Evolution Of The Human Race Biology Essay

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Evolution is an idea that envelops the theory that plants and animals had developed, the way they were by a process of gradual and continuous change. (Futuyma, 1997) This theory, also known as 'descent with modification', and effects all organisms (Futuyma, 1997).

1.2 Charles Darwin

Against many assumptions, evolutionary theory did not begin in the 18th century with Charles Darwin's "The Origin of Species" (Coyne, 2009). Similar ideas to evolution have existed since the times of the ancient Greeks, and had been in and out of favor in the periods between ancient Greece and the Victorian era (Coyne, 2009). By Charles Darwin's time the idea of evolution had been called "descent with modification" which was not especially controversial; several other evolutionary theories had also been proposed (Coyne, 2009).

1.3 Fossilization

In order for fossils to form, an animal or a plant must be trapped into a material. In many instances, fossils are found in the layers of sediment (Larsen, 1998). The layers of sediment will surround the remains; this can continue for thousands of years (Judkins, 2010). The actual remains that are found aren't the original organism at all (Larsen, 1998). Instead, the remains begin to decay within the sediment (Judkins, 2010). The minerals then replace the space left by decay (Larsen, 1998). The mineral hardens and makes an impression of the animal or plant. The remains are what are known as fossils (Judkins, 2010).

1.3 Evidence from paleontology

Paleontology aids in proving the 'Succession with modification" from fossils it is possible to find out how a particular group of organisms evolved by arranging its fossil records in a chronological sequence (Gros, 1979). Such sequences can be determined because fossils are mainly found in sedimentary rock, in different layers (Clark, 1973). Sedimentary rock is formed by layers of silt or mud on top of each other; thus, the resulting rock contains a series of horizontal layers, or strata therefore each layer contains fossils which are typical for a specific time period (Gros, 1979).

A succession of animals can also be seen from fossil records. Studying the number and complexity of fossils at different strata levels, it has been shown that older fossil-bearing rocks contain fewer types of fossilized organisms, and they all have a simpler structure, whereas younger rocks contain a greater variety of fossils, with more complex organisms.

In the past, geologists could only roughly estimate the ages of various strata and the fossils found (Gros, 1979). They did so, for instance, by estimating the time for the formation of sedimentary rock layer (Gros, 1979). More modern ways of finding the age of a fossil is done by measuring the proportions of radioactive and stable elements in a given strata level, the age of the fossil can be more precisely dated by scientists (Clark, 1973). This method is named as 'radiometric' dating (Baadsgaard, 1993).

1.4 Origins and evolution of humans

The recent African origin of modern humans is the most endorsed model representing the origin and early dispersal of modern humans and hominids (Tian, 2005). The concept was only a theory until the 80's; it was then corroborated by a study of mitochondrial DNA, combined with evidence based on physical structure of ancient specimens (Tian, 2005). According to genetic and fossil evidence, ancient H. sapiens evolved to anatomically modern humans entirely in Africa, between 200,000 and 100,000 years ago (Tian, 2005).

2.0 Evolution of hominid features

2.1 Brain Size

Cranial complexities had increased over evolutionary time, with the hominine brain increasing 3-fold over the last 3,000,000 years (Dehanene, 2009). Trends among humans show a strong correlation and trends in the brain size and general cognitive ability (Allen, 2009). It is okay to theorize that increasing brain size defines a higher intelligence (Dehanene, 2009). Intern larger craniums have higher associated costs, taking longer to develop and require greater energy to run. The current paper documents that in hominoids, as brain size increased from 300 to 1800 cm3 over several hominoid groups; it was accompanied by changes in seventy four muscular and skeletal traits (Allen, 2009). It is concluded that in the evolutionary competition to discover and fill the new niches, there was also a need for greater behavioral complexity and larger brain size, leading to domino-effects on other psychological and neurological traits (Dehanene, 2009).

2.2 Tooth Size

Parabolic ArcadeThere are strong trends in the size of human teeth, it can be seen that the diet heavily affects the dental attributes. The evolution of hominid teeth started with the Ardipithecus ramidus and Ardipithecus kadabba 5.8 to 5.2 MYA, are probably ancestral to the Australopithecus and subsequently the ancestors of humanity (Oxnard, 1987).  A. ramidus's similarity to chimpanzees includes the thinner tooth enamel and smaller molars of a fruit and vegetable eater, while its lower canines and upper premolars display hominid traits. Apes generally have large canines, while human canines are small (Oxnard, 1987). 

