Since the mid-nineteenth century as the understanding of evolution began, from Darwin's ground-breaking theory of natural selection in 1859 through to the modern day, the idea of evolution has been one surrounded in controversy. Its implications can be felt throughout the scientific and theological worlds with evidence for our evolutionary history arguably challenging many religious beliefs about our purpose and origin. Alongside widespread objections to evolutionary sciences, even loyal supports of the idea of evolution are divided upon their ideas on the exact emergence of humans. Many contradictory theories have been presented, each with validating evidence e.g. the out of Africa and regional evolutionary hypotheses, despite the myriad of potential evidence being available.
Considering the time-period involved, the biological degradation of soft organic tissues means most work is carried out on the hard, mineralised tissues - i.e. bone and teeth. This review will attempt to link methods of analysis and key evidence of archaeological dental specimens with the theories of the emergence of mankind.
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Perhaps the main reason for the usage of dental artefacts is their durability and preservation over even millions of years. (Kordos and Begun, 2001), (Gomez-Robles et al, 2008). Teeth have been found to be resistant to ecological changes and far less vulnerable to developmental effects in the body than bone. (Tafforeau et al, 2007). Consequently for many species, only teeth have been recovered (Kordos and Begun, 2001) and therefore, all information including species identification has to be done using the same. As a result, a wide range of techniques have been formulated to be able to extract different information from dental remains. The use of development incremental lines, comparing enamel thicknesses, tooth morphology, biochemical analyses of teeth etc. will be discussed. Information surrounding specimen ageing (Ackerman and Rogers, 2006), (Alt and Buitrago-Téllez, 2004), (Olejniczak et al, 2008), (Hillson, 1996), life history (Ackerman and Rogers, 2006),( Gibbons, 2002), (Gibbons, 2007), (Martinon-Torres et al, 2008), (Smith and Hublin, 2008), (Sponheimer et al, 2005), (Stringer et al, 1997), diet (Dean, 2006) (Sardi and Rozzi, 2007), (Scott and Lockwood, 2004), (Skinner et al, 2008), (Tafforeau et al, 2007), (Teaford and Ungar, 2000), details of dental disease (Ackerman and Rogers, 2006), (Alt and Buitrago-Téllez, 2004) (Bromage et al, 1995) etc can then be used to help one confirm taxonomic statuses, understand phylogenetic relationships and ultimately build up a picture of the emergence of modern man.
Finding the age at death of remains and specimen dating
Finding the age of excavated fossils at death can be very useful to anthropologists. Applications include an insight into changes in the longevity of life in intermediate species (Caspari and Lee, 2004), the proportions of various age groups within a population (Alt and Buitrago-Téllez, 2004) etc - an understanding of hominin society through time.
Widely used ageing techniques include measuring skeletal changes to the calcified regions of pelvic joints, changes in cranial sutures and levels of calcification near the sternum end of the fourth rib (Hillson, 1996). These methods are not considered to be highly accurate with dental analyses methods being shown to often be more reliable (Dean, 2006). Currently used methods are generally the original 'Gustafson technique' and its modifications (Hillson, 1996). The original system combined known indicators of age on teeth to combine them and use linear regression methods to gain an average estimate. The six used indicators are dental attrition, periodontosis, levels of secondary dentine, cement apposition, root resorption and the level of translucent secondary dentine in the root.
Dental attrition measurements can be enhanced by the usage of microwear analyses (Ungar et al, 2006) to more accurately show changes in toothwear. The durability of enamel (Smith and Hublin, 2008) reduces the level of ecological wear after death, however, questions have though been raised concerning the correlation between dental attrition and diet (Olejniczak et al, 2008), and whether species-wide variation in diet allows a fair comparison in the usage of wear and ageing. Evidence suggests that for example, the diet of australopithecines contained less resistant and hard foods than that of paranthropus (Teaford and Ungar, 2000), which is likely to affect levels of attrition on the teeth of the two genera. Similarly, it has been suggested that attrition-based ageing overages those under the age of 35 and overages those over the age of 50. (Caspari and Lee, 2004) Despite these counter-arguments, dental attrition is often still seen as a reliable indicator of age and is even used on its own to estimate age.
