanks to chemical evolution polymerized to make polymeric units which gave rise to the unicellular microorganisms and eventually gave rise to multicellular more complex species.
This is called biogenesis.

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Theories of Evolution
There were many theories of evolution but the most accepted ones are as follows:
Lamarck’s Theory of inheritance of Acquired Characters
(i) This theory of evolution was published in the book ‘Philosophie Zoologique’ by French scientist Jean Lamarck.
(ii) 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.
(iii) 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.
(iv) His theory also said that the organs or structures modified themselves to suit the new needs of the organism. His theory contained three postulates:

  1. New needs
  2. Acquisition of new characters, organs or structures
  3. Inheritance of acquired characters

Example: Originally, giraffes did not have long necks. But, the trees were very tall and it couldn’t feed on the minimal number of short trees. So, as an adaptive measure to be ready to eat leaves on the tall trees, the giraffe’s neck started growing longer. This was an adaptive mechanism thanks to new needs.
Lamarck’s theory of evolution was disproved by scientists like Weismann and Mendel by completing their own experiments which weren’t according to Lamarck’s postulates.

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Darwin’s Theory of Natural Selection
Charles Darwin alongside Wallace postulated this theory of evolution. Darwin believed that life thrives and multiplies by the principle of survival .
The theory of survival had the following postulates:

  1. Increased reproduction – He said that each organism or species tends to breed to let its species live and multiply. He said that populations grow geometrically.
  2. 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 constant. With both these limitations to the growing population of different species puts a tab on the population growth.
  3. 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’s ready to fight its own species or the opposite species or the environment itself lives.
  4. Variations – Thanks to this struggle for food and space, each organism or species tends to urge a plus over the opposite. 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.
  5. Survival of the fittest – Darwin’s theory of survival believes that those that can survive in all types of conditions are the ones that nature selects.


Evidences of Evolution
(i) Evolution is a biological theory according to which all organisms on Earth, i.e. plants, and animals have a common origin and their differences today are the result of modifications that occurred in successive generations.
(ii) There are evidences of evolution that can prove this.
(iii) Evolution claims that there has been a genetic variation that brings about the difference in the physical characteristics of different species.
Evidences of evolution can be broadly divided into 5 broad categories:

  1. Relationship between organisms
  2. Anatomy and morphology
  • Homologous Organs
  • Analogous Organs
  • Vestigial Organs
  • Atavism
  1. Genetics
  2. Paleontology (Study of fossils)
  3. Embryology
  • Relationship Between Organisms

The theory of evolution states that all organisms alive today have originated from a single ancestor. There exists many similarities to prove these common origins. These are:

  1. All organisms are made up of cells.
  2. The structure and functioning of individual organelles of cells remain the same in all organisms except with a few variations.
  3. Cells of the same kind and function form tissues in all organisms.
  4. All organisms grow, reproduce and multiply.
  5. All organisms are able to perform metabolism and generate energy to be able to live and thrive.
  • Anatomy and Morphology

Evolution also is proved by the similarities in the anatomy and morphology of different animals and plants due to the presence of the following:

  • Homologous organs

These are organs that are similar embryologically but serve different functions in different organisms. This phenomenon is known as homology.
(i) Homology in plants: Thorns of a Bougainvillea are homologous to tendrils in creepers such as a money plant.
(ii) Homology in animals: The structure of the heart in animals is an example of homology. Chambered heart is present in most animals; 2, 3 and 4 chambered-hearts are seen in higher vertebrates. The forelimbs are present in all animals but modified to perform different functions, namely, fins in aquatic animals, wings in birds, hands in humans, etc.

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b. Analogous organs
These organs perform the same function but develop from different structures in different organisms.
(i) Analogy in plants: In some plants leaves and in some their stems perform photosynthesis. Here, two differently originated structures perform the same function.
(ii) Analogy in animals: Wings of an insect, bird and a bat (mammal) originate from different structures but all perform the same function of enabling the organism to fly.

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c. Vestigial organs
(i) These organs are non-functional and rudimentary in nature. However, they were very functional in the ancestors of the organism.
(ii) This occurs due to the decreasing use of the organ which leads it to become small or non-functional in nature.
Examples of such a structure are the appendix and the wisdom teeth in human beings.
d. Atavism
(i) This can be shown as the reverse of vestigial organs where a structure or organ which has become extinct in the species has appeared in the future generations.
(ii) Example of such a trait is the appearance of a short tail in human babies.

  1. Genetics

(i) The genetic code is made up of nitrogen bases.
(ii) These base combinations are almost the same in all the organisms. Certain triplets of amino acid sequences also produce the same proteins in different organisms.
(iii) This genetic code called the ‘universal genetic code’remains identical to a large extent in most organisms proving the possibility of a common ancestry.

  1. Paleontology

(i) The study of fossils is known as palaeontology. Fossils are formed when certain remains of organisms or plants get embedded in the soil or water and are preserved for many hundreds of years.
(ii) They appear either as skeletal remains, footprints, moulds or intact structures as found in the snow.
(iii) By studying fossils, we are able to establish similarities between the organism in the present to its ancestor in the past.
(iv) There can be many similarities that prove the common origins between different closely related animals and the differences can be studied to establish how they differ now and why.
(v) Fossils are very important evidence to prove the theory of evolution and common ancestry.