A. robustus as well as A. africanus both had teeth different to the modern apes. Apes have teeth with pointed cusps as adaptations to a diet of fruit and other soft vegetation (Ungar, 2010). The Australopiths have teeth flattened into grinding surfaces. This suggests a diet different to apes and consisting of tougher foods (Oxnard, 1987).

Overall in apes, the teeth are arranged in a rectangular shape, where the left and right cheek teeth are in two parallel lines (Ungar, 2010). Australopiths dental shapes tend to be more rectangular but have accents of parabolic shape, but in Humans the dental shape is a full parabola (Ungar, 2010). The trends reveal that there is also a strong trend in tooth size, such that the cheek teeth of modern humans are smaller than those of Australopithecus genus. Even in Humans there has been a decrease in tooth sizes over the last 30,000 years (Oxnard, 1987).

2.3 Forehead shape

There is a general trend towards a flatter facial angle with the appearance of more recent hominids, culminating in the vertical face of Homo sapiens (Lieberman, 2001). However there is considerable variation even among the older members of the lineage: Kenyanthropus platyops is named for its relatively flat face, its name means "flat-faced ape-man from Kenya" (Lieberman, 2001). Table 1 shows the different slopes of the homininae species, the greatest slopes are seen the chimpanzee and the gorilla, this possibly due to the size of their skull (it is much smaller than the hominids). The two species which have a flat forehead are the H. neanderthalis and H. sapien, a possible reason for a flat forehead is the large frontal lobe development in these two species. The older hominids have a more slanted forehead whereas newer hominid species have flatter foreheads.

2.4 Eyebrow Ridge

The brow ridge functions to reinforce the weaker bones of the face in much the same way that the chin of modern humans reinforces their comparatively thin mandibles (Endo, 1965). This was necessary in pongids and early hominids because of the tremendous strain put on the cranium by their tremendous chewing apparatus, which is best demonstrated by any of the members of the genus Paranthropus (Endo, 1970). The brow ridge was one of the last traits to be lost in the path to modern humans, and only disappeared with the development of the modern pronounced frontal lobe (Endo, 1965). This is one of the most salient differences between Homo sapiens and Homo neanderthalensis (Endo, 1970).

3.0 A comparison of the Chimpanzee and Human skull

The chimpanzee and human skulls are somewhat similar in their appearance, but when specificities are looked at there are many differences which are crucial to the animal's anatomy. The human skull consists of 28 bones and the chimpanzee has around 30 (Carlson, 1999). Several major differences between humans and chimpanzee skulls include: 

Sagittal crests

Muscles that attach to the cranial base help to position, move, and stabilize the skull of the chimpanzee (Carlson, 1999). The sagittal crest provides the skulls movement and stability to the back and shoulder (Carlson, 1999). It is marked by nuchal lines and markings that represent attachment surfaces (Carlson, 1999). The nuchal crests of the chimpanzee are more extreme, this is because the chimpanzee's small skull combined with a much more muscular structure produces stronger crests (Carlson, 1999).

Premaxilla

The chimpanzee's premaxilla is lengthened to support a larger and more forward projecting incisor set which helps in the chewing of food (Schultz, 2005). The premaxilla unites with the maxilla after birth (Schultz, 2005). In comparison the human premaxilla is a lot smaller due to the lack of large incisors (Schultz, 2005).

Chin

The human mandible is heavily reinforced by a bar of bone, since the size of the cranium is much larger which strengthens the symphysis (Carlson, 1999). The ape mandible lacks a mental protuberance and is reinforced by an inferior transverse torus. Viewed from above, the contrasting shape of the dental structure is very different (Carlson, 1999).

