Introduction to Evolution



Evolution is the law of life. It is the gradual change in the inherited traits of a biological population over subsequent generations. Evolutionary changes such as a change in physical appearance, mental skills, and emotional development occur as a result of the genetic variations passed through the genes. There are multiple factors involved in evolutionary science out of which, the genetic variations play a key role. It is said that all organisms have come from a single cell. But, the question here is why some organisms are still unicellular and some are multicellular with complex structure and functions? It’s again because of the depth of variation that a particular species undergone through generations. Multiple cell differentiations in the individuals due to the influence of genes combined with mutation lead to the evolutionary process. Among all the organisms, human beings are the most evolved and highly advanced species. In other words, we are one of the oldest organisms undergone a lot of variations witnessing many variations and differentiations.


Evolution as a discipline

Evolution is the branch of biology dealing with the study of the history of life forms on earth and how they have changed over a period. In order to understand the changes in the lives of plants and animals, one should be clear about the complete picture of the origin and evolution of the earth, sun, moon, and stars as the changes in these planetary bodies are directly or indirectly linked with the evolution of life forms.


Origin of life

It still a debate of how and when exactly the life forms had begun, however, some evidences have drawn the conclusion that this universe has begun forming 20 billion years ago. Earth is aged about 4-5 billion years. What we are witnessing today on the earth was entirely in its primitive form at once. Evolutionary changes have influenced on both living and non-living forms resulted in the modification of organisms to what we see today. For instance, the soil was formed out of rock-solid masses when the natural rain and sunlight hit against them. Some of the basic evolutionary theories are explained as under.


1. Big-bang theory

It was speculated that the whole universe including the earth, sun, moon, and stars were formed as a result of a single explosion of a huge solid mass. The big-bang theory postulated that a gigantic and unknown object has exploded and scattered into fragments. Each fragment has converted into small pieces. Each piece underwent a lot of changes to gain its shape, size, and structure. therefore, the earth is just a speck out of many planetary bodies. 


2. Formation of hydrogen and helium gases

Big-bang theory states that, after the surprise explosion, the universe has drastically expanded. Such an expansion has distributed the heat evenly across the planets resulting in the gradual dropping of core temperatures. Hydrogen and the Helium were the first gases formed. Gradually, the gases started expanding. Hydrogen and helium are lightweight gases, they react with the heat quickly making them float higher( ozone layer)  in the atmosphere. In the next phase, gases condensed slowly to form a cool earth sphere. Big-bang theory predicted that the explosion was the main reason behind the formation of the Milky-Way galaxy. The theory also estimated that the earth is 4.5 billion years old.


3. Formation of the atmosphere

When the earth was formed, there was no atmosphere. However, water vapours, methane, carbon dioxide and ammonia have released as a result of the molten mass explosion. Such elements released out of molten mass were not conducive for the survival of creatures, so millions of years later,  the evolution of atmosphere has brought in right conditions in which we all are living today.


4. Formation of the atmospheric gases


The ultraviolet rays from the sun broke H2O into Hydrogen and Oxygen. In the early days of earth formation, extreme volcanism has hit the earth. Volcanoes had led to the formation of a few gases known as the volcanic gases. The mechanism involved in the formation of gases is called degassing of molten mass where the combination of 2 or more gaseous elements led to the formation of a new gas. For instance, oxygen combined with ammonia and methane to form water and CO2. As the gases evolved into an advanced form, the ozone layer was formed. Again when the ozone layer cooled, the water vapour fell as rain. Heavy rains have filled the reservoirs such as ponds, oceans etc. The picture explains how the H2O releases H+OH and hydrogen escapes from hydrolysis. 


5. Formation of life or Abiogenesis

Primitive lives appeared over 4-5 billion years ago. There are still so many questions about how were first life forms formed?  and they have undergone differentiation into many species, colours, classes?  Early scientists believed that life on the earth came from outside. But, the evidence suggests that the earliest known life-forms are putative fossilized microorganisms. However, this evidence couldn’t be accepted by many modern theorists.


Oparin and Haldane concept

Oparin and Haldane have proposed that the early life forms were made up of  RNA, DNA and proteins. This was preceded by chemical evolution. The chemical evolution states that a diverse number of organic molecules have formed as a result of inorganic constituents combining with the sunlight and volcanic storms.


S.L. Miller`s laboratory experiment


Miller has created an artificial laboratory setting in the year 1953 to demonstrate the evidence of the formation of sugars, nitrogen bases, pigments, fats, and amino acids. He created a high-emission electric source in the form of a spark as shown in the picture. The electric discharge was produced instantly and it was enclosed within the flask containing CH4, H2, NH3 and water vapour at 8000C. During his experiments, he recognized the formation of sugars, nitrogen bases, pigments,  fats, and amino acids that ensemble the gases in the atmosphere. Similar experiments were conducted further from many scientists, they also observed the same phenomenon. Based on the series of experiments and observations, the first non-cellular forms of life are assumed to be originated around 3 billion years back when the atmospheric gases were freely available. 


A Brief About Human Evolution

Evolution is not an accidental phenomena that takes place overnight. Instead, it is a series of  gradual progressions, changes and modifications of  primates into the humans what we see today. The modern humans are otherwise called as Homo Sapiens, who can bear an upright-walking posture. The first humans were assumed to be evolved  in Africa about 315,000 human years ago. The following are the stages of human evolution:


1. Dryopithecus

These were assumed to be the ancestors of both men and apes lived in specific geographical areas of Africa, China, Europe, and India. The term Dryopithecus means oak wood apes inhabited in the tropical lowlands with dense forests therefore they were predominantly herbivores.


2. Ramapithecus

It was believed that they lived in the Shivalik range in Punjab and later immigrated towards Africa and Saudi Arabia. Some of the evidences such as, A distinct jaws, thickened tooth enamel,shorter canines, ability to use hands for food and defence have proved their existence.  Furthermore , they exhibited extrapolations of upright posture .


3. Australopithecus

The early  fossil  of Australopithecus  was first identified  in South Africa in the year 924. Australopithecus  genus  lived on the ground, used some natural stones as their weapons, and walked erect. One of the most notable feature is that they were as short as 4 feet and weighed 60-80 pounds.


4. Homo Erectus

The evidence for this genus was identified  in Java during 1891. They were characterized by their advanced and large cranial capacities . They were believed to have lived in communities. They were capable of using  tools made of quartz, bones, wood and many natural substances for their survival. .They have also proved the evidence of collective huntings.Homo erectus used fire for the first time, and they used  to dwell in caves.


5. Homo Sapiens Neanderthalensis

The Homo Erectus have advanced into Homo Sapiens. They have evolved into 2 sub-species , namely, Homo sapien Neanderthal and Homo sapiens sapiens. They had the cranial capacity of  1200 to 1600 cc. Their species of hominids were capable of hunting big games like mammoths.


6. Homo Sapiens Sapiens

The fossils of Homo Sapiens were first identified in the Europe. They had well-shaped jaws that are quite reduced, almost turned into the modern man’s chin .The skull was rounded, and they had the cranial capacity of 1350 cc. 


Further reading


 Evidences for evolution


Darwin`s theory of evolution


Accumulation of variation


mechanism of evolution


Convergent and divergent evolution



  1. What is evolution?

  2. What is the relationship between the genetic variations and evolution ?

  3. What is big-bang theory explain its significance in the formation of earth?

  4. Explain the Miller`s experiment 

  5. Explain the formation of the atmosphere







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