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Similarly, translucent (sclerotic) root dentine is used as an ageing method by both palaeontologists and forensic scientists. The main method involves the usage of bright light being shone on intact roots and measuring the level of translucent dentine. (Hillson, 1996) Bang and Ramm (1970) measured the area of translucent dentine seen in teeth, measuring the extension of this zone and devised a table mapping age and average extension and in 1994, Lucy et al confirmed the usage of the table with independent but agreeable results. (Hillson, 1996) Another technique, developed by Azaz et al, utilised the cross-sectional area of translucent dentine in teeth instead and again, using regression formulae to link the measurements with age. Both techniques have been found to be similarly accurate and if results are standardised effectively, show good results (Dean, 2006). Combining the two and using averages could prove an even more reliable method.
Considering secondary dentine deposition, constant deposition of dentine throughout life at a constant pace, should in theory, show a direct correlation to age. A system of measuring the decreased size of the pulp chamber and root canals in section has been seen to be the best method and Kvaal et al in 1994, found the usage of radiographs to be of great use in doing so. (Dean, 2006) However, it has been found that measurements carried out on specimens of known ages have delivered a low co-efficient of correlation between age and pulp chamber reduction. (Hillson, 1996) Also if the effect of tertiary dentine is not factored in, standardising results could become difficult.
Adding the other three factors, Gustafson was able to link the position of the tooth and the six factors to find a regression formula with an error of +/- 3.63 years. Independent analyses by Maples and Rice (1979) suggested the actual error was 11.28 years (Hillson, 1996) highly questioning the reliability of the method and its actual applications. Consequently, Johnasen ranked the six factors in terms of their correlative index to age and modified the formula to reflect this which would be independently verified by Lucy et al in 1996. (Hillson, 1996)
Dental microstructure of dentine and enamel, can also be an invaluable tool in finding the age at death of an individual. Incremental lines within dental hard tissues form as a result of circadian or weekly changes in dentinogenesis and amelogenesis; lines of Retzius and cross-striations being weekly and circadian markings respectively in enamel, whilst, Andresen's lines and Von Ebner's lines are the respective equivalents in dentine. Using this method has been found to be very reliable and accurate. (Dean, 2006) (Tafforeau et al, 2007) To calculate an estimated age from enamel, one is able to determine the number of Retzius lines either internally, or their external manifestation as perikymata (especially on incisors). From this, the developmental lines seen from before the neonatal line are subtracted age from birth onwards only. (Smith, 2008) To ensure the presence of the neonatal line, this study can only be done on deciduous teeth or permanent teeth which begin mineralising before birth (first molars in humans). By multiplying this with the interval between Retzius lines, i.e. 7-9 days and adding an estimated time for enamel cusp formation, an age can be found. (Tafforeau et al, 2007)
For the purposes of looking at trends in infant deaths and the relative proportions of adults and children in populations, a simpler method may be implemented. An investigation used the theory that that first molars erupt during infancy whilst third molar eruption can be linked to the reaching of adulthood. (Caspari and Lee, 2004) The investigation was able to use this to compare adults across homo genus species and a clear correlation could be seen of age longevity increasing steadily over time but rapidly after the emergence of early homo sapiens. This may be used as evidence for the grandmother theory with older individuals able to reach a menopausal stage to increase life-longevity and aid in the upbringing of youngsters in later homo species. (Kordos and Begun, 2001). However, this is only weak evidence as the study did not look at the age at death of the adults, just their status as adults.
