What is Evolution?

The history and science of origin and evolution comprise two events, the beginning of life and expansion of life. Our earth originated about 4.5 billion years ago. The term evolution is derived from two Latin words and means the act of unfolding or unrolling. English philosopher Herbert Spencer first used it. Evolution is defined as the change in heritable traits of biological populations over successive generations. These traits are the manifestations of genes that are passed down from parent to offspring during reproduction.

Theories of Origin of Life

There are four major hypotheses of the evolution of life on our planet, according to science.

Special Creation

According to this theory, living organisms were formed by the supernatural power of a creator called God. Example: Human beings.

Abiogenesis

This theory prompts that life evolved from non - living things by spontaneous generation.

Panspermia

As per the theory of Panspermia, Life originated from alien spores that came on earth from space.

Chemical Evolution

This theory was first proposed by a Russian biochemist A.I. Oparin (1894-1980), and an English biologist J.B.S. Haldane (1892-1964). It is popularly known as the "Oparin - Haldane hypothesis." According to this, life originated from non - living matter (about 3500 million years ago) by chemical evolution.

Stanley Miller and Harold Urey's simulation experiment

• The most significant evidence in support of chemosynthesis of life was given by Urey and Miller.
• Miller set up the apparatus and recreated the probable conditions of primitive earth in the laboratory.
• The apparatus, as shown in the figure, is filled with water vapor, methane, ammonia, and hydrogen.
• The mixture in the flask was kept circulating at 80o C and passed through the condensers.
• The gaseous mixture in the flask was subjected to high electric discharge like that of lightning. Due to this, conditions similar to those that prevailed in the early atmosphere were duplicated in the flask.
• The experiment was run continuously for about a week.
• After analysis, the content showed amino acids like glycine and alanine essential constituents in living organisms.
• Thus, it was proved that biologically macromolecules could be formed under conditions present on the primitive earth.
• By stimulating the primal earth's ambiance and supplying it with enough energy, scientists synthesized synthetic amino acids, nitrogen bases, and simple sugars, which are the essential building blocks in the laboratory.

Organic Evolution

The science of slow and gradual changes by which today's complex forms of life are developed from pre-existing simple ones is called organic evolution.

A British Naturalist Charles Darwin put forth some evidence in paleontology, anatomy, embryology, morphology, etc.

Paleontology- The study of fossils and their importance is known as paleontology.

Fossils- Fossils are the dead residues or impressions through hard parts of the organisms which got decayed.

Fossils are more common in sedimentary rocks.

Paleontological Evidence in Favor of Organic Evolution

Geological time scale

Chronological order or history of evolution based on the research on fossils is termed geological time scale.

• The age of the earth is about 5000 million years.
• Geologists have divided the history of the earth into eras, periods, and epochs.
• The history of the earth is divided into seven eras.
• The distribution of fossils in different strata of rocks shows that the oldest stratum contains fossils of simple organisms. The strata formed later contains fossils of more and more complex forms. This indicated that the present complex form had been evolved from past simple forms of life.

Connecting links

Connecting links are those extinct organisms that have the characters of two different groups. Example: Archaeopteryx is a connection which is in reptiles and birds.

• This was a fossil bird of the Mesozoic era of about 180 million years old.
• The fossil was discovered by paleontologist Andreas Wagnar in 1861 in Bavaria, Germany.

Reptilian characters

Presence of teeth in jaws, scales, claws, and a long tail with caudal vertebrae and keel less sternum with incompletely divided ventricle in the heart.

Avian characters

Presence of feathers, wings, pneumatic bones, beak, etc.

Homologous Organs

Homologous organs are the ones that are similar in their origin and basic plan of development but differs in their function in science. Example: the forelimb of man, bat, horse, and birds.

