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HISTORY OF LIFE ON EARTH Physical conditions - last 4 billion years major changes: •oxygen in atmosphere •climate change •ocean currents •sea level changes •continent movement •volcanism •meteorites How do we know ages? Relative ages of rock layers (sedimentary): their relative positions their embedded fossils How do we know ages? Absolute ages: radioisotopes carbon 14 - half life of 5,700 y. less than 50,000 years potassium 40 - half life of 1.3 b.y. the most ancient events http://id- archserve.ucsb.edu/Anth3/Courseware/Chronology/09_Potassiu m_Argon_Dating.html AGE DIVISIONS Earth’s geological history is divided into eras (eg. Cenozoic) and periods (eg. Cretaceous). boundaries between units based on differences between their fossil biotas Cenozoic: 65 m.y.a. to present Mesozoic: ~250-65 m.y.a. Paleozoic: ~550-250 m.y.a. Precambrian: ~550 m.y.a. to 4.5 b.y.a. Crust movement Continental drift: Throughout Earth’s history the continents have shifted, sometimes separating, at other times colliding. Powered by radioactive decay within the Earth! Climate change (temperature) volcanism sea level changes ocean currents carbon dioxide levels ......dramatic effects on the evolution of life Sea level changes... up to 200 meters difference Meteorite collisions also have changed conditions on Earth. cause of abrupt mass extinction at the end of the Cretaceous period (65 m.y.a) -- iridium -- Yucatan crater (180 km diameter) Crater - in Yucutan Mexico Meterorite or asteroid 10 miles in diameter 70% of living species wiped out Debris blown into air caused dramatic cooling- nuclear winter Global firestorm from methane gas released from ocean The Fossil Record Importance: Much of what we know about the history of life on Earth comes from the study of fossils. The Fossil Record Reveals broad patterns in the evolution of life. Fossils show that many evolutionary changes are gradual. Incomplete but getting better... The Fossil Record The Fossil Record The Fossil Record Life in the Remote Past The fossil record for Precambrian times is fragmentary, but fossils from Australia show that many animals that evolved disappeared forever. Ediacara Hills Life in the Remote Past Diversity exploded during the Cambrian period. The Cambrian Explosion. Why? All modern phyla present ... and more chinese fossil beds + Burgess Shales Life in the Remote Past Hallucigenia A phylum body plan that disappeared? Rates of Evolutionary Change Gradualism Punctuated equilibrium None? Coelacanth ... more later – molecular clock Rates of Evolutionary Change Mass extinctions (5 so far) 99% spp. have become extinct, some during mass extinctions major changes in flora and fauna subsequent radiations after extinctions eg. Cenozoic radiations of modern mammals Patterns of Evolutionary Change Cambrian - all major Animal body plans (phyla) have evolved! The Future of Evolution The forces of evolution continue to operate today eg. in humans: West Nile Virus, sexually transmitted diseases.... The Future of Evolution Human intervention now plays an unprecedented role in the history of life. Biodiversity crisis - the sixth great extinction
"HISTORY OF LIFE ON EARTH"