Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET 2020
After about how many years of the formation of the earth, life appeared on this planet?
1. 500 billion years
2. 50 million years
3. 500 million years
4. 50 billion years
Answer:3. 500 million years
ORIGIN OF LIFE
👉The universe is very
old – almost 20 billion years old.
👉The Big Bang theory attempts
to explain to us the origin of universe.
👉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. Water vapour, methane, carbondioxide
and ammonia released from molten mass covered the surface.
👉Life appeared 500
million years after the formation of earth, i.e., almost four billion years back.
👉Early Greek thinkers thought units of life called spores
were transferred to different planets including earth. ‘Panspermia’
👉This was also the theory of spontaneous generation.
👉Spontaneous generation theory was dismissed by louis Pasture
👉Oparin of Russia and Haldane of England proposed that the first form
of life could have come from pre-existing non-living organic molecules
(e.g. RNA, protein, etc.) and that formation of life was preceded by chemical
evolution
👉The conditions on earth were – high temperature, volcanic
storms, reducing atmosphere containing CH4, NH3, etc.
👉In 1953, S.L. Miller,
an American scientist created similar conditions in a laboratory scale
(Figure 7.1). He created electric discharge in a closed flask containing
CH4, H2, NH3 and water vapour at 800°C. He observed formation of amino
acids.
👉
👉In similar experiments others observed, formation of sugars,
nitrogen bases, pigment and fats.
👉The first non-cellular forms of life could have originated
3 billion years back
👉The first cellular form of life did not possibly originate till about 2000
million years ago.
👉All life forms were in
water environment only.