Cranial volume

The human skull has a 3 times greater endocranial volume which reflects a larger brain size, about 1200cc in human and about 400cc in chimpanzees (Cantalupo, 2009). The larger human skullcap creates for a larger surface area for the attachment of the M. temporalis (Cantalupo, 2009). Fibers from this muscle attach to the skull in the F. temporalis and pass behind the zygomatic arch to intercourse in the coronoid process and interior margin of the ascending ramus of jaw (Cantalupo, 2009). The temporal lines on the calvarium signify the terminal fibers of M. temporalis (Cantalupo, 2009). If cranial volume is small (in the case of the chimpanzee), there won't be enough surface area on the calvarium for fibers to join (Cantalupo, 2009). Where fibers from the opposite muscle meet, the sagittal crest is created. (Cantalupo, 2009)

Facial prognathism

Prognathism is a term used to describe the relationship of the mandible and maxilla to the skeleton base where either of the jaws protrudes or intrudes beyond a predetermined line (Kano, 1992). The projection of the face is greater in chimpanzees than humans (Kano, 1992). Flat faces are called orthognathic and projecting faces are called prognathic. (Kano, 1992) The craniofacial angle is used to quantify the extent to which the face projects beyond the neurocranium (Kano, 1992).

4.0 A. Boisei, H. Habilis, A. Lucy, A. Africanus and A. Robustus (odd one out)

Australopithecus africanus 

A. africanus existed between 3 and 2 MYA (Kreger, 2010). Brain size ranged between 420 and 500 cc (Kreger, 2010). This is a little larger than chimp brains, but still not advanced in the areas necessary for speech. The back teeth were a little bigger than in the A. afarensis (Kreger, 2010). Although the teeth and jaws of A. africanus were larger than those of H. sapiens, they were far more similar to human teeth in comparison apes (Kreger, 2010).

Australopithecus robustus 

A. robustus had a body similar to A. africanus, but a larger and more robust skull and teeth (Wood, 2004). It existed between 2 and 1.5 MYA (Wood, 2004). The massive face is flat or dished, with no forehead and large brow ridges. It had small front teeth, but it had large molar teeth in a larger lower jaw (Wood, 2004). The average brain size is about 530 cc. Bones excavated with robustus skeletons indicate that they may have been used as digging tools (Wood, 2004).

Australopithecus boisei 

A. boisei existed between 2.1 and 1.1 MYA (Tobias, 1967). It was similar to A. robustus, but its face and cheek, teeth were bigger (some molars being up to 2 cm across) (Tobias, 1967). The brain size was similar to A. robustus; with an average of about 530 cc (Tobias, 1967). A few experts consider boisei and robustus to be variants of the same species (Tobias, 1967).

Homo habilis 

H. habilis existed between 2.4 and 1.5 MYA (Tobias, 2009). It is similar to australopithecines in many ways; The face is still primitive, but it projects less than in A. africanus (Tobias, 2009). The back teeth are smaller, but still larger than in modern humans. The average brain size was larger than the australopithecine; at 650 cc (Tobias, 2009). The cranial shape is also more humanlike (Tobias, 2009). The bulge of Broca's area, which is needed for speech, is visible in one H. habilis brain cast, and indicates it was possible for it to speak (Tobias, 2009). H. habilis was thought to have been about 127 cm tall and weighed around 45 kilogram; females may have been smaller (Tobias, 2009).

Lucy (Australopithecus)

Lucy was found in Africa between 4 to 1 MYA (Johanson, 1982). Among them they shared many anatomical traits; they were fully bipedal (Johanson, 1982). The structural features were a small and apelike braincase, comparable in size to those of gorillas and other great apes, when compared to the overall body size (Johanson, 1982).

The comparison

The odd one out of these species is H. habilis, since it's the only descendant of the homo genus. The earliest known human was Homo habilis; it can be clearly seen in the timeline that the first remnants of this ancient ancestor date back to 2.5 MYA. H. habilis evolved earlier than A. boisei and A. Robustus which is odd because these two species are considered ancestors of the Homo genus, H. habilis's existence dates back earlier than these species. It can be seen that H. Habilis also dates back (to the same time or earlier) when compared to A. garhi. The existence of H. habilis disrupts the previously linear human evolution to a more networked based thought, the divergence between pongids and members of the Homo genus is not as clear as previously perceived. Habilis has been a controversial species. Originally, some scientists did not accept its existence as a separate species, believing that all habilis specimens should be assigned to either the australopithecines or H. erectus (Tobias, 2009). H. habilis is now fully accepted as a species, but it is widely thought that the H. habilis specimens have too wide a range of variation for a single species, and that some of the specimens should be placed in one or more other species (Tobias, 2009).

