History of life on earth

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

History of life on earth