Evolution
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Evolutionary Biology:
It is the study of history of life forms i.e. the changes in flora and fauna that have occurred over millions of years on earth.
What is Evolution?
The word evolution means to unfold or unroll orto reveal hidden potentialities. Evolution simply
means an orderly change from one condition
to another.
Evolution term was introduced by Herbert
Spencer.
It can be best explained by Darwin's concept of
'Descent with modification'.
This is the story of origin of life and evolution
of life forms or biodiversity on planet earth in
the context of evolution of earth and against
the background of evolution of universe itself.
What is Universe?
The universe is a huge cluster of galaxies.
Galaxies contain stars and clouds of gas and
dust. Considering the size of universe, earth is
indeed a speck i.e. The universe is vast.
Stellar distances are measured in light
years. When we look at stars on a clear night
sky we are, in a way, looking back in time. What
we see today is an object whose emitted light
started its journey millions of year back and from
trillions of kilometers away and reaching our
eyes now. However, when we see objects in
our immediate surroundings we see them
instantly and hence in the present time.
Therefore, when we see stars we apparently
are peeping into the past.
The Big Bang theory attempts to explain to
us the origin of universe
Big Bang theory- Proposed by Abbe Lemaitre
According to it, the universe originated about 20 billion years ago due to a thermonuclear explosion of a dense
entity. This single huge explosion which is unimaginable in physical terms, is called as big bang.
The universe expanded and hence, the temperature came down.
The gaseous clouds which were formed by big bang condensed under gravitation and converted into many flat
discs like structures called nebula, made up of atoms and small particles. Solar nebula was one of them,
which formed our solar system.
The very hot central part of solar nebula became still hotter and converted into the sun.
Later on, due to condensation of atoms and dust particles moving around the sun other planets were formed.
In the solar system of the Milky Way galaxy, earth was supposed to have been formed about 4.5 billion years
back.
There was no atmosphere on early earth. It was formed later.
Theories for origin of life:
1. Theory of special creation -
The greatest supporter of this theory was Father Suarez. This is a mythology based theory.
This theory has three connotations-
(a) All living organisms (Species or types) that we see today were created as such.
(b) The diversity was always the same since creation and will be the same in future.
(c) The earth is about 4000 years old.
All these ideas were strongly challenged during the nineteenth century based on observations of Charles
Darwin, Wallace etc. They believed that life forms varied over the periods of time.
From fossils records and their dating, we can conclude that earth is very old, not thousands of years as was through earlier but bilions of years old
2. Cosmic panspermia theory -
Some scientists believe that life came from outer space.
Early Greek thinkers thought units of life called spores were transferred to different planets including earth.
'Panspermia' is still a favourite idea for some astronomers
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