Trying to establish knowledge of intermediary hominins and social change would be impossible without dating species; the study of chronology of the appearance and extinction of various taxa allows one to track the rate of change in various evolved features. U-series dating has been shown to be fairly accurate in tooth enamel. (Scott and Lockwood, 2004) On the basis that teeth , whilst the host is alive, contains no Uranium, one can assume that any uranium taken up and that has undergone nuclear decay must have occured since death. Therefore, by calculating the ratio of Uranium/Thorium, one is able to ascertain using half-life calculations, the time that has elapsed since the death of an individual. Although other common methods e.g. carbon dating are also widely used. (Scott and Lockwood, 2004)
Life histories of Hominins
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Human life history differs considerably from extant apes; the longevity of life and delayed maturation has been vital to the emergence of homo sapiens. (Caspari and Lee, 2004) The life history of a species can be linked to keeping a "balance between its birth and death rates as well as the species' continuous ability to use resources across a given period of time." (Dean, 2006) As life history itself is displayed as the rate of growth and development, this variable can be deduced and analysed by comparing the rate of growth of teeth.
Circadian and weekly (7-9 days) developmental markings can be found in both enamel and dentine in the form of cross-striations and striae (Dean, 2006) (Kelley, 2004) (Smith, 2008). It has been suggested that these striae are linked to adrenal cortical hormones and therefore, the duration over which these fluctuations occurs will impact the short term-growth. (Dean, 2006) The rate of tooth development can then be linked to the expected longevity of life of an individual, and along with average ages at death of species, one can estimate the periods of time spent in various stages of life. (Raghavan et al, 2009) Various studies have been done using this technique, one such was carried out on Neanderthal teeth to compare their life history with ours to question their classification as a separate species rather than a sister species. (Kelley, 2004) Using perikymata on incisors showed their growth patterns were much more like archaic Homo species rather than like early H. Sapiens with a much higher death rate. Compared to modern humans, a large difference can be seen in the time taken to reach adulthood, it is therefore argued that the classification of Neanderthals as a separate species is justified. Similarly, another study concentrated on the entire Homo genus. The results showed a distinct divide between archaic and later Homo with H. Ergaster being the intermediate; H.Antecessor, H. Erectus and H. Sapiens showed a progressively far more elongated development pattern than the ancestral. (Raghavan et al, 2009)
Specific dental events have been linked to various developmental stages such as tooth eruption and mandibular development, e.g. the eruption of the permanent first molar. Other teeth e.g. central incisors have shown similar eruption times for primates and humans with very different life histories, invalidating their usage. (Raghavan et al, 2009) The eruption of the first molar in primates has been correlated to brain growth, significant craniofacial development, reproductive development etc. (Dean, 2006) The presence of permanent teeth is also suggested to allow children to feed upon more of an adult diet and so it may be considered the point of moving into the juvenile stage as opposed to an infant who is totally dependent upon its parents. (Sardi and Rozzi, 2007) This is simply an idea rather than a theory with concrete evidence. Comparative studies, in the past, of extinct species shows a gradual change from australopithecines to H. Erectus followed by significant change in later H. Erectus and early H. Sapiens with reference to eruption times. (Dean, 2006) This validates the theory that modern human life history has evolved relatively recently.
However, a study looking into the correlation between first molar eruption and cranial development produced results to question the widely held opinion as stated above. (Sardi and Rozzi, 2007) Looking at human development, it was found that most cranial development occurred in the infant's first three years, three years before the eruption of the molars begins. (Sardi and Rozzi, 2007) If the two events are not linked, delaying the eruption of the first molar has no advantage in terms of brain and central nervous system development as the brain gains the vast majority of its weight beforehand. The authors do however accept that CNS growth is at its highest in the infant's third year and the first molar crown formation begins at two and a half years so perhaps, the alveolar bone accommodating this could be linked to the development of the brain. (Sardi and Rozzi, 2007)
Information extracted about locations and migration
Migration patterns of early H. Sapiens and earlier hominids is an area of special interest as it has been rather shrouded in controversy; the out of Africa and regional evolutionary theories not being fully agreed upon by experts. (Quam et al, 2009) (Stringer et al, 1997) Therefore, being able to track migration movements of various species and especially, H. Erectus and early H. Sapiens could help to provide concrete evidence for the more widely accepted 'out of Africa hypothesis' or cast doubts over the same. Actual evidence of individuals crossing from the African continent to the Eurasian landmass would be the critical link.