• The above organs having similarities in their origin and basic structure plan, but their functions are different. Ex- forelimb of hand used for handling or grasping, forelimb of horse for running, and forelimb of bats and birds for flying.
• In each case, the forelimb consists of the upper arm having a single bone, humerus, the forearm containing two bones, radius, ulna, and the hands having carpels in the wrist, meta carpels in the palm, and phalanges in the digits.
• The internal structure of the forelimbs of vertebrates are similar; surprising uniformity is found in the arrangement of muscles, nerves, and blood vessels.
• All such examples suggest a common evolutionary origin of the animals and support in growth and evolution of organisms. The above explanation clarifies that dissimilarity developed in closely related groups due to adaptive radiation called homology or divergent evolution.

Vestigial Organs

Vestigial organs are the non-functional organs in organisms, but these were functional in the ancestors.

• These organs are small and functionless organs present in organisms that are homologous to fully formed and functional structures in related organisms. The presence of these organisms can be explained only by presuming that the organs become useless to possess due to alternate development. They, therefore, reduce with time.
• Human beings have 180 vestigial organs.

Embryological Evidence

• Sir Ernst Haeckel gave a thesis on recapitulation, also called biogenetic law. In science, this law states that an organism during its development (ontogeny) tends to repeat (recapitulate) the evolutionary history (phylogeny) of its ancestors. Thus, the phylogenetic relationship is well exhibited by embryology.
• The presence of gill slits is one of the salient characters of chordates. Gill slits remain in primitive chordates.
• The early embryos of the vertebrates like fish, salamander, tortoise, chick, rabbit, and human beings resemble one another so closely that it becomes difficult to distinguish them. This proves that land vertebrates evolved from the fishes.

Lamarckism

The main objective of Lamarckism is that acquired characters are not inherited because these are changes in the somatic cells and not in germ cells. Jean Baptist de Lamarck gave various examples in support of Lamarckism.

Example: Present day, limbless snakes with long slender bodies developed from the limbed ancestors due to continued disuse of limbs and stretching their bodies to suit their limbs.

Lamarckism was the first theory to be put forth, which was reasonable and convincing at that time. Lamarckism is a stressed adaptation to the environment as a primary product of evolution. 

Darwin's Thesis on Natural Selection

Darwinism is the theory of organic evolution on every species, not only humans.

• Species grow over the years by accumulating small variation, which provides the species with structural and functional superiority over other species in their survival and differential reproduction.
• Darwin observed that each species has a natural tendency to overproduce and establish itself on the earth.
• Individuals multiply in geometric ratio, but the resources are limited, which leads to a struggle for existence. Here are various kinds of struggle.
• The kind of struggle between the people of a similar type of species is called an intraspecific struggle.
• This kind of struggle that occurs between the members of different species is called an interspecific struggle.
• The struggle between the species and various environmental factors like heavy rainfall, famine, etc., is called an environmental struggle.

Variation and Inheritance

The morphological, physiological, behavioral, and structural differences amongst the individuals of the same species are called a variation. Some variations are caused due to external conditions such as temperature, food abundance, etc. such variations are called somatic variations. Heritable variations are the genetic variations that are passed on to the offspring of the parents.

Variations are produced due to the action of few major genes or due to mutations. Variations may be harmful, useful, or neutral. Darwin believed that continuous variation and useful variations provided the raw material for evolution. Darwin's concept variation is the only cause of evolution, but now it has been established that it is one of the causes of evolution.

Survival of the fittest

In science, according to Darwin, in the struggle for existence, only those species survive which possess the most useful variations and the best adapted to the changing environment. Nature eliminates the unfit.

Origin of new species (Speciation)

Charles Darwin described that; useful variations are inherited from generation to generation. After a certain period, these variations become prominent, and the organism appears so different from the original species that a new species is ultimately evolved. We can see this in humans; too earlier humans were in the ape form, they evolved to become humans.

Hugo de Vries Theory of Mutation

Dutch botanist Hugo de Vries proposed it in 1901; it states that new species are not formed by continuous variation but by the sudden appearance of variation named mutation. 

A mutation is defined as sudden, abrupt, and permanent changes that appear in genes. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biochemistry
• Masters of Science in Biochemistry
• Masters in Biotechnology
• Molecular Genetics

Related Concepts

1. RNA world
2. Human evolution