5.0 Evaluation

5.1 Australopithecus

From the evidence gathered by archeologists and paleontologists, it appears that the Australopithecus genus evolved in eastern Africa around 4 MYA before spreading throughout the continent and eventually becoming extinct 2 MYA.

The possible cause for this evolution to take place in Africa was seen in the critically statistical analysis presented by Martin Trauth, Juan Larrasoan and Manfred Mudelsee, in the journal "Trends, rhythms and events in Plio-Pleistocene African climate".

The journal was aimed at detecting trends, rhythms and events in representative dust flux records from the subtropical Atlantic, eastern Mediterranean Sea, and the Arabian Seas (Trauth, 2009). The journal sheds light on the significance of patterns of tropical African climate variability (Trauth, 2009). The results demonstrate that neither the proposed gradual increase in African aridity 3,000,000 years ago (Trauth, 2009). A comprehensive interpretation of the dust flux records in terms of Landscape variability in the Saharo-Arabia desert, together with their comparison with lake, pollen and isotopic records of African climate variability, indicate that the hydrological cycle in tropical Africa is mainly controlled by low-latitude heating via its impact on monsoon dynamics (Trauth, 2009). An examination of the fossil record indicates that the key clashes in hominin evolution reported nowadays at 2 (Trauth, 2009).6 MYA, 1.8 MYA and 1 MYA, which suggests that periods with enhanced speciation and extinction events coincided with periods of maximum climate variability on high moisture levels (Trauth, 2009).

5.2 Use of tools by the Australopithecus

Most species of Australopithecus were not able to use tools. Scientists have argued that A. garhi used stone tools due to the discovery of butchered animals and its remains in locations of A. garhi fossils (Asfaw, 1999). From Cranial capacity estimations of intelligence of a hominid species can be made, most species had cranial capacities between 800cc and 1830cc. On the other end of the spectrum the members of the Australopithecus had capacities between 350cc and 800cc although brain size does not necessarily entirely affect the intelligence of a creature it is a basis. The theory is that members of the Australopithecus species were not developed enough to develop and use tools that members of the Homo family used.

5.3 Knuckle walk

Chimpanzees and gorillas are the two main apes which use knuckle walking as a way of movement (Richmond, 2001). This form of hand-walking posture allows these tree climbers to use their hands while moving on the ground with retaining long fingers for climbing (Richmond, 2001). It may also allow small objects to be carried in the fingers while walking. Knuckle-walking tends to evolve when the fingers of the forelimb are specialized for tasks other than movement on the ground (Richmond, 2001). In the gorilla the fingers are used for the manipulation of food, and in chimpanzees for the manipulation of food as well as climbing (Richmond, 2001).

5.4 Nomadic society - Hunting and gathering.

A hunter/gatherer society is one whose primary method of subsistence involves the direct acquirement of edible plants and animals (Mayell, 2003). The Neolithic Revolution is the first agricultural revolution it was the transition from hunting and gathering to agriculture and settlement (Mayell, 2003). Archaeological data indicate that different forms of domestication of plants and animals arose independently in six separate locales worldwide, with the earliest known developments taking place in India the Middle East around 10,000 BC (Mayell, 2003). Until about 50,000-40,000 years ago the use of stone tools seemed to have been used primarily (Mayell, 2003). Each member of the homo genus seemed to have started at a higher level than the previous one, but once that phase started further development slowed down (Mayell, 2003). These Homo species were conservative, but after 50,000 BC modern human had started to change at a much greater speed. Jared Diamond, the author of 'The Third Chimpanzee', and some other experts characterize this as a great Leap Forward. Modern humans started burying their dead, making clothing, developing complex hunting techniques, and engaging in cave painting (Diamond, 1992). That was something that had not been seen in human cultures prior to 50,000 BC (Diamond, 1992).

A model proposed by Geoffrey Miller who argues that human intelligence is unnecessarily sophisticated for the needs of hunter gatherers, since they just needed to survive. He argues that the manifestations of intelligence such as language, music and art are of no value to the survival of older hominids (Miller, 2000). Rather, intelligence may have been an indicator of attractiveness (Miller, 2000). Hominids would have selected for intelligence as a pathway for healthy genes to be seen in the offspring, sexual selection would have led to the evolution and increase of human intelligence in a short period. (Miller, 2000) Intelligence is a possible contributing factor which effected the transition from hunting and gathering to a more agriculture based sedentary society (Miller, 2000).

