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Chapter 9- From Tree Shrew to Ape.doc

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Ben Evans

Chapter 10: From Tree Shrew to Ape - during Permian and early Triassic periods, much of the world’s fauna was dominated by therapsids, a diverse group of reptiles - at end of Triassic, most therapsid groups disappeared, and dinosaurs radiated most lands - at end of Mesozoic, placental and marsupial mammals dominated - modern humans have many complex adaptations, like grasping hands, bipedal locomotion (walking upright on two legs), toolmaking, language, and cooperation Continental Drift and Climate Change - evolution produces adaptation, but what is adaptive in one environment may not be adaptive in another environment - if environment varied over time, then evolution would have to track a moving target instead of moving towards a fixed goal - the positions of the continents have changed relative to each other and to the poles - one factor that has caused the world to change a lot is the movement of continents, or continental drift - continents are not fixed, but are enormous, relatively light plates of rock that make up the continents slowly wander around the glove, floating on the denser rock that forms the floor of the deep ocean - about 200 mya, all the land that exists today was once a giant landmass called Pangaea - about 125 mya, Pangaea broke off into a northern half, called Laurasia (North America and Eurasia minus India), and a southern half, called Gondwanaland - Gondwanaland separated into Africa and India - India headed north and crashed into Eurasia - continental drift is important in the history of human lineage because: 1. oceans serve as barriers that isolate certain species from others, so the position of the continents plays an important role in the evolution of species 2. continental drift is one of the engines of climate change, and climate change has fundamentally influenced human evolution - size and orientation of continents have effects on climate o very large continents tend have severe weather The Methods of Paleontology - much of our knowledge of history of life comes from study of fossils, the mineralized bones of dead organisms - bones of dead organisms may be preserved long enough for the organic material in the bones to be replaced by minerals (mineralized) from surrounding rock o natural copies of bones are called fossils - there are several radiometric methods of estimating the age of fossils - radiometric methods provide one of the most important ways to date fossils 1. potassium-argon dating  used to date age of volcanic rocks found in association with fossil material  molten rock emerges from volcano at high temperatures  all of argon gas is boiled out of rock  any argon present in rock must be due to decay of potassium  ratio of potassium to argon can be used to date volcanic rock  argon-argon dating • allows more accurate dating of single rock crystals 2. carbon-14 dating (radiocarbon dating)  based on unstable isotope of carbon that living animals and plants incorporate into their cells 1  measure ratio of unstable carbon-14 to the stable isotope carbon-12  once animal dies, carbon-14 decays into carbon-12 at a constant rate 3. thermoluminescence dating  based on an effect of high-energy nuclear particles traveling through rock  particles come from decay of radioactive material in and around the rock and from cosmic rays that bombard the Earth from outer space  heating a rock relaxes bonds holding that atoms in the crystal lattice together  all trapped electrons are recaptured by their respective atoms, giving off light  possible to estimate number of trapped electrons in these flints of heating them in laboratory and measuring amount of light given off 4. electron-spin-resonance dating  used to determine age of apatite crystals, an inorganic component of tooth enamel  crystals are preserved in fossil teeth and are bombarded by a flow of high-energy particles that generate trapped electrons in the crystal lattice  scientists estimate number of trapped electrons by subjecting teeth to a variable magnetic field (using a technique called “electron spin resonance”) - methods based on isotopes that decay very slowly, such as potassium-40, work well for fossils from distant past - potassium-argon cannot be used to date samples less than about 500 000 years old - carbon-14 can be used only to date sites that are less than about 40 000 years old - absolute radiometric dating is supplemented by relative dating methods based on magnetic reversals and comparison with other fossil assemblages - radiometric dating methods are problematic for two reasons: 1. a particular site may not always contain material that is appropriate for radiometric dating 2. radiometric methods have relatively large margins for error - relative methods are used to supplement these absolute methods: 1. based on remarkable fact that, every once in a while, the Earth’s magnetic field reverse itself 2. make use of fact that sometimes the fossils of interest are found in association with fossils of other organisms that existed for only a limited period of time The Evolution of Early Primates - evolution of flowering plants created a new set of ecological niches o primates were among the animals that evolved to fill these niches - during first two-thirds of Mesozoic, forest of world were dominatd by gymnosperms - breakup of Pangaea during Cretaceous gave rise to flowering plants, called angiosperms - angiosperms depend on animals pollinating them - ancestors of modern primates were small-bodied nocturnal quadrupeds much like contemporary shrews - the plesiadapiforms give us information what earliest primates were like o from Paleocene epoch, 65-54 mya o varied from tiny, shrew-sized creatures to animals as big as a marmot o well-developed sense of smell o probably terrestrial 2 - Carpolestes simpsoni had an opposable big toe with a flat nail, but claws on its other digits o claws and hands helped it climb large-diameter tree trunks o had low-crowned molars, which are suited for eating fruit o possess some but not all of the traits that characterize modern primates  argued that forward-facing eyes that provide binocular stereoscopic vision, grasping hands and feet, and nails on the toes and fingers all evolved together to enhance visually directed predation on insects in branches of trees  discovery that grasping hand and feet evolved in a frugivorous plesidapiform species before eyes shifted forward presents problems in this hypothesis - discovery of C. simpsoni helps explain why natural selection favored the basic features
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