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 ( mya) Cambrian explosion ( 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 ( mya) By 400 mya, land plant and animals began to evolve First insects and seed plants evolve

Mesozoic Era ( mya) Triassic ( mya)—first dinosaurs and first true mammals

Mesozoic Era ( mya) Cretaceous ( 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