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Human evolution has been a fine-tuned process over 7 million years from the first divergence of chimpanzees to the modern-day human. Homo Sapien the only surviving Homo, possess the ability to pass on knowledge and experience, which differentiates humans from species that came before. Early Hominins diverged first. Australopithecus second and lastly, the genus Homo. The main competing scientific models to which humans originated and dispersed are “Out of Africa” and “Multiregional Evolution”. The fossil record provides the best insight and evidence for human evolution.
The first step in human evolution was the divergence of the Early Hominies. This has been determined as fossils have been discovered which exhibit the beginning of human-like features such as walking bipedally. Sahelanthropus tchadensis partial cranial remains (Brunet et al. 2002)points to bipedalism as the position of the foramen magnum suggests that that it walked upright (Zollikofer et al. 2005) and not horizontally such as in apes. Supporting evidence from the fossil Ardipithecus ramidus, shows more ape like features in its hand and feet. However, the upper pelvis points to partial bipedalism (White et al. 2009). Although still significantly ape like, early hominins were the first transitional form from ape to human.
Around 4 mya the course of evolution gave rise to a new genus, the Australopithecus. “Lucy”, the most famous fossil of her kind, shows remnants of chimpanzee in her flexible ankle joints (Richmond and Strait 2000) and small brain size compared to humans (Cameron and Groves 2005). However, her femur faces inward toward the knee, unique to upright walking. The mixture of traits suggests that Australopithecus sent time in trees but also walking. As it will never be truly known if her ape-like characteristics is only a relic of adoral decent or still vital to her survival, (Cameron and Groves 2005) confirming her true place in human evolution challenging.
The Genus Homo
The Australopithecines were the precursor to the genus Homo which gave rise to the most successful of the species: Homo Sapiens. It is thought that the first Homo to exist is Homo Habilis (Leakey et al. 1964). The Australopithecines shared many traits with the Australopithecus, Wood and Collard (1999) argued that this fossil should not be classed in the genus Homo. However, was classed as Homo due to tool use (Blumenschine et al. 2003)and supposed ancestry to Homo eructs. Homo eructs, developed the ability to control fire (Steven 1989) increasing the nutritious value of food. Which may have led to the increased cranial capacity and intelligence in later homos.
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It is thought that Neanderthals and the modern human diverged from Homo Heidelbergensis, a descendent of H. erectus. Other models suggest H. erectus gave rise to H. antecessor, then diverged into H. Heidelbergensis and H.rhodesiensis. H. Heidlebergensis become H. Neanderthalensis and H. Rodolfensis become H. Sapiens (Rightmire 2008). However, given the limitations with the fossil record, it remains improbable that definitive relationships will be made across the species. New techniques in DNA sequencing and studying the dispersal of the hominids may give more inferences on the evolution of Humans.
Competing models of Human Dispersal in Human Evolution
The most substantiated model of human evolution is the Out of Africa (OofA) or Replacement theory. The model suggests that H. Sapiens had first evolved in Africa and then moved out to other continents. During their dispersal, they came across earlier hominids such as H. Neanderthalensis. However, no interaction took place between species until the other hominids went extinct (López et al. 2016). OofA is mostly supported by genetic evidence. Studies comparing Mitochondrial DNA of modern humans to African apes (Horai et al. 1995) estimated the divergence of last common ancestors and concluded Africa as the main source of genetic information. In addition, other genetic tools such as linkage disequilibrium to reconstruct the dispersal (McEvoy et al. 2011) show agreement to the origin of human DNA. There are debates as to which route was taken by the H. Sapiens when they left Africa via the north or the south. However, yet to be resolved.
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In contrast to the OofA theory, is the multi-regional theory. The theory agrees with OofA that the H. Sapien did originate within Africa first. However, it states that’s humans integrated with other homninds came before H. Sapien took over. Within the fossil record, the main evidence lies within Dial Mab in China (Wu XZ 1981). He was thought to show a transitional form as the similarities between early hominin fossils and modern Chinese men (López et al. 2016). However, the evidence does not hold reliable due to the poor quality of the bones. Making this supporting evidence for multi-regional theory redundant. Other DNA evidence proves interbreeding with other hominids such asH. Neanderthalensis. out with Africa (Yotova 2011). Human populations from other continents than Africa show 1%-4% shared DNA with H. Neanderthalensis. But populations in Africa show very minuscule amounts, if not shared DNA withH. Neanderthalensis (Gibbons 2010). Therefore, suggests that H. Sapiens did interbreed with H. Neanderthalensis once migrations began Out of Africa.
Based on the evidence for both theories, OofA hold the most scientific rigidity. However, yet undiscovered remains may change the current theories dramatically. Therefore, models must remain fluid , to allow new archaeological and DNA to add to the current theories.
Over the 7 million years, there is a clear pattern to how humans evolved. First the divergence of the early hominins where the first human traits appeared. Then Australopithecus and then Homo genus. Each contributing to the evolution of the modern-day human. The competing models of evolution: Out of Africa and the multi-regional give clues to how humans dispersed out of Africa. Nevertheless, the evidence shown in the fossil record and DNA studies is still inconclusive. There are still too many missing links and key questions to be resolved. Therefore, more convulsive evidence must be brought forward to make definitive statements about how modern humans evolved.
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