Looking at biochemical analysis of the mineralised contents of enamel, the ratio of the atomic mass isotopes of 87 and 86 of strontium can help point to show provide location based information and even migratory evidence. The ratio of these two isotopes is closely linked to the geological features and makeup of a unique area. (Berkovitz et al, 2009) As teeth are constantly being remodelled, careful sectioning of teeth and looking at growth incremental lines and measuring the isotopic ratios at various levels, one could in practice, map the migratory patterns of throughout the life of a hominid specimen. However, the data regarding strontium ratios across the world and especially of Africa is not available yet and such a database requires meticulous effort. (Berkovitz et al, 2009)
Similarly, oxygen incorporated into the hydroxyapatite complex of enamel must ultimately have been derived from the water drunk by the individual. It is therefore possible to measure the oxygen isotopes O-18 and O-16 and similar to above, match the ratio to the region of water found. (Ungar et al, 2006) (Berkovitz et al, 2009) Likewise, it would be possible to look at changes in the O18:O16 ratios across various points in the enamel to be able to build up migratory patterns. Whilst again data for all the world's waterways are not available, it can be seen that moving towards tropical climates and oceanic water, O18 levels increase and vice versa for lower O18:O16 ratios. (Ungar et al, 2006) A likely consequence is very high O18 levels at equatorial and coastal regions of Africa and moving away from either the coast or equator would cause the ratio to fall.
A practical example of when both ratios were used was for the famous 'Iceman' found in the Alps near the Austro-Italian border in 1991. (Berkovitz et al, 2009) O18:O16 ratios were initially found to show that 'Ortzi the Iceman' predominately lived close to the Alps rather than on or very close to the mountain range; it was also deduced that he lived at lower altitudes when he was younger and as he grew older, lived at higher altitudes. Strontium ratio tests were also carried out to find that he in fact, originated 60km due South of his point of death in Southern Italy. (Berkovitz et al, 2009) The results showed a very precise result could be pinpointed. This example helps to show the viability of using these biochemical tests, if carried out on entire tribes one could also show how human beings populated the globe.
Analysis of diet and dietary change
Considering the need for food, most organisms' habits place the obtainment of foods to be of the highest priority. Therefore, by studying the diet of our ancestors, much can be learnt about their behavioural patterns and hunting patterns.
Toothwear through the process of abrasion can be a very good indicator of diet as the type of foods eaten would lead to characteristic patterns of toothwear. (Teaford and Ungar, 2000) (Ungar et al, 2006) A microwear study (Ungar et al, 2006) was carried out on early Homo first molars (species not designated), as well as molars from H. Habilis and H. Erectus specimens to track the evolutionary changes through our genus. Experiments were also carried out on extant primates with known diets to justify the link between diet and toothwear. Statistical analyses showed a direct correlation between that of specific dietary features such as food abrasiveness and resistance with the density and size of microwear features. (Ungar et al, 2006) Early Homo showed a low percentage of worn pits suggesting a low consumption of resistant foods as was the same with H. Habilis. The authors suggest fruits without tough outer shells could have been the food of choice. Comparatively, the H. Erectus molars showed a higher level of wear, indicative of a more abrasive diet. Whilst such foods are not preferred, the authors theorise that H. Erectus may have eaten meat in accordance with the changing environment at the time which allowed this species to adapt. (Ungar et al, 2006) Indeed, Gibbons used microwear studies to show H. Erectus having a carnivorous diet. (Gibbons, 2007) However, Gibbons also commented upon the small canines of H. Erectus compared to other carnivores; Wrangham in 1999 suggested that this could be used as 'weak-evidence' for the species eating less raw foods and so, cooking their meats which has the biological advantage of conserving energy which would otherwise be used for digestion. (Gibbons, 2007) The idea was also put forward for such energy being used instead for expansion and extensive usage of the cerebral hemispheres of the brain. (Gibbons, 2007) This surely can simply be considered no more than a theory as no actual proof exists, more an attempt to collate a few known facts.
