Theory Of Evolution Essay Research Paper Theory — страница 5

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areessentiallysimilar to those in reptile and bird eggs. The human yolk sac remainssmall andfunctionless, and the exhibits have no development in the human embryo.Nevertheless, the presence of a yolk sac and allantois in the humanembryo is oneof the strongest pieces of evidence documenting the evolutionaryrelationshipsamong the widely differing kinds of vertebrates. This suggests thatmammals,including humans, are descended from animals that reproduced by means ofexternally laid eggs that were rich in yolk.The reptiles, and in particular the dinosaurs, were the dominantlandanimals of the Earth for well over 100 million years. The Mesozoic Era,duringwhich the reptiles thrived, is often referred to as the Age of Reptiles.In terms of evolutionary success, the larger the animal,

thegreater thelikelihood that the animal will maintain a constant Body Temperatureindependentof the environmental temperature. Birds and mammals, for example,produce andcontrol their own body heat through internal metabolic activities (astate known asendothermy, or warm-bloodedness), whereas today’s reptiles are thermallyunstable(cold-blooded), regulating their body temperatures by behavioralactivities (thephenomenon of ectothermy). Most scientists regard dinosaurs aslumbering,oversized, cold-blooded lizards, rather than large, lively, animals withfast metabolicrates; some biologists, however–notably Robert T. Bakker of The JohnsHopkinsUniversity–assert that a huge dinosaur could not possibly have warmedup everymorning on a sunny rock and must have relied on internal

heatproduction.The reptilian dynasty collapsed before the close of the MesozoicEra.Relatively few of the Mesozoic reptiles have survived to modern times;thoseremaining include the Crocodile,Lizard,snake, and turtle. The cause ofthe declineand death of the large array of reptiles is unknown, but theirdisappearance isusually attributed to some radical change in environmental conditions.Like the giant reptiles, most lineages of organisms haveeventually becomeextinct, although some have not changed appreciably in millions ofyears. Theopossum, for example, has survived almost unchanged since the lateCretaceousPeriod (more than 65 million years ago), and the Horseshoe Crab,Limulus, is notvery different from fossils 500 million years old. We have noexplanation for theunexpected

stability of such organisms; perhaps they have achieved analmostperfect adjustment to a unchanging environment. Such stable forms,however, arenot at all dominant in the world today. The human species, one of thedominantmodern life forms, has evolved rapidly in a very short time. The Rise of MammalsThe decline of the reptiles provided evolutionary opportunitiesfor birds andmammals. Small and inconspicuous during the Mesozoic Era, mammals rosetounquestionable dominance during the Cenozoic Era (beginning 65 millionyearsago).The mammals diversified into marine forms, such as the whale,dolphin,seal, and walrus; fossorial (adapted to digging) forms livingunderground, such asthe mole; flying and gliding animals, such as the bat and flyingsquirrel; andcursorial animals (adapted for

running), such as the horse. Thesevariousmammalian groups are well adapted to their different modes of life,especially bytheir appendages, which developed from common ancestors to becomespecializedfor swimming, flight, and movement on land.Although there is little superficial resemblance among the arm ofa person,the flipper of a whale, and the wing of a bat, a closer comparison oftheir skeletalelements shows that, bone for bone, they are structurally similar. Biologists regardsuch structural similarities, or homologies, as evidence of evolutionaryrelationships.The homologous limb bones of all four-legged vertebrates, for example,areassumed to be derived from the limb bones of a common ancestor. Biologists arecareful to distinguish such homologous features from what they

callanalogousfeatures, which perform similar functions but are structurallydifferent. Forexample, the wing of a bird and the wing of a butterfly are analogous;both areused for flight, but they are entirely different structurally. Analogousstructures donot indicate evolutionary relationships.Closely related fossils preserved in continuous successions ofrock stratahave allowed evolutionists to trace in detail the evolution of manyspecies as it hasoccurred over several million years. The ancestry of the horse can betracedthrough thousands of fossil remains to a small terrier-sized animal withfour toes onthe front feet and three toes on the hind feet. This ancestor lived inthe EoceneEpoch, about 54 million years ago. From fossils in the higher layers ofstratifiedrock, the horse is