BIOLOGY NOTES FOR NEET-BIOLOGY NOTES FOR NEET PDF
ORIGIN OF LIFE
Big–Bang Theory : BIOLOGY NOTES FOR NEET
- According to it, the universe originated about 20 billion years ago due to thermonuclear explosion.
- This thermonuclear explosion is called Big-bang.
- Flat-disc like structure is called SOLAR-NEBULA formed.
- About 4.5 billion years ago, the origin of our solar system took.
- The very hot central part of this solar Nebula became still hotter & converted into the sun. Now, due to condensation of atoms & dust particles moving around the sun, formation of the other planets took place [ Mercury, venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).
- The solid part of our planet earth was called Lithosphere & the gaseous part was known as atmosphere.
- When the earth’s surface cooled down and its temperature decreased to 100°C, water formed on it.
Ancient Theories for origin of life:
1. Theory of special creation –
The greatest supporter of this theory was father Suarez According to Bible life and everything was created by god in 6 days.
first day : Earth and heaven
second day : Sky and water
third day : Land and plants
forth day : Sun, moon and stars
fifth day : Fishes and birds
sixth day : Land animals and first man Adam and from his 12th Rib first woman Eve.
According to hindu mythology the world was created by God Brahma. (The first man was Manu and the first woman was Shraddha)
According to it life has not changed ever since its origin. Special creation theory lacks scientific evidences so is not accepted.
Theory of Spontaneous Generation (Abiogenesis or Autogenesis) –
- This hypothesis was supported by ancient Greek philosophers like Thales, Anaximander, Xenophanes, Plato, Empedocles, Aristotle.
- According to this theory life was originated from nonliving things spontaneously.
- They believed that the mud of the Nile river could give rise to frogs, snakes, crocodiles.
- Abiogenesis was strongly supported by Von Helmont. He claimed formation of mice in 21 days. If a sweat soaked dirty shirt is kept in wheat barn.
3.Cosmozoic Theory –
Proposed by Richter.
Protoplasm reached on earth in the form of spores or other simple particles from some
unknown part of the universe with cosmic dust and they gave rise to various forms of life.
4. Cosmic panspermia theory –
Proposed by Arrhenius.
According to this theory organisms existed throughout the universe and their spores could freely travel through space from one star to the other.
5. Theory of Eternity of Life –
Helmholz believed that life is immortal.
6. Theory of Biogenesis –
Harvey (1651) and Huxley (1870)
New organisms can be originated on earth only by pre–existing life. This theory rejected the theory of Spontaneous generation but cannot explain origin of life. To prove Biogenesis and to disprove abiogenesis experiments were performed by –
Francesco Redi (Italian 1668) –
He took cooked meat in three jars, one was uncovered, the second was covered with parchment and the third was air tight. He observed that maggots developed only in the uncovered jar while maggots could not develop in the meat in closed jars. This proved that larvae were formed from eggs laid by the flies in open jars. Since the meat in closed jars could not be visited by flies so no larvae could develop.
Lazzaro Spallanzani (Italian 1767) –
He boiled vegetables and meat to prepare a sterlized nutritive soup and he kept some of it in air sealed flasks and some in loosely corked flasks. He observed that the soup in sealed flask remained sterile while micro organisms appeared in the soup in loosely corked flasks. Thus even micro organisms were formed from pre existing ones in the air rather then spontaneously.