Looking at the latter stages of human evolution, the enlargement of pitting and a difference in angles of toothwear can be seen at the change from hunter-gatherer based behaviour to agriculture (Hillson, 1996) suggesting a new avenue of analysis using the angle of wear. Similarly, an increase in caries can also be seen (Alt and Buitrago-Téllez, 2004) (Hillson, 1996); as this was unlikely to be due to changes in oral hygiene, it can be further attributed to a change in diet to one containing more fermentable carbohydrates possibly, as agricultural control was established.
Much more work has been done on paranthropus and australopithecine species with reference to diet than homo (Ungar et al, 2006), and so I will now concentrate more upon these. Early Australopithecines had small incisors compared to primates extant at the same period as them. (Gibbons, 2002) (Teaford and Ungar, 2000) This has been suggested to show that these early hominins avoided fruits with heavy husks and hard seeds which require gripping and biting forces. Such a hypothesis is based upon the widely-accepted idea that Australopithecines were herbivores; their flat molars (Teaford and Ungar, 2000) and relatively low density of wear features (Hillson, 1996), both point towards a less resistant, vegetarian diet. This can be contrasted with the sharper crested molars of paranthropus species which show more typical carnivorous microwear features. (Teaford and Ungar, 2000) The same trend can be seen applied to the later species of both Australopithecines and Paranthropus, with early Homo seeming to be herbivorous (Ungar et al, 2006) (Gibbons, 2002) - this can possibly be used as evidence to support the ancestral link to Australopithecines rather than Paranthropus.
As well as physical analysis of teeth, biochemical tests have proven to be invaluable in fossil-dietary investigations. Although This incorporation of strontium impurities into enamel has been shown not to be random but highly dependent upon dietary behaviour (Sponheimer et al, 2005) (Berkovitz et al, 2009) and so looking at Sr/Ca ratios can be used to compare the diets of various species. Such tests were also carried out on bone previously but it has been shown that fossilisation can cause the Sr/Ca signal to degrade, whereas enamel is far more resistant.( Sponheimer et al, 2005) Using previously completed experiments on extant African primates, it has been ascertained that low Sr/Ca ratios point towards a carnivorous diet and a higher ratio corresponds to a herbivorous diet (Berkovitz et al, 2009) (Sponheimer et al, 2005); it is believed that biochemical analyses of extant primates can be very accurately compared to that of extinct primates (Teaford and Ungar, 2000). Another study looked at the ratios of various Paranthropus and Australopithecus species and confirmed the wear-based evidence described above that Paranthropus had a carnivorous diet as opposed to the herbivorous diet of Australopithecines. (Sponheimer et al, 2005) However, a diet based largely upon leaves can lead to a lowered Sr/Ca ratio (Sponheimer et al, 2005), reducing the reliability of the technique and a full understanding of the vegetation of the African savannah is not known to conclusively link the Sr/Ca ratio to food types. (Berkovitz et al, 2009) Whilst the Australopithecus specimens did show grazer-like high ratios, additional Ba/Ca ratios were also carried out. In this case, the australopithecine ratios again suggested grazing, however a match could be seen for both ratios with that of extant mole rats and warthogs. The authors theorise that this could suggest that like the latter extant species, australopithecines could have been utilising underground food resources e.g. root vegetables as a back-up food during less plentiful seasons. It is however emphasised that this is simply a theory and requires more concrete evidence. (Sponheimer et al, 2005) If this is true, along with the unusually flat and uncrested molars, Australopithecines had begun to show a change in dietary behaviour compared to other hominoids of the same period; this would therefore be a highly important period in human evolution, arguably the beginning of intelligent exploitation of our environment.
