Untitled Essay Research Paper BODYINTRODUCTION TO EVOLUTION — страница 5

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interpreted as an indication that life originatedin the sea and that the balance of salts in various body fluids did not change verymuch in evolution. The membranes found in the human embryo are essentiallysimilar to those in reptile and bird eggs. The human yolk sac remains small andfunctionless, and the exhibits have no development in the human embryo.Nevertheless, the presence of a yolk sac and allantois in the human embryo is oneof the strongest pieces of evidence documenting the evolutionary relationshipsamong the widely differing kinds of vertebrates. This suggests that mammals,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 dominant landanimals 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, the greater thelikelihood that the animal will maintain a constant Body Temperature independentof the environmental temperature. Birds and mammals, for example, produce andcontrol their own body heat through internal metabolic activities (a state known asendothermy, or warm-bloodedness), whereas today’s reptiles are thermally unstable(cold-blooded), regulating their body temperatures by behavioral activities (thephenomenon of ectothermy). Most scientists regard dinosaurs as lumbering,oversized, cold-blooded lizards, rather than large, lively, animals with fast metabolicrates; some

biologists, however–notably Robert T. Bakker of The Johns HopkinsUniversity–assert that a huge dinosaur could not possibly have warmed up everymorning on a sunny rock and must have relied on internal heat production. The reptilian dynasty collapsed before the close of the Mesozoic Era.Relatively few of the Mesozoic reptiles have survived to modern times; thoseremaining include the Crocodile,Lizard,snake, and turtle. The cause of the declineand death of the large array of reptiles is unknown, but their disappearance isusually attributed to some radical change in environmental conditions. Like the giant reptiles, most lineages of organisms have eventually becomeextinct, although some have not changed appreciably in millions of years. Theopossum, for example, has survived almost

unchanged since the late CretaceousPeriod (more than 65 million years ago), and the Horseshoe Crab, Limulus, is notvery different from fossils 500 million years old. We have no explanation for theunexpected stability of such organisms; perhaps they have achieved an almostperfect adjustment to a unchanging environment. Such stable forms, however, arenot at all dominant in the world today. The human species, one of the dominantmodern life forms, has evolved rapidly in a very short time. The Rise of Mammals The decline of the reptiles provided evolutionary opportunities for birds andmammals. Small and inconspicuous during the Mesozoic Era, mammals rose tounquestionable dominance during the Cenozoic Era (beginning 65 million yearsago). The mammals diversified into marine forms, such

as the whale, dolphin,seal, and walrus; fossorial (adapted to digging) forms living underground, such asthe mole; flying and gliding animals, such as the bat and flying squirrel; andcursorial animals (adapted for running), such as the horse. These variousmammalian groups are well adapted to their different modes of life, especially bytheir appendages, which developed from common ancestors to become specializedfor swimming, flight, and movement on land. Although there is little superficial resemblance among the arm of a person,the flipper of a whale, and the wing of a bat, a closer comparison of their skeletalelements shows that, bone for bone, they are structurally similar. Biologists regardsuch structural similarities, or homologies, as evidence of evolutionary relationships.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 call analogousfeatures, which perform similar functions but are structurally different. Forexample, the wing of a bird and the wing of a butterfly are analogous; both areused for flight, but they are entirely different structurally. Analogous structures donot indicate evolutionary relationships. Closely related fossils preserved in continuous successions of rock stratahave allowed evolutionists to trace in detail the evolution of many species as it hasoccurred over several million years. The ancestry of the horse can be tracedthrough thousands of fossil remains to a small