Theory Of Evolution Essay Research Paper Theory — страница 4

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dominated by arthropods calledtrilobites and largescorpionlike forms called eurypterids. Common in all Paleozoic periods(570-230million years ago) were the nautiloid ,which are related to the modernnautilus, andthe brachiopods, or lampshells. The odd graptolites,colonial animalswhosecarbonaceous remains resemble pencil marks, attained the peak of theirdevelopment in the Ordovician Period (500-430 million years ago) andthenabruptly declined. In the mid-1980s researchers found fossil animalburrows inrocks of the Ordovician Period; these trace fossils indicate thatterrestrialecosystems may have evolved sooner than was once thought. Many of the Paleozoic marine invertebrate groups eitherbecame extinct ordeclined sharply in numbers before the Mesozoic Era (230-65 millionyears

ago).During the Mesozoic, shelled ammonoids flourished in the seas, andinsects andreptiles were the predominant land animals. At the close of the Mesozoicthe once-successful marine ammonoids perished and the reptilian dynastycollapsed, givingway to birds and mammals. Insects have continued to thrive and havedifferentiatedinto a staggering number of species. During the course of evolution plant and animal groups haveinteracted toone another’s advantage. For example, as flowering plants have becomelessdependent on wind for pollination, a great variety of insects haveemerged asspecialists in transporting pollen. The colors and fragrances of flowershave evolvedas adaptations to attract insects. Birds, which feed on seeds, fruits,and buds, haveevolved rapidly in intimate association

with the flowering plants. Theemergence ofherbivorous mammals has coincided with the widespread distribution ofgrasses,and the herbivorous mammals in turn have contributed to the evolution ofcarnivorous mammals. Fish and AmphibiansDuring the Devonian Period (390-340 million years ago) the vastland areasof the Earth were largely populated by animal life, save for rarecreatures likescorpions and millipedes. The seas, however, were crowded with a varietyofinvertebrate animals. The fresh and salt waters also containedcartilaginous andbony Fish. From one of the many groups of fish inhabiting pools andswampsemerged the first land vertebrates, starting the vertebrates on theirconquest of allavailable terrestrial habitats. Among the numerous Devonian aquatic forms were the

Crossopterygii,lobe-finned fish that possessed the ability to gulp air when they roseto the surface.These ancient air- breathing fish represent the stock from which thefirst landvertebrates, the amphibians, were derived. Scientists continue tospeculate aboutwhat led to venture onto land. The crossopterygians that migrated ontoland wereonly crudely adapted for terrestrial existence, but because they did notencountercompetitors, they survived.Lobe-finned fish did, however, possess certain characteristicsthat servedthem well in their new environment, including primitive lungs andinternal nostrils,both of which are essential for breathing out of the water.Such characteristics, called preadaptations, did not develop because theothers werepreparing to migrate to the land; they were

already present by accidentand becameselected traits only when they imparted an advantage to the fish onland.The early land-dwelling amphibians were slim-bodied with fishliketails, butthey had limbs capable of locomotion on land. These limbs probablydevelopedfrom the lateral fins, which contained fleshy lobes that in turncontained bonyelements.The ancient amphibians never became completely adapted forexistence onland, however. They spent much of their lives in the water, and theirmoderndescendants, the salamanders, newts, frogs, and toads–still must returnto water todeposit their eggs. The elimination of a water-dwelling stage, which wasachievedby the reptiles, represented a major evolutionary advance. The Reptilian AgePerhaps the most important factor contributing to the

becoming ofreptilesfrom the amphibians was the development of a shell- covered egg thatcould be laidon land. This development enabled the reptiles to spread throughout theEarth’slandmasses in one of the most spectacular adaptive radiations inbiological history.Like the eggs of birds, which developed later, reptile eggscontain acomplex series of membranes that protect and nourish the embryo and helpitbreathe. The space between the embryo and the amnion is filled with anamnioticfluid that resembles seawater; a similar fluid is found in the fetusesof mammals,including humans. This fact has been interpreted as an indication thatlife originatedin the sea and that the balance of salts in various body fluids did notchange verymuch in evolution. The membranes found in the human embryo