Other biochemical analyses on enamel have been carried out on carbon atoms present in enamel proteins. A commonly held belief is that environmental change has been one of the major drivers of human evolution (Lee-Thorp et al, 2007) (Raghavan et al, 2009). One of the biggest impacts of environmental change is that on dietary availabilities and therefore, a studies have been carried out to evaluate the changes in the African savannah and the effects of this on hominins at the time. About 2 million years ago, it was believed a drier climate began to persist in the African landscapes leading to a shift towards the more dry-tolerant C3 grasses etc from the C4 plants. (Lee-Thorp et al, 2007) The result was a more open environment where bipedality of hominins would have become a real advantage. (Lee-Thorp et al, 2007) The study compared the Carbon13/Carbon12 isotopic ratios of early Homo specimens and Australopithecines against previously measured ratios of extant C3 and C4 consumers. (Lee-Thorp et al, 2007) (Ungar et al, 2006) (Berkovitz et al, 2009) With C3 type plants showing a lowered level of Carbon13 usage than C4 plants, C4 plants show a higher C13/C12 ratio, allowing the ratio to be used to show the relative abundance of both grasses in an environment. A herbivore's preference for one or the either shows through, as the carbon in the enamel must be obtained from the plants eaten. (Berkovitz et al, 2009) This technique is especially useful as it can be used to shows average dietary changes in an individual as the tooth matrix is constantly being remodelled. (Ungar et al, 2006) This study showed a high consumption of C3 plants by Australopithecines as expected with these hominins living primarily before the shift to C4 plants. However, charting the results chronologically, a major shift can be seen in the C13/C12 ratio in H. Habilis to that of C3 grasses, about 1.6 million years ago. (Lee-Thorp et al, 2007), whilst a gradual shift can be seen in the intermediary. This agrees with and further validates environmental evidence of the shift to a more open and dry plain dominated by C4 grasses. It also pinpoints the adaptation of earlier hominin diets to the available vegetation, i.e. the ability of early Homo to thrive in the new conditions which other species may not have been able to. (Lee-Thorp et al, 2007)
For many primate species and even some hominins, either due to only dental remains being found or the bony remains being damaged, many species have been identified and classified solely on dental evidence. (Kordos and Begun, 2001) Similarly the classification of a species as a hominin is partly done upon the basis on their dentition, with many anthropologists regarding the 'canine-3rd premolar' complex with a less sharp canine as important as bipedality for hominid classification. (Gibbons, 2002) Therefore it is clear that dental remains have a vital taxonomic role.
Enamel thicknesses have been seen to vary between hominins and the rest of the hominidae family; our enamel can be seen to be far thicker than extant and extinct apes. (Olejniczak et al, 2008) A study looked at this very fact, focusing on human and chimpanzee enamel but also looking at a few earlier hominins and reviewing past findings. (Olejniczak et al, 2008) The authors did acknowledge that in regions of teeth where enamel was missing, reconstructions were made using morphological evidence. The results showed "marked" differences in the enamel thicknesses between humans and chimpanzees with an intermediate seen in the very few early hominin teeth. Enamel thicknesses are now known of almost extant and extinct great apes, which facilitates the taxonomic classification of new found fossils. (Olejniczak et al, 2008)
Other factors must though be considered for classification and in the case of dental remains, crown sizes, cusps sizes and shape and as well as tooth morphology are being used. (Bromage et al, 1995) (Gomez-Robles et al, 2008) (Quam et al, 2009) As an example, an early hominin (dated) mandible along with some posterior teeth was found near Malawi in (Bromage et al, 1995) but could not be immediately classified. As each genus shows a unique angle, angular measurements were taken of the Hunter-Schreger bands in the premolar enamel relative to the EDJ which showed Paranthropus like results. (Bromage et al, 1995) Furthermore, the large crown bases and thick enamel of the molars pointed to the same, however, the bucco-lingulally reduced molars and molar enamel thicknesses pointed more towards H. Habilis. It was therefore believed to be H. Rudolfensis (suggested to be an intermediate) and four out of the seven features on the mandible suggested the same. (Bromage et al, 1995) This cannot however be considered a fully conclusive classification.
