History of life on earth
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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?.

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History of life on earth

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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) 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)

  • 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 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


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


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