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  1. Embryology

(i) It has been observed that the embryos of different organisms appear similar in early stages of embryo development. 
(ii) Some animals show structures present during embryonic life but these disappear in adult life or immediately before being born.
(iii) And it is said in the ‘Recapitulation Theory’ or ‘the Biogenetic Law’ by Ernst Haeckel that ‘Ontogeny repeats phylogeny’ which means that the embryo of an organism undergoes all the stages in the embryonic life as its evolutionary history.
(iv) For example, a reptile embryo undergoes transformations from a fish-like appearance to amphibian-like features and then to a reptile embryo.
(v) There are many theories that prove evolution is a fact and the truth of life on Earth and this evidence of evolution only proves them more strongly.

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Industrial Revolution and Adaptive Radiation
Dark Winged Moths
(i) A study of moths before industrialization in England showed that there have been more white-winged moths than dark-winged moths.
(ii) The gathering after industrialization showed that there have been more dark-winged moths than white-winged moths.
(iii) The tree trunks became dark, thanks to pollution.
(iv) So, white-winged moths might be easily spotted by predators. Dark wings gave survival advantage.
Hence, the number of dark-winged moths increased rapidly. This provided another proof that evolution can happen.

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Development of Resistance
(i) Doctors use various antibiotics to control infections.
(ii) Over a period of your time, bacteria develop resistance against an antibiotic. This also shows that evolution can happen.
(iii) This also shows that evolution isn’t an immediate process.
Adaptive Radiation
(i) The method of evolution of the many varieties from one sort of organism during a given geographic area is named adaptive radiation.
(ii) It happens within a short span of time.
Darwin’s observation on finches of the Galapagos Islands showed interesting things.
(i) Finches of that island show a good variety; in terms of sorts of beaks suited to different eating habits.
(ii) Darwin proposed that each one of the varieties evolved on the island itself. From the first seed-eating finches, many other forms (with altered beaks) arose.
This shows adaptive radiation.

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Another example of adaptive radiation is seen in Australian marsupials.
(i) Different types of marsupials evolved from one ancestral stock within the Australian island continent.
(ii) Placental mammals in Australia also exhibit adaptive radiation.
Examples of marsupial mammals: pouched mole, numbat, pouched mouse, spotted cuscus, flying opossum, thylacine cat.
Examples of placental mammals of Australia: Mole, ant-eater, mouse, lemur, flying phalanger, bobcat, wolf.

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The rate of appearance of the latest forms is connected to the life cycle or lifespan. Microbes can multiply rapidly to become many individuals within hours. So, a replacement species can emerge during a microbe within a brief time. But life cycles of a posh animal are often from a few months to several years. Evolution of a replacement species in such animals would take many years.


Hardy-Weinberg Principle
(i) It states that, “Allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences”.
(ii) These influences included factors such as choice of mate, mutation, selection, genetic drift, gene flow, and meiotic drive.
(iii) The gene pool remains constant.
(iv) The total genes and their alleles in a population constitute the gene pool.
(v) Sum total of all the allelic frequencies is 1.
Let us assume there are two alleles ‘A’ and ‘a’ of a gene.
Let us assume that ‘p’ represents the frequency of allele A and ‘q’ represents the frequency of allele a.
Frequency of diploid AA individuals in population =p2
Frequency of diploid aa individuals in population =q2
Frequency of diploid Aa individuals in population =2pq
Hence,p2+2pq+q2= 1
When frequency differs from expected values, the difference (direction) shows the degree of evolutionary change.
Thus, disturbance in Hardy-Weinberg Equilibrium would then be said to result in evolution.
Factors affecting Hardy-Weinberg Equilibrium

  1. Gene migration or gene flow
  2. Genetic Drift
  3. Mutation
  4. Genetic Recombination
  5. Natural Selection.
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(i) The process of origin of a new species is called speciation.
(ii) A species is a group of organisms in which most of the characters are similar and members of a species are able to breed among themselves.
(iii) Speciation can occur if two groups of the same species are somehow prevented from interbreeding for several generations.
(iv) This can take place because of geographical segregation or because of some genetic changes.
(v) Evolution of new species, because of geographical segregation is called genetic drift.


Origin and Evolution of Man
(i) Dryopithecus and Ramapithecus existed around 15 mya. Ramapithecus was similar to man, while Dryopithecus was more ape-like.
(ii) Man-like primates existed in eastern Africa around 3 to 4 mya.
(iii) Australopithecines mostly lived in East African grasslands around 2 mya.
(iv) Homo habilis is an example of Australopithecines. Scientists are still confused if Homo habilis should be kept under Australopithecines or among hominids. Its brain was of 650-800 cc capacity.
(v) Homo erectus existed about 1.5 mya. Its brain’s capacity was 900 cc.
(vi) Neanderthals lived near east and central Asia between 100,000 to 40,000 years back. The brain capacity of Neanderthal was 1400 cc.
(vii) Modern Homo sapiens arose in Africa between 75,000 to 10,000 years back.

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