THE PATHS TRAVERSED BY LIFE:sslc biology notes

A. Origin of first life on primitive earth.

  1. Panspermia theory : Life has originated in some
    other planet in the universe and accidentally
    reached the earth. (Evidence : The organic
    substances obtained from the meteors)
  2. Theory of chemical evolution : Life originated
    as a result of the changes that occurred in the
    chemical substances in seawater, under specific
    conditions in primitive earth, where free oxygen
    was absent. (by A.I. Oparin and J.B.S Haldane).
    (Support: Urey-Miller experiment)
    Major events in the evolving of organic molecules:
  • Earth forms – formation of primitive gases in
    primitive atmosphere
  • condensation of water vapour to form rain
  • formation of primitive ocean
  • formation of simple organic molecules
  • formation of complex organic molecules
  • formation of primitive cell from nucleic acids and
    Major energy sources for chemical evolution:
    Lightning, UVradiations and volcanic eruptions.
    Experimental support by by Stanley Miller and
    Harold Urey :
    They re-created an experimental set
    up, in which the glass flask as the
    primitive atmosphere that contained
    methane, ammonia and water vapour.
    Instead of lightning or other energy
    sources, they passed high voltage
    electricity through the gaseous
    mixture.They condensed this gaseous
    mixture to water, that was considered
    as the primitive ocean. Organic molecules like
    amino acids were found in this.
    B. Evolutionary stages after the origin of earth.
    4500 m.y back – origin of earth
    3800 m.y back – origin of primitive cell (life)
    3500 m.y back – origin of prokaryotes
    1500 m.y back – origin of eukaryotes
    1000 m.y back – origin of multicellular organisms
    C. Theories related to organic evolution.
  1. Inheritance of Acquired characters. ( Lamarckism )
    The characters developed during the life time of
    organisms (acquired characters) accumulate
    through generations and lead to the formation of
    new species.
    According to Lamarck, when giraffes with short
    neck faced food scarcity, they stretched their necks
    to reach out to tall trees. Thus giraffes with long
    necks emerged through generations .
    Scientists critisized lamarckism as acquired
    characters were not inheritable.
  2. Theory of Natural Selection (by Charles Darwin)
    When over production of organisms occur, they
    compete for food, space, mate, and other limited
    resources (Struggle for Existence). In this struggle,
    only organisms with favourable variations survive
    in that nature. Over a long period,the favourable
    variations accumulate, resulting the formation of
    new species. (Natural selection).
    Observations in Galapagos Islands and the
    population theory of Robert Malthus
    were influenced Darwin.
    The finches in Galapagos
    had beaks adapted to their feeding
    habits. Only beaks with favourable
    variations (or adaptations) to that
    nature might have survived there.
    Ship: HMS Beagle
    Book published: The Origin of Species by means of
    Natural Selection.
    Limitation in Darwin’s theory ?
    Darwin could not explain the reasons for variations
    in organisms. Later, Hugo deVries explained one
    reason as mutation.
  3. Neodarwinism : is the modified version of
    Darwin’s theory in the light of new information
    from the branches of genetics, cytology, geology
    and paleontology about the reasons of variations
    occurred in organisms.
    (eg:- Mutation theory of Hugo deVries) which states
    that sudden and heritable changes (mutation) lead
    to evolution.
    D. Evidences to organic evolution.
  • Palaeontology (fossil study),
  • Comparative morphology,
  • Biochemistry and Physiology,
  • Modern molecular biology.
    1.Primitive fossils have simple structure.
    Recently formed fossils have complex structure.
    (Complex structured organisms are evolved from
    primitive simple organisms.)
    Certain linking fossils reveal the evolution of one
    form of organisms from another form.
  1. The evidences from the comparative morphological
    studies of homologous organs justify that all
    organisms were
    evolved from a
    common ancestor.
    [Homologous organs
    are organs that are similar in structure but perform
    different functions

Biochemistry and physiology justify that organisms
are made up of cells with similar protoplasm, cell
organelles and cellular activities. Enzymes control
chemical reactions and energy is stored in ATP
molecules in all organisms. Hereditary factors are
gene , seen in DNA and the structure of DNA is
alike in all. Carbohydrates, proteins and fats are the
basic substances. There are similarities in growth,
excretion etc.
[All organisms are evolved from common ancestor]

  1. Study of modern molecular biology (comparative
    study of same protein molecules of different
    organisms) reveals the evolutionary relationship
    among organisms as well as the period of
    separation of different group of organisms from
    their ancestor.
    For instance, we can analyse the similarities or
    differences in the sequence of amino acids in the
    beta chain of haemoglobin molecules of different
    mammals and there by we can understand about the
    evolutionary relationship (how close/how far)
    among them.

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