A Quick Look at the History of Life on Earth Part 2 The Phanerozoic Eon Including the Paleozoic, Mesozoic and Cenozoic Eras

Paleozoic Era Paleozoic = old life - bounded by two major events in life history - begins with the Cambrian explosion 543mya -ends with the Permian extinction over 90% of life disappeared 250mya

The Cambrian Explosion a seemingly sudden appearance of large numbers of multicellular organisms in the fossil record. - Rocks older than this appeared to have no fossils (because they are too small to see, and because the multicellular organisms didn’t have hard parts to preserve. Most types of multicellular organisms present today appeared at this time. Many other groups of organisms appeared and then went extinct during this period Successful forms tended to be large with shells or exoskeletons for support Occurred in 5 – 10 million years

Other Paleozoic Highlights Early Paleozoic dominated by marine invertebrates Appearance and evolution of fish during the middle period of the Paleozoic – first fish had no jaws and cartilage skeletons - Jaws developed - cartilage skeletons give way to bones.

Paleozoic Highlights continued Major change in vertebrates: tetrapods (animals with 4 limbs) appear in the mid-Paleozoic. - First as paired fins on fish, then later as legs. Amphibians and reptiles appeared. - Reptiles were not common until the next era, the Mesozoic.

Paleozoic Highlights the move to land Life moved from the sea to land during the middle of the Paleozoic Era. - First bacteria and fungi – make soil - followed by plants - then arthropods (insects, etc.) - finally vertebrates. Large areas of swamp vegetation from this period were converted into coal after being buried and compressed.

Paleozoic Highlights- the move to land Why now? - The oceans had been the only shield life had against UV light - UV light damages DNA and threatens life - by the mid Paleozoic, oxygen in the upper atmosphere formed ozone (O3) - ozone blocks out much of the sun’s UV light – making life on land possible

Permian Extinction-the Dramatic End of the Paleozoic Era Largest mass extinction in earth history - 250mya – 90% of species disappeared Multiple ideas exist about the cause - massive volcanic eruptions in Siberia lasting 1 million yrs coincided with the extinction - climate change - evidence for both global warming and glaciation exists The mass extinction cleared the way for the rise of the dinosaurs during the Mesozoic era Evidence for massive volcanic eruption exists. Evidence to support global warming – animals with shells suffered most evidence for glaciation – glacial grooves etc in austrailia and other areas –pangea was found over south pole

Mesozoic Era 250mya – 65mya Mesozoic = middle life Age of the dinosaurs - reptiles were the largest animals on land and in the sea during this period. major diversification of flowering plants late in the Mesozoic. Supercontinent Pangea breaks into many continents allowed speciation to occur in many different habitats. Birds and mammals appear but aren’t prominent. Ends with a mass extinction – Cretaceous extinction

Cretaceous Extinction Thought to have resulted from a large asteroid striking the earth - drove huge amounts of dust to into the atmosphere and blocked out sunlight – reducing photosynthesis - reduced primary productivity - Evidence to support - worldwide layer of iridium found at correct age in rocks – very rare in earth rocks common in meteors, asteroids, comets etc…. - massive impact crater found off of the Yucatan penninsula of Mexico - 100 mi in diameter - correct age Iridium- very rare element in earth rock but much more common in

Cenozoic Era- 65mya - present Cenozoic = New Life Mammals become the dominant life form on land. An adaptive radiation that took advantage of the sudden loss of dinosaurs. Another large group evolves: the grasses. Adaptive radiation of birds and flowering plants. Fish, amphibians and reptiles don’t change much during the Cenozoic. Geologically, continents that had been separated started to collide: Africa with Europe, North America with South America, India with Asia Hominids appear 5 mya –modern humans (genus Homo)2 mya - Homo Sapiens approx 200,000 ya

Cenozoic Life

Summary Earth forms 4.6 billion years ago Solid surface forms 4 billion years ago Life starts (?) 3.8 billion years ago Age of Bacteria: Archaean era Oxygen atmosphere develops 2 billion years ago. Eukaryotes develop. Proterozoic era Edicarian life: 650 million years ago. First multicellular life, forms unknown today Cambrian explosion: most current life forms appear 550 million years ago Paleozoic era: 550 – 250 million years ago. Marine invertebrates, fishes, amphibians, invasion of the land. Coal formation. Permian mass extinction: 250 million years ago. 95% of all life dies; end of Paleozoic Mesozoic: 250-65 million years ago. Age of the dinosaurs (reptiles). Mammals, birds, and flowering plants appear Cretaceous mass extinction: asteroid hits the Earth, killing much of life, including the dinosaurs. Cenozoic era: 65 million years ago till present. Mammals dominant