Lesson 1 of 13 · 7 min
Early earth and early ideas
NCERT §6.1
Before sunrise, on the bus out of Bhopal, Ms Fatima points at a bright star low in the sky. 'The light you are seeing left that star long before any of you were born,' she says. 'Today we look at the past all day.'
The story this chapter follows: The Bhopal field trip
The lesson in notes
In short
Evolutionary biology studies the history of life forms on earth. To follow it we need the backdrop: how the universe, the stars and the earth themselves came about.
Starlight left its source millions of years ago, so looking at stars is looking into the past. Distances between stars are measured in light years.
The universe is almost 13.8 billion years old. The Big Bang theory explains its origin as a single, enormous explosion; the universe then expanded and cooled, hydrogen and helium formed, and gases condensed under gravity into galaxies.
The earth, in the solar system of the Milky Way galaxy, is thought to have formed about 4.5 billion years ago. Early earth had no atmosphere.
Water vapour, methane, carbon dioxide and ammonia came out of the molten mass and covered the surface. UV rays split water into hydrogen and oxygen; the light H₂ escaped, and oxygen combined with ammonia and methane to give water, CO₂ and other compounds. The ozone layer formed.
As the earth cooled, water vapour fell as rain and filled the hollows to make oceans. Life appeared about 500 million years after the earth formed, that is, almost 4 billion years ago.
Panspermia: some scientists hold that life came from outer space. Early Greek thinkers imagined units of life called spores moving between planets, and some astronomers still like the idea.
Spontaneous generation held that life arises from decaying matter such as straw and mud. Louis Pasteur showed that life comes only from pre-existing life: in pre-sterilised flasks killed yeast gave rise to nothing, while a flask left open to air gave new organisms. The theory was dismissed for good.