Quam et al argued for the reclassification of H. Habilis as an Australopith, early H. Erectus as H. Ergaster with classification of early Homo being highly controversial. (Quam et al, 2009) To do so, their study looked at maxillary first molars (as they are the least variable teeth and show a clear evolutionary reduction in size from the primitive to modern phenotypes). The results showed how a slight decrease in molar size can be seen in early Homo compared to australopithecines and paranthropus, however a sharp decrease of 17% can be seen in H. Erectus and a further 10% in early H. Sapiens. This is mainly due to a decrease in the disto-buccal cusp, such that in primitive hominins, the disto-buccal cusp was larger than the mesio-buccal but late Homo showed the opposite trait. 80% of the H. Ergaster specimens showed the derived trait, so the authors suggested the possibility of H. Ergaster being at the point of division and showing dominance for the modern trait (Quam et al, 2009) Either way, both observations help to justify the opinion that early Homo should have their genera changed. Early H. Sapiens show a very small proportion of the primitive trait, however H. Neanderthalensis and H. Heidelbergensis show none (Quam et al, 2009) which supports the idea that Neanderthals evolved from H. Heidelbergensis and that they had a separate lineage of evolution from humans. H. Antecessor on the other hand was shown not to be a part of the late stages of the H. Sapien lineage as they showed no sign of the derived trait, showing an ancestry not linked to H. Ergaster. (Quam et al, 2009)The study however could not conclusively classify the earlier species of Homo or the settle on the classification of H. Ergaster.
An investigation has though questioned the validity of the tooth morphology based taxonomy. This looked at crown variation in a variety of teeth from the primate groups: Homo, Pongo, Gorilla and Pan specimens to look at patterns of variation. (Scott and Lockwood, 2004) Following statistical analysis of the frequencies of expected patterns and anomalies, the authors report an inconsistency in variation, commenting on how in instances inter-genus variation was actually less than that seen within certain genera. Similarly, a common statistical pattern could be seen in mandibular dentition variation between species within a genus however, Pongo did not follow this and so doubts may be created on the usage of this 'common pattern'. (Scott and Lockwood, 2004) It can therefore be concluded that the sole usage of tooth morphology to separate species may not be fully reliable (Scott and Lockwood, 2004) and in certain cases, it may not be possible to distinguish between two closely related species.
Hybridisation and phylogenetic applications
The relatively late evolutionary changes amounts to a large level of sudden genetic variation (Ackerman and Rogers, 2006) (Caspari and Lee, 2004) (Tafforeau et al, 2007); a possible source is hybridisation. (Ackerman and Rogers, 2006) Suggestions have even been made that H. Neanderthalensis was able to breed with H. Sapiens with, modern humans being the resultant hybrid. (Kelley, 2004) (Raghavan et al, 2009) This issue therefore becomes critical to the story of our evolution.
