WHAT IS ORIGIN OF LIFE ? | BIOLOGY
WHAT IS ORIGIN OF LIFE ? | BIOLOGY

The origin of life on Earth is a set of paradoxes. In order for life to have gotten started, there must have been a genetic molecule—something like DNA or RNA—capable of passing along blueprints for making proteins, the workhorse molecules of life.
These are important theories of life which have been proposed.

THEORY OF SPONTANEOUS GENERATION

1.Organism formed automatically from non-living matter.
2.Van Helmont (1652) stated that young mice could arise from wheat grains when these are put in the dark along with a moist shirt.

THEORY OF BIO-GENESIS

Theory of spontaneous generation (life) was not accepted by Francesco Redi (1668). Lazzaro Spallanzani (1765) and Louis Pasteur (1861).
Spontaneous generation of flies from rotting meat was disproved by F Redi.
Spallanzani stated that air carried microorganisms.
Pasteur is famous form “Germ Theory of Diseases”.
Theory of bio-genesis states that the living things can only arise from previously existing living things.

Read More: CLASSIFICATION OF PLANTS AND ANIMALS

MODERN THEORY (OPARIN’S HYPOTHESIS)

The most important condition for the origin of life is presence of water.There is no life on moon due to absence of water.
Hydrogen atoms were most numerous and most active in primitive atmosphere, may be a plant product.
It was hypothesized by Al Oparin. Oparin wrote “The Origin of Life” in 1936.
The first experiment support to Oparin-Haldene’s theory of origin of life came from Urey and Miller’s simulation experiment in 1853.

EVOLUTION

The term ‘Evolution’ means to reveal hidden potentialities.

EVIDENCES OF ORGANIC EVOLUTION

Homologous structures organs are similar in basic structure and origin but dissimilar in functions.
Wings of bat, cat’s paw, front foot of horse, human hands and wings of birds are homologous organs.
Analogous organs are the organs similar in shape and function but their origin,basic development plan are dissimilar.
Wings of insects, birds and bats are analogous organs.
Analogous organs are also called as homoplastic organs.
Vestigial organs are degenerate, non-functional organs which  were functional earlier.
Human body has been discribed to posses about 90 vestigial organs. Some of these are muscles of ear pinna, canine teeth and third molar teeth, body hairs, vermiform appendix (degenerated terminal part of Caecum ) nicitating membrane of eye, caudal vertebral (coccyx or tail bone), etc.

ORGANISM
CONNECTING LINK BETWEEN
Virus
(Euglena)
Living and non-living
Plants and animals
Proterospongia
(Protozoa)
Protozoa and Porifera
Peripatus
(Arthropoda)
Annelida and Arthropoda
Neopilina
(Mollusca)
Annelida and Mollusca
Balanoglossus
(Chordata)
Non-chordata and Chordata
Dipnoi (Lung fish)
Pisces and Amphibia
Archaeopteryx
(Aves)
Reptiles and Aves
Prototheria
(Mammalia)
Reptiles and Mammalia

Atavism or reversion is the sudden reappearance of some ancestral features.
Appearance of thick body hair, large canines, monstral face, short temporary tails, additional pairs of nipples, etc., are examples of atavism.

THEORIES OF EVOLUTION

Evolution is generally progressive and variation is most important requirement for it.
The ultimate source of organic variation is mutation (sudden change in genes ).

Read more: PLANT LIFE CYCLE | BIOLOGY

LAMARCKISM 

The french biologist Jean Baptiste de Lamarck (1744-1829) suggested a complete theory of evolution.
Lamarckian theory is also known as “Theory of ingeritance of acquired characters” or “Theory of use and disuse of organ”.
Lamarck’s theory was published in Philosophie Zoologique in 1809.
Lamarck arranged his theory in four postulates.

a.Internal forces tend to increase size of the body.
b.Formation of new organs is the result of the need.
c.Development of organs is based on the continuous use and disuse.
d.All changes acquired by the organism are transmitted to offspring by the process of inderitance.

Some examples of uses and disuses of organs are
a.Webbed feet of swimming birds.
b.Rudimentary eyes of cave dwellers.
c.Elongated limbless body of snake.
d.Long thin neck in giraffe.
e.Vestigial organs of living animals.

DARWINISM

Charles Darwin (1809-1882) and alfred Russel Wallace (1823-1913)  proposed,the “Theory of Natural Selection”. Darwin’s theory is based of the five principles.

a.Over production is shown by every organism.
b.Organism shows struggle for existence.
c.Struggle for existence leads to variations and their inheritance.
d.The variations in the organisms leads to the survival of the fittest.
e.Natural selection and species formation is due to variations.

One major criticism against Darwin’s theory was his failure to give an explanation for variations.

MUTATION THEORY 

Hugo de Vries proposed the theory of mutation.
Mutations are discontinuous variation.
Mutation are due to changes in chromosomes, genes and DNA.
Mutations are the changes which may or may not be inherited.

SYNTHETIC THEORY

Initial basis of synthetic theory was given by Dobzohansky (1937).
Modern synthetic theory of evolution was designated by Huzley in 1942.
According to it the basic factors are:
a.Gene Mutation
b.Changes in chromosome structure and number
c.Genetic recombination
d.Natural selection
e.Reproductive isolation

First three factors are responsible for genetic variability.

MODERN VIEWS OF EVOLUTION 

Hardy-Weinberg Equilibrium

Mutations introduce new genes into a species resulting a change in gene frequencies.
If certain conditions existed gene frequencies would remain constant.
Conditions necessary for gene frequencies would remain constant.
Conditions necessary for gene frequency to remain constant are:

a.Mating must be completely random.
b.Mutation must be very large.
c.The population must be very large.
d.All genes must have an equal chance of being passed to the next generations.

Constant gene frequencies over several generations indicate that natural selection and evolution are not taking place.
Changing gene frequencies would indicate that evolution is in progress.

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