history of life on earth 4.6 bya—when it all began (precambrian) earth’s atmosphere changed over...
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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
• 1.4 bya (still in Precambrian) multi-cellularity 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)
• Cretaceous (144-66 mya) first flowering plants evolved
• Dinosaurs and other groups went extinct
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 ago-present)
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
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
• 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
• 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
• 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
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)
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