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History of life on earth
• 4.6 bya—when it all began (Precambrian)
• Earth’s atmosphere changed over time
• First organisms were likely prokaryotes
(3.4 by old fossils)
• Photosynthetic organisms probably
evolved next
How did life first arise?
• Non-living molecules may have developed
the ability to self-replicate
• 1.9 bya eukaryotes evolved, perhaps
through symbiotic relationships with
prokaryotes
• 1.4 bya (still in Precambrian) multicellularity evolved—unicellular organisms
may have become colonial and then
individuals within a colony specialized on
certain tasks
Paleozoic (570-245 mya)
• Cambrian explosion (570-505 mya)—huge
numbers of new species evolved
• Ancestors of modern animal groups
evolved
Present theories as to why Cambrian
Explosion took place (Dr. Raff)
• Groups of animal genes evolved to the
point that they could direct the
development of complex organisms
• During the Cambrian, the waters' oxygen
levels increased.
• Biological arms race—more species =
more pressure to diversify.
Paleozoic Era (570-245 mya)
• By 400 mya, land plant and animals began
to evolve
• First insects and seed plants evolve
Mesozoic Era (245-66 mya)
• Triassic (245-208 mya)—first dinosaurs
and first true mammals
Mesozoic Era (245-66 mya)
• Cretaceous (144-66 mya) first flowering
plants evolved
• Dinosaurs and other groups went extinct
Cenozoic Era (66 mya-present)
• Mammals radiated
Cenozoic Era (66 mya-present)
• Pleistocene Epoch (2 mya-10,000 ago)—
humans evolved as well as many species
that exist today
• Holocene Epoch (10,000 years agopresent)
Overall increase in species
richness over time with five
massive extinction events
Today we are in the midst of a six
massive extinction event
Climate change linked to most extinction
events--cooling that caused the extinction of
the dinosaurs may have been due to
• Asteroid hitting earth
• Plate tectonics
• Volcanic activity
Massive extinctions may pave
the way for new species
Continental drift has resulted from
plate tectonics
• First proposed by Alfred Wegener—
German meteorologist and astronomer—in
1912
• Continents ride on plates that float on the
mantle of the earth.
Wegener’s evidence
• Fossil and coal beds indicating Europe
and North America previously had tropical
climates
Wegener’s evidence
• Shapes of continents on either side of
Atlantic
Wegener’s evidence
• Parallel stratigraphy and fossils on both
sides of Atlantic
Wegener’s evidence
• Glacial deposits in subtropical Africa and
South America
Wegener’s evidence
• Marsupials present in both South America
and Australia (and previously in North
America and New Guinea)
Wegener’s conclusions
• Continental rocks are less dense than
those of ocean floor and float on mantle of
earth
Wegener’s conclusions
• Pangaea existed
Wegener’s conclusions
• Break up of Pangaea occurred at a rift
valley which became an ocean
Wegener’s conclusions
• Continental shapes, fossils and
stratigraphy not explainable with fixed
positions of continents
• Continents moved .3 to 36 m/year
• Radioactive heat in mantle responsible for
continental movement
Problems with Wegener’s ideas
• Continental movement rates were too fast
• Geographical and biogeographical
evidence was sparse
• Mechanisms of plate movement were not
plausible
By 1929 Wegener had suggested some
mechanisms of plate movement
• Centrifugal forces on Earth’s surface due
to its revolution around the sun
• Gravitational effects of Earth, moon, sun
• Convection of liquid rock beneath Earth’s
crust
• Wegener died 1930
Evidence accumulated in favor of
plate tectonics theory
• Ocean rocks are younger than continental
rocks
• High temperatures detected in rift valleys
Evidence accumulated in favor of
theory
• Heat and gravity lower in trenches
Evidence accumulated in favor of
theory
• Improved stratigraphic information from
Gondwanaland, Paleozoic-Mesozoic
boundary
• Glacial movements in Gondwanaland
made more sense if one assumed
continents had been in different positions
than found today
Evidence accumulated in favor of
theory
• Lystrosaurus fossils found in Antarctica,
Africa, and India
Evidence accumulated in favor of
theory
• Orientation of rock crystals on land
masses showed different orientations,
based on Earth’s magnetic fields when
rocks cooled
Evidence accumulated in favor of
theory
• Ocean floor shows magnetic reversals
Recognized forces responsible for
plate movements today
• Ridge push
• Mantle drag
• Slab pull
Current patterns at plate
boundaries
• Oceanic plates sink beneath less dense
continental plates (west coast of North
America)
Current patterns at plate
boundaries
• Mountains form when plates are of equal
densities (Himalayas at Indian and
Eurasian plate boundaries)
History of continents
• Gondwanaland formed 650 mya (southern
continent landmass)
• Laurasia formed 400 mya (northern
continent landmass)
At end of Paleozoic (245 mya)
• Pangaea formed, also Panthalassa
History of continents
• 180 mya continents started to move apart
History of continents
• Laurasia, during the Cretaceous, had vast
shallow seas
History of continents
• During the last 74 my, connections have
existed on and off between Europe and
North America and North America and
Asia
History of continents
• Gondwanaland split up during the
Cretaceous
• 160 mya Africa and South America split
from the rest of Gondwanaland
• 130 mya Madagascar and India split from
Antarctica-Australia
• 105 mya South America and Africa split
• 100 mya Australia and New Zealand split
from Antarctica
History of continents
• Drift from plate tectonics continues to take
place today