Three controversial human ancestors

Homo Floresiensis (The 'Hobbit')

Homo floresiensis was recently discovered species in Indonesia. Significant controversy has arisen regarding this species (Richards, 2006).  H. Floresiensis is a possible species, now extinct, in the genus Homo. The remains were discovered in 2003 on the island of Flores in Indonesia (Cauchi, 2004). Partial skeletons of nine individuals have been recovered, including one complete cranium (Salleh, 2007). These remains have been the subject of intense research to determine whether they represent a species distinct from modern humans, and the progress of this scientific controversy has been closely followed by the news media at large (Salleh, 2007). This hominin is remarkable for its small body and brain and for its survival until relatively recent times as recent as 12,000 years ago (Cauchi, 2004).

Jacob Teuku known as the "grand old man" of Indonesian paleontology studied H. floresiensis 's remains and concluded that the remains did not belong to a new hominid species, but were those of H. sapiens after all (Richards, 2006). Teuku stated that the specimen suffered from microcephaly, a disorder resulting in abnormal development of the brain and, often, body (Richards, 2006). His diagnosis was backed by Indonesian colleagues and also a few Australian and American researchers whom he permitted to study the bones (Richards, 2006).

A. Ramidus

Ardipithecus Ramidus is a very early hominin genus. Two species are described in the literature: A. ramidus, which lived about 4.4 MYAduring the early Pliocene, and A. kadabba, dated to approximately 5.6 MYA(Perlman, 2010). The first fossil find was dated to 4.4 MYAbased on its interval between two volcanic strata: the basal Gaala Tuff Complex and the Daam Aatu Basaltic Tuff (Perlman, 2010). Its distinguishing characteristics are bipedalism incorporating an arboreal grasping hallux or big toe, reduced canine teeth and a smaller brain size comparable to that of the modern chimpanzee (Perlman, 2010).

Controversy struck when A. ramidus a purported human ancestor that was dubbed Science magazine's 2009 "Breakthrough of the Year" which is now coming under fire from scientists who say there is scant evidence for her discoverers' claims that there were dense woodlands at the African site where the creature lived 4.4 MYA(UNU, 2010). Instead, "there is abundant evidence for open savanna habitats," says University of Utah geochemist Thure Cerling, lead author of a critique published as a "technical comment" an issue of Science Magazine (UNU, 2010). However the issue is still disputed by many scientists and geologists alike.

Sahelanthropus tchadensis

Sahelanthropus tchadensis is an extinct hominid species that is dated to about 7 million years ago (BBC, 2006). Whether it can be regarded as part of the Hominine tree is unclear; there are arguments both supporting and rejecting it (BBC, 2006). Another complication in its classification is that it is older than the human-chimpanzee divergence which was estimated between 6.3 and 5.4 MYA) seen in genetic data, and that there are few if any specimens other than the partial cranium known as Toumaï(BBC, 2006). The fossils were discovered in the Djurab desert of Chad, in July 2001 to March 2002 at three sites (BBC, 2006).

Controversy surrounding Sahelanthropus arose when a consensus of the specie's lineage was not reached. Sahelanthropus may represent a common ancestor of humans and chimpanzees. The original placement of this species as a human ancestor but not a chimpanzee ancestor would complicate the picture of human origins (Wolpoff, 2006). The researchers and the journals touted the fossils then as belonging to the earliest member of the human family so far discovered, proclamations that the scientific community knew would come under scrutiny (Wolpoff, 2006). The controversy began when; Chris Stringer, head of the Human Origins Program at the National History Museum in London, told National Geographic News in July that discoveries such as this are always complex because evidence is usually incomplete and there is little agreement about what key features characterize a distinct human ancestor (Wolpoff, 2006).

The analysis by Wolpoff and colleagues centers on the argument that the fossil, formally known as Sahelanthropus tchadensis and nicknamed Toumaï, does not have a feature which is needed for it to be identified a hominid (Wolpoff, 2006). Traits like bipedalism or erect posture are absent in the species (Wolpoff, 2006). It could of course be an ancestor of both humans and chimpanzees, it certainly is early enough, but there is no reason to be sure it is the ancestor of any surviving species (Wolpoff, 2006).

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