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CBSEScience (English Medium) Class 12
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Syllabus

Overview of Evolution

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Topics

CBSE: Class 12

Key Points: Origin of Life

  • The universe originated about 13.8 billion years ago by the Big Bang, and Earth was formed about 4.5 billion years ago in the Milky Way galaxy.
  • Early Earth had a reducing atmosphere with gases like methane, ammonia, hydrogen and water vapour; oceans formed as the Earth cooled.
  • Life appeared about 4 billion years ago, initially in water, after the formation of oceans and the ozone layer.
  • The theory of spontaneous generation was disproved by Louis Pasteur, who showed that life arises only from pre-existing life.
  • Oparin–Haldane theory proposed chemical evolution of life; Miller’s experiment (1953) proved that organic molecules like amino acids could form from inorganic substances under early Earth conditions.
CBSE: Class 12

Theory: Evolution of Life Forms

  1. The theory of special creation claimed that all life forms were created as they are, diversity remained unchanged, and Earth was only about 4000 years old; these ideas were challenged in the 19th century.
  2. Charles Darwin, based on observations from the voyage of H.M.S. Beagle, proposed that life forms have evolved gradually, many became extinct, and all organisms share common ancestors.
  3. Darwin proposed natural selection as the mechanism of evolution, where organisms with better reproductive fitness survive and produce more offspring, leading to evolution over long geological time.
CBSE: Class 12

Key Points: Evidences Supporting the Theory of Evolution

Evidence Type Basis of Evidence Key Observation Example(s) Conclusion
Palaeontological (Fossils) Study of fossils in sedimentary rocks Different fossils found in different geological layers Dinosaurs, extinct organisms Life forms existed at different times and evolved over time
Embryological Evidence Study of embryos Early embryos of vertebrates show similar features Gill slits in vertebrate embryos Indicates common ancestry
Comparative Anatomy (Homology) Similar structure, different functions Same basic structure adapted for different uses Forelimbs of human, whale, bat, cheetah Divergent evolution; common origin
Comparative Anatomy (Analogy) Different structure, same function Different organs evolved for same function Wings of bird & butterfly; potato & sweet potato Convergent evolution
Biochemical Evidence Similarity in genes and proteins Common molecular sequences in different organisms Similar proteins/genes Supports common ancestry
CBSE: Class 12

Definition: Saltation

Evolution occurring due to sudden, large mutations leading to the formation of new species in a single step is called saltation.

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