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