A. Origin of first life on primitive earth.
- 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) - 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
lipids.
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.
- 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. - 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. - 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.
- 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]
- 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.
