The Origin and Evolution of the Earth

How the universe, the stars and the earth formed, and how the earth’s layers, air, oceans and life evolved.

Key ideas

The Big Bang: all matter was in a tiny, very dense ball that exploded about 13.7 billion years ago, and the universe has been expanding since.
Hubble showed in 1920 that galaxies are moving apart; the rival steady-state idea (Hoyle) is now less favoured.
Stars form in nebulae, huge clouds of mainly hydrogen gas; stars group into galaxies.
Planets formed in stages: cores with discs of gas and dust, small rounded bodies, planetesimals, and then a few large planets by accretion.
By differentiation, heavy iron sank to the centre and lighter material rose, giving the earth its layers.
The present atmosphere formed in three stages: the first hydrogen–helium air was lost to solar winds, degassing added gases and water vapour, and photosynthesis added oxygen.
Water vapour condensed and rain filled the oceans; life began about 3,800 million years ago.

Remember

Big Bang	about 13.7 billion years ago
The earth formed	about 4,600 million years ago (4.6 billion)
Life began	about 3,800 million years ago
Oxygen flooded the air	about 2,000 million years ago
Speed of light	300,000 km per second (18,000,000 km per minute)
Light year	a DISTANCE: about 9.461 × 10¹² km
Kelvin to Celsius	°C = K − 273
Nebular hypothesis	Kant, revised by Laplace (1796); revised again in 1950 by Otto Schmidt and Carl Weizsäcker

Watch out

Calling a light year a unit of time: it is the distance light travels in a year.
Adding 273 when going from kelvin to °C (subtract it).
Forgetting that the earliest atmosphere had almost no free oxygen.

Worked example

Suppose a space probe 200,100,000 km away sends a signal at the speed of light (300,000 km per second). (a) How many seconds does it take to arrive? (b) How many minutes and seconds is that? (c) Why is a light year a measure of distance, not of time?

Working: 200,100,000 ÷ 300,000; ÷ 60

Answer: (a) 667 seconds (b) 11 min 7 s (c) It is the distance light travels in a year, about 9.461 × 10¹² km.