Evolution

This assignment attempts to explain what evolution is and how it has brought about changes in organisms over millions of years. Evolution is the change in the inherited traits of a population from generation to generation. These changes occur over extremely long periods of time. Mutations in the genes can produce new or altered traits, resulting in genetic variations between organisms. New traits can also occur from transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. Evolution occurs when these genetic variations crop up in a population either through non-random natural selection or through random genetic drift. Charles Robert Darwin was a British naturalist who constructed the modern evolutionary theory with his concept of the development of all forms of life through the slow-working process of natural selection. British scientist Charles Darwin embarked on a 5 year expedition on the English surveying ship, the HMS Beagle. The expedition took place between 1831 and 1836, near the end of which he explored the Galápagos Islands. He discovered there that each of the islands sustained its own form of tortoise, mocking bird and finch. The various forms were closely related but varied in structure and eating habits. These observations led him to write his masterpiece, On the Origin of the Species. Being a naturalist aboard the Beagle, Darwin had the opportunity to observe the various geological formations found on different continents and islands along the way, as well as a huge assortment of fossils and living organisms. In his geological observations, Darwin was impressed with the effect that natural forces had on shaping the earth’s surface. He noted that certain fossils of supposedly extinct species closely resembled living species in the same geographical area. These observations raised the question for Darwin, of possible links between different but similar species. At the heart of the evolutionary theory is the basic idea that life has existed for billions of years and has changed a great deal over time. Overwhelming evidence supports this fact. Scientists continue to argue about details of evolution, but the question of whether life has a long history or not was answered positively at least 2 centuries ago. The history of living things is documented through multiple lines of evidence that converge to tell the story of life through time. There are many evidential processes that support evolution like fossil evidence, biogeography, comparative anatomy, etc. Fossil records provide snapshots of the past and when assembled, illustrate a whole scenario of evolutionary change over the past few billion years. The picture may be smudged in places and may have bits missing, but fossil evidence clearly shows that the earth is old and has changed quite significantly over time. Fossils are the preserved remains of once-living organisms (refer to diagram1). Fossils are created when first, the organism becomes buried in sediment, then, the calcium in the bones or other hard tissue mineralizes, and, finally, the surrounding sediment eventually hardens to form rock. The process of fossilization probably occurs rarely. Usually, animal or plant remains will decay or be scavenged before the process can begin. In addition, many fossils arise in rocks that are inaccessible to scientists. When they are attainable, they are often destroyed by erosion and other natural processes before they can be collected. As a result, only a fraction of the species that have ever existed (estimated to be around 500 million) are known from fossils.

Nonetheless, the fossils that have been discovered are sufficient to provide detailed information on the course of evolution through time. Comparative embryology is a branch of biology dealing with the development of the animal embryo. The field of embryology involves the development of the fertilized egg and embryo and the growth of the fetus. Many animals including humans, sea urchins and frogs are quite similar in their early development. All begin with a single cell that divides into 2 cells, the first step in the process of cleavage (refer to diagram 2), in which cell division occurs so rapidly that the cells don’t have time to grow between divisions, resulting in smaller and smaller cells. Scientists have illustrated that in the earliest stages of development, the embryos of organisms that shared a recent common ancestor are very similar in appearance. As the embryos develop, they grow less alike. For example, the embryos of cats and dogs, both members of the mammal group Carnivora, are more similar in the early stages of development than just before birth. The same is true for human and ape embryos. Modern human beings, like gorillas, tarsiers, and chimpanzees, are primates. Sometime along the course of primate evolution, human development deviated from that of gorillas and other primates. Scientific evidence shows that the physical and behavioral traits shared by people evolved over a period of at least 6 million years. Physical and genetic similarities show that the modern human species, Homo sapiens sapiens, has a very close relationship to the apes, another group of primate species. Humans and the great apes (large apes) of Africa—chimpanzees and gorillas—share a common ancestor that lived sometime between 8 and 6 million years ago. The earliest humans evolved in Africa, and much of human evolution occurred there. The fossils of early humans who lived between 6 million and 2 million years ago come entirely from Africa. Humans have experienced major structural changes over the course of evolution. Diagram 3 depicts Australopithecus afarensis (center), the earliest of the three species; Homo erectus (left), a transitional species; and Homo sapiens sapiens (right), a modern human. Homo erectus and modern humans are much taller than Australopithecus afarensis and have flatter faces and much larger brains. Modern humans have a larger brain than Homo erectus and an almost flat face beneath the front of the braincase. Unlike their ape ancestors, early humans had anatomical adaptations for upright walking (refer to diagram 4). The early human species Australopithecus afarensis had a wide and short pelvis and femurs (upper leg bones) that inclined inward toward the knees. These adaptations provided side-to-side balance and a support for the hip muscles to hold the torso erect. Scientists have several ideas about why Australopithecus first split off from the apes, initiating the course of human evolution (Diagram 5). Practically all hypotheses suggest that environmental change was an important factor, especially in stimulating the evolution of bipedalism (walking on 2 legs).

References:Wikipedia, the free encyclopedia: Evolution :Futuyma, Douglas J. (2005). Evolution. Sunderland, Massachusetts: Sinauer Associates, Inc. (accessed on 10th August) http.wikipedia.search/evolution.com Encarta: Charles Darwin: Voyage of the Beagle :Allen, Garland E., and Bird, Randy. "Charles Darwin." Microsoft® Encarta® 2006. Redmond, WA: Microsoft Corporation, 2005. (accessed on 12th August) Encarta: Sidebars: “From ‘Voyage Of The Beagle’” :(Unknown)(Accessed on 10th August) Encarta: Fossil Evidence :Clark, Neil. "Fossil." Microsoft® Encarta® 2006. Redmond, WA: Microsoft Corporation, 2005. (Accessed on 11th August) The Evolution website: Fossil Evidence :Dr. George Johnson’s Backgrounders (accessed on 9th August) (unknown) Encarta: Embryology :Boell, Edgar J., and Roche, Jean. "Embryology." Microsoft® Encarta® 2006. Redmond, WA: Microsoft Corporation, 2005. (accessed on 12th August) Wikipedia, the free encyclopedia: Human Evolution :Dart RA (1925). "The Man-Ape of South Africa". (accessed on 10th August) http.wikipedia.human+evolution/search.com Encarta: Human Evolution :Potts, Richard B. "Human Evolution." Microsoft® Encarta® 2006. Redmond, WA: Microsoft Corporation, 2005. (Accessed on 11th August)

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Diagram 4 Diagram 2

Diagram 5