Human Evolution And The Fossil Record

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Human Evolution and the Fossil RecordScientists continue to debate the history of man. It is generally agreed upon by the scientific community, however, that humans evolved from lesser beings, and this essay will function to provide evidence to support this claim. Several points will be outlined, including the general physical changes that occurred between several key species on the phylogeny of man, and a discussion of dating methods used to pinpoint the age of the fossils. This essay will begin with a brief discussion of dating techniques. In the study of hominid evolution, two main methods of dating are used: carbon-14 and potassium-argon dating. Carbon-14 dating involves the decay of radioactive C-14, which has a half life of 5770 years. This makes this method useful for dating of recent fossils, with good accuracy, up to 50,000 years back. After 5770 years, half of the carbon-14 in a fossil decays to nitrogen-14. Since the ratio of carbon-12 to carbon-14 in a living organism remains the same as in the environment around them because the organism constantly eats and replenishes it, if it were to die, the ratio would change greatly after many years. It is the difference between this ratio now and the time is died that allows a date for it to be established. Potassium-argon dating, another dating method, is possible due to volcanic ash and rocks found near many fossil sites. Rocks and ash created in this manner contain potassium-40, but no argon. As time passes, the potassium-40 decays into argon-40. In the laboratory, the sample is reheated, and since argon-40 is a gas, it is released. The ratio of argon-40 released to potassium-40 still present allows for a date to be assigned to objects near the sample. However, due to potassium's high half-life (1.3 billion years), it is only useful as a dating technique for finds older than 500,000 years old. Also, it is only useful where volcanic activity existed. Both these methods have error margins, ranging from a few thousand years in carbon-14 dating to tens of thousands of years, or more, for potassium-argon dating. However, thanks to scientific breakthroughs, these two processes can be used with reasonable security in establishing a time for fossils. Our farthest believed ancestor is believed to be Australopithecus afarensis. This species, which lived between three and four million years ago, is believed to be the first real hominid because it is the oldest, and "most primitive of any definite hominid form thus far found."(Turnbaugh, 281) Evidence from fossilized footprints, as well as pelvic and leg bones which were similar to modern hominids, led scientists to believe that they could walk upright. Its teeth resembled more those of primates, due to their large size. Its skull capacity ranged from 350 to 500 cm3. This species, though it had some hominid characteristics, was still more like an ape. Its face portruded outwards near the mouth region, and it did not have a definable chin. Finally, their craniums had large, portruding ridges over either eye.Another important being in the human timeline is Australopithecus africanus. Many scientists believe that it is the next in the sequence leading to man, however, a few believe that it belongs to a lineage on its own. A. africanus fossils have been dated back to the time period between two and three million years ago. It had a greater body size than A. afarensis, and a skull volume ranging between 420 and 500 cm3. It averaged a little higher in height than the 3 + to 5 feet believed for A. afarensis. Its jaws also portruded out. The "keel" effect is very distinguishable on this species, as it is with many of the older hominid species - a slight peak on the top of the cranium. Ridges over the eyes were also prominent on this hominid. The next species believed to be in our line of descent is Homo habilis. This is the first being with the distinction of having Homo as its genus. This species, which is dated back to between 1.5 and 2.4 million years ago, had a face which portruded less than A. africanus and A. afarensis. Its teeth, though still larger than modern humans, were smaller than those of its ancestors. Finally, its fossil fragments displayed "an average increase in cranial size of 21 percent and 43 percent, respectively, over [A. africanus and A. afarensis],"(Turnbaugh, 288) with an average cranial capacity of 650 cm3. Skulls found of this hominid also feature a bulge of "Broca's area," an area essential for human speech. It was also taller than the previous hominids, averaging around 5 feet high. At about the same time as Homo habilis and some of the other Homo species, other hominid species belonging to the Australopithecus genus, are believed to have co-existed. These include A. robustus, A. boisei, A. aethiopicus, and A. robustus . Though similar to the Homo line in structure, their bones were thicker and more robust. These other hominids are believed to have developed on a different lineage than the Homo line, and all of these streams died out at around the time of Homo erectus, the next key hominid on the human lineage. Because they are believed to have evolved apart from Homo hominids, it is not important to cover these species in detail. Homo erectus lived between 300,000 and 1,800,000 years ago, and still had portruding jaws and a "keel" effect on the top of the cranium. It, like its predecessors, had no definable chin, and thick brow ridges. However, skull capacity in these hominids jumped from an average of 650 cm3 in H. habilis to an average of 900 cm3 in early specimens and 1100 cm3 in later specimens. The skeleton "is more robust than those of modern humans, implying greater strength."(Foley, n.pag.) Due to their larger brain sizes, they are believed to have posessed greater intelligence, and evidence of this has been found in their probably use of fire, as shown by traces of burnt bones in cave floors, and the finding of more sophisticated tools than H. habilis. They were shorter, on average, than Homo sapiens, and their craniums showed a Nuchal torus, or a ridge, across the back of the head. This species also had keeled craniums. Archaic Homo sapiens, which first appeared 500,000 years ago, are believed to be our most rescent relatives. By this time, the "keel" that existed on their skulls is non-existent, and the supraorbital torus (the brow ridge) has begun to recede. Cranial volume has been measured at an average of 1200 cm3, and their brain shape was probably most similar to our own. Fossil evidence shows a trend for their posterior teeth to have reduced in size, and the anterior teeth to have increased in size, from previous Homo species, while late archaic Homo sapiens finds show a general reduction in the size of both areas. The face and jaw areas also showed a reduction in size from previous species. It is at this point that Homo sapiens neanderthalensis enters the picture. Commonly known as Neanderthal Man, this species is believed by most scientists to have existed at the same time as late archaic Homo sapiens and early Homo sapiens sapiens, our own species. Many scientists theorize that either we killed them off, or interbred with them to produce modern humans. Their craneal volume is in fact higher than modern humans, at an average of 1450 cm3. Their bones were also thicker, which implies greater bulk in body. They also had larger nose cavities, a weak chin, and a portruding jaw area. "Neandertals would have been extraordinarily strong by modern standards, and their skeletons show that they endured brutally hard lives."(Foley, n.pag.) Neandertal skeletons have been dated to between 30,000 and 230,000 years ago. Finally, our own species is encountered. Scientists have dated the earliest Homo sapiens sapiens fossils back 120,000 years. Our species showed an increase in skull capacity up to an average of 1350 cm3. The supraorbital ridge is all but gone with modern humans, and other features seen in earlier Homos, such as the "keel" and the craneal ridges on the back are also gone. The cranium is more rounded, as opposed to the general "pentagon" shape seen in earlier hominids. Teeth size for modern humans shows a decrease in size from archaic Homo sapiens. Also, bone size shows a trend towards reduced robustness, with thinner bones and smaller jaws. From all the fossil evidence, a rough line can be drawn for human evolution, starting from A. afarensis and ending in H. sapiens sapiens. A clear progression of features, especially in the cranial region, can be seen. Features such as brain size are seen to have developed and increased from our earliest ancestors

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