4,600,000,000

Earth formed

4,400,000,000

Gases discharged into the atmosphere from the molten surface

4,300,000,000

First continents formed. (Oldest known crystals found in Australia)

4,200,000,000

Early seas in places as water begins to condense out of the atmosphere

4,000,000,000

Earth's crust starts forming

3,900,000,000

Bacterial cells start to form (in water, protected from ultraviolet radiation)

3,300,000,000

Small traces of free oxygen found in the atmosphere and in sediments

3,000,000,000

Large numbers of bacteria present

2,600,000,000

Significant crust on the Earth's surface

2,500,000,000

Oxygen gas starts to accumulate in the atmosphere

2,100,000,000

Ultraviolet light causes ozone to be formed from oxygen and the Earth's surface becomes protected from harmful ultraviolet light

2,000,000,000

Oxygen in the atmosphere is abundant. Iron reacts with some of it to make iron oxide, and silicon changes to silicon dioxide, the quartz that forms sand

1,600,000,000

Under the protection of ozone there is a large diversification of aerobic life

600,000,000

Appearance of simple 'animals' (sponges and arthropods)

570,000,000

Marine animals appear (trilobites) and some simple plants and fungi

500,000,000

Algae and insects appear on land

440,000,000

First land plants

323,000,000

Large trees and swamps leading to the formation of coal (Carboniferous age)

290,000,000

Large amphibians and reptiles

250,000,000

Dinosaurs appear

245,000,000

Major extinction of much life (Triassic age)

208,000,000

Start of the Jurassic age

145,000,000

Start of the Cretaceous age (when chalk is formed)

100,000,000

Flowering plants appear. Primates appear

65,000,000

Another major extinction of much life including all dinosaurs except bird-like forms

4,000,000

First human ancestors

200,000

Evidence of modern humans

50,000

Date of earliest cave painting known

5,000

Humans are living in cities