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which covers much of Scandinavia in Europe. More than a third of Aus-
tralia is Precambrian shield. Sizable shields exist in Africa, South America,
and Asia as well.
Many shields are fully exposed where flowing ice sheets eroded their
cover of sediment during the Pleistocene ice ages.The exposure of the Cana-
dian Shield from Manitoba to Ontario is attributed to uplifting of the crust
by a mantle plume and the erosion of sediments in the uplifted area. Some of
Figure 23 The
Precambrian shields
comprise the oldest rocks
on Earth.37ARCHEAN ALGAETABLE 3 EVOLUTION OF LIFE AND THE ATMOSPHEREEvolution Origin (million years) AtmosphereOrigin of Earth 4,600 Hydrogen, helium
Origin of life 3,800 Nitrogen, methane, carbon dioxide
Photosynthesis 2,300 Nitrogen, carbon dioxide, oxygen
Eukaryotic cells 1,400 Nitrogen, carbon dioxide, oxygen
Sexual reproduction 1,100 Nitrogen, oxygen, carbon dioxide
Metazoans 700 Nitrogen, oxygen
Land plants 400 Nitrogen, oxygen
Land animals 350 Nitrogen, oxygen
Mammals 200 Nitrogen, oxygen
Humans 2 Nitrogen, oxygen