A study therefore investigated the level of hybridisation within hominins. (Ackerman and Rogers, 2006) Looking at the literature, the authors were able to find many examples of hybrids within other primates but none within our lineage and little discussion about the same. (Ackerman and Rogers, 2006) The skulls of baboons were studied as their population structure was believed to be similar to that of later species of Homo. Looking at variation in known baboon hybrids, it was found that amongst other factors, tooth overcrowding and supernumerary mandibular fourth molars (in over 50%) were common. In normal specimens, supernumerary teeth were far less common (less than 5%) and usually maxillary anterior teeth. (Ackerman and Rogers, 2006) However, looking at hominin fossils, only two supernumerary teeth have been discovered and only one of which was a mandibular fourth molar. Similarly, in human populations, even maxillary anterior supernumerary teeth are rare (Ackerman and Rogers, 2006) suggesting a lack of hybridisation in our evolutionary tree. The authors do however accept that this should not be made a final judgement as phenotypes of hybrids are not fixed, always visible or even fully understood - more work is required. (Ackerman and Rogers, 2006)
The exact evolutionary tree is possibly the least agreed upon aspect of human evolution (Smith, 2008), with many different theories and counter-theories surrounding phylogenetic relationships. (Lycett and Collard, 2005) Whilst all of the above aspects of human emergence are themselves topics of interest, compounding patterns in each aspect help to show phylogenetic patterns especially, changes in life histories, ages of death, diet and tooth morphology.
Limitations and possible future developments
As with using all other forms of evolutionary evidence, homoplasies and convergent evolution may be brought into question but a review study of many older studies showed a distinct lack of evidence for these in extinct and extant primate skulls. (Lycett and Collard, 2005) As discussed above, questions have been raised about inter-species and intra-species variation and how this varies between species. With species such as H. Rudolfenesis, few specimens have been identified and therefore, once cannot be certain that the dental remains found are representative of the entire population or simply a recessive phenotype. Similarly, with a lack of full agreement on species allocation, e.g. some regarding H. Ergaster as being part of the H. Erectus species (Kordos and Begun, 2001), studies on species are unlikely to be fully standardised and therefore, comparisons become difficult. Finally, as with all scientific evidence, interpretation is key and the wide breadth of theories and ideas using the same facts and evidence is testament to this.
The use of radiology via computed tomography (CT) and panoramic radiography and is a newer study tool, to test the usage of this technology, three individuals dating from Australopithecines to humans of the medieval era were analysed as a "pilot study". (Alt and Buitrago-Téllez, 2004)
Beginning with the australopithecine jaw, measurements of the mandible and maxilla suggested a link to the Paranthropus genus and electron microscopy showed microwear patterns not usually expected from Australopithecus. (Alt and Buitrago-Téllez, 2004) The second specimen, a H. Erectus mandible had to be studied with the use of a plastic cast model. The morphology of the dental tissues were outlined and allowing the authors to comment on how this showed how cast models could also be used. (Alt and Buitrago-Téllez, 2004) The third, a mandible from a medieval human displayed carious lesions as well as an osteomyelitis with evidence of periapical disease - all likely to have begun with the caries. It can be ascertained that this individual is likely to have died from this condition as without antimicrobial treatment, osteomyelitis does not arrest and so may become fatal. (Alt and Buitrago-Téllez, 2004)
This study helped to show the applications of radiology in taxonomic classification using morphology and anatomy studies, pathological studies and infection diagnosis etc. (Alt and Buitrago-Téllez, 2004) The radiographs could help increase the detail of anatomical measurements and features and allow surveying of disease prevalence and healing in various populations as well as possible causes of death. Increases in caries seen using radiography (Alt and Buitrago-Téllez, 2004) can help to pinpoint how different populations made the key dietary and social transition towards agriculture and fermentable carbohydrate consumption. Its non-invasive nature enables the study of even the most fragile fossils.
It can be supposed that the future will bring many new analytic techniques and refinement of the same. More studies and more data will allow for more reliable comparisons; data on dietary analysis of extant taxa increases the conviction of conclusions drawn for extinct primates whilst a more comprehensive record of tooth morphology will allow for more conclusive species recognition etc. The use of radiological techniques will allow for possibly bone and endodontic analysis without invasive damage, to open up new routes. Key to conclusiveness though will be standardisation of measurements and basic taxonomy to allow widespread comparisons and finally, agreement; universally accepted theories are a major target to make the evolutionary story a more concrete and convincing argument.
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