History of Life on Earth
Grade 10
Changes in the composition of the atmosphere
• When the earth formed, oxygen levels in the atmosphere were very low
• Fossil records show that the first living organisms i.e. bacteria (prokaryotes)
were anaerobic i.e. they did not need oxygen to survive
• Blue-green bacteria appeared and they used carbon dioxide in the atmosphere
and released oxygen i.e. they could produce their own food through
photosynthesis
• The levels of oxygen started to increase in the atmosphere and aerobic
organisms started to develop
• The increase in oxygen resulted in an increased variety of living organisms on
earth
Changes in climate e.g. the ice age
• An ice age is a long geological period of drastic decrease in temperature of the
earth’s surface and atmosphere
• Many species died out during the ice ages and some migrated to warmer
areas on earth.
• When the ice forms, the level of the ocean will decrease exposing more land
• Ice ages therefore affected life on earth due to the extinction and redistribution
of species
Geological events
• The theory of continental drift proposes that the earth’s continents moved and
are still moving slowly.
• About 250 million years ago all continents were joined to form one large
continent, called Pangaea
• Pangaea eventually broke up into two supercontinents i.e. Laurasia in the north
and Gondwanaland in the south
• These two supercontinents eventually broke up further into the continents that
we know today.
• The climate and habitats changed because of continental drift
• A large number of life forms became extinct.
• By means of biogeography (the study of the distribution of present day and
extinct living organisms) evidence has been found that the continents were
once joined. Closely related species occurring on different continents probably
shared a common ancestor e.g. flightless birds like the ostrich and emu.
Grade 10
Changes in the composition of the atmosphere
• When the earth formed, oxygen levels in the atmosphere were very low
• Fossil records show that the first living organisms i.e. bacteria (prokaryotes)
were anaerobic i.e. they did not need oxygen to survive
• Blue-green bacteria appeared and they used carbon dioxide in the atmosphere
and released oxygen i.e. they could produce their own food through
photosynthesis
• The levels of oxygen started to increase in the atmosphere and aerobic
organisms started to develop
• The increase in oxygen resulted in an increased variety of living organisms on
earth
Changes in climate e.g. the ice age
• An ice age is a long geological period of drastic decrease in temperature of the
earth’s surface and atmosphere
• Many species died out during the ice ages and some migrated to warmer
areas on earth.
• When the ice forms, the level of the ocean will decrease exposing more land
• Ice ages therefore affected life on earth due to the extinction and redistribution
of species
Geological events
• The theory of continental drift proposes that the earth’s continents moved and
are still moving slowly.
• About 250 million years ago all continents were joined to form one large
continent, called Pangaea
• Pangaea eventually broke up into two supercontinents i.e. Laurasia in the north
and Gondwanaland in the south
• These two supercontinents eventually broke up further into the continents that
we know today.
• The climate and habitats changed because of continental drift
• A large number of life forms became extinct.
• By means of biogeography (the study of the distribution of present day and
extinct living organisms) evidence has been found that the continents were
once joined. Closely related species occurring on different continents probably
shared a common ancestor e.g. flightless birds like the ostrich and emu.
