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H. antecessor    H. heidelberge

H. erectus (Asia)

   
H. sapiens

   

H. neanderthalensis

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H. neanderthalensis

    In 2006, two teams attempted to extract DNA from a tooth discovered in 2003, but both teams were unsuccessful. It has been suggested that this happened because the dentine was targeted; new research suggests that the cementum has higher concentrations of DNA. Moreover, the heat generated by the high speed of the drill bit may have denatured the DNA.[38]

Congenital disorder hypotheses

The small brain size of H. floresiensis at 417 cc has prompted hypotheses that the specimens were simply H. sapiens with a birth defect, rather than the result of neurological reorganisation.[39]

Microcephaly
[39]

Microcephaly
Prior to Jacob's removal of the fossils, American neuroanthropologist Dean Falk and her colleagues performed a CT scan of the LB1 skull and a virtual endocast, and concluded that the brainpan was neither that of a pygmy nor an individual with a malformed skull and brain.[40] In response, American neurologist Jochen Weber and colleagues compared the computer model skull with microcephalic human skulls, and found that the skull size of LB1 falls in the middle of the size range of the human samples, and is not inconsistent with microcephaly.[41][42] In 2006, American biologist Robert Martin and colleagues also concluded that the skull was probably microcephalic, arguing that the brain is far too small to be a separate dwarf species; he said that, if it were, the 400-cubic-centimeter brain would indicate a creature only one foot in height, one-third the size of the discovered skeleton.[43]

A 2006 study stated that LB1 probably descended from a pygmy population of modern humans, but herself shows signs of microcephaly, and other specimens from the cave show small stature but not microcephaly.[44]

In 2005, the original discoverers of H. floresiensis, after unearthing more specimens, countered that the skeptics had mistakenly attributed the height of H. floresiensis to microcephaly.[4] Falk stated that Martin's assertions were unsubstantiated.[45] In 2006, Australian palaeoanthropologist Debbie Argue and colleagues also concluded that the finds are indeed a new species.[44]

In 2005, the original discoverers of H. floresiensis, after unearthing more specimens, countered that the skeptics had mistakenly attributed the height of H. floresiensis to microcephaly.[4] Falk stated that Martin's assertions were unsubstantiated.[45] In 2006, Australian palaeoanthropologist Debbie Argue and colleagues also concluded that the finds are indeed a new species.[46] In 2007, Falk found that H. floresiensis brains were similar in shape to modern humans, and the frontal and temporal lobes were well-developed, which would not have been the case were they microcephalic.[47]

In 2008, Greek palaeontologist George Lyras and colleagues said that LB1 falls outside the range of variation for human microcephalic skulls.[48] However, a 2013 comparison of the LB1 endocast to a set of 100 normocephalic and 17 microcephalic endocasts showed that there is a wide variation in microcephalic brain shape ratios and that in these ratios the group as such is not clearly distinct from normocephalics. The LB1 brain shape nevertheless aligns slightly better with the microcephalic sample, with the shape at the extreme edge of the normocephalic group.[49] A 2016 pathological analysis of LB1's skull revealed no pathologies nor evidence of microcephaly, and concluded that LB1 is a separate species.[50]

A 2007 study postulated that the skeletons were those of humans who suffered from Laron syndrome, which was first reported in 1966, and is most common in inbreeding populations, which may have been the scenario on the small island. It causes a short stature and small skull, and many conditions seen in Laron syndrome patients are also exhibited in H. floresiensis. The estimated height of LB1 is at the lower end of the average for afflicted human women, but the endocranial volume is much smaller than anything exhibited in Laron syndrome patients. DNA analysis would be required to support this theory.[51]

Endemic cretinism
In 2008 Australian researcher Peter Obendorf—who studies endemic cretinism—and colleagues suggested that LB1 and LB6 suffered from myxoedematous (ME) endemic cretinism resulting from congenital hypothyroidism (a non-functioning thyroid), and that they were part of an affected population of H. sapiens on the island.[52] Cretinism, caused by iodine deficiency, is expressed by small bodies and reduced brain size (but ME causes less motor and mental disablement than other forms of cretinism), and is a form of dwarfism still found in the local Indonesian population. They said that various features of H. floresiensis are diagnostic characteristics, such as enlarged pituitary fossa, unusually straight and untwisted humeral heads, relatively thick limbs, double rooted premolar, and primitive wrist morphology.[52]

However, Falk's scans of LB1's pituitary fossa show that it is not larger than usual.[53] Also, in 2009, anthropologists Colin Groves and Catharine FitzGerald compared the Flores bones with those of ten people who had had cretinism, and found no overlap.[54][55] Obendorf and colleagues rejected Groves and FitzGerald's argument the following year.[56] A 2012 study similar to Groves and FitzGeralds' also found no evidence of cretinism.[57]

Down syndrome

In 2014,

However, Falk's scans of LB1's pituitary fossa show that it is not larger than usual.[53] Also, in 2009, anthropologists Colin Groves and Catharine FitzGerald compared the Flores bones with those of ten people who had had cretinism, and found no overlap.[54][55] Obendorf and colleagues rejected Groves and FitzGerald's argument the following year.[56] A 2012 study similar to Groves and FitzGeralds' also found no evidence of cretinism.[57]

In 2014, physical anthropologist Maciej Henneberg and colleagues claimed that LB1 suffered from Down syndrome, and that the remains of other individuals at the Flores site were merely normal modern humans.[58] However, there a number of characteristics shared by both LB1 and LB6 as well as other known early humans and absent in H. sapiens, such as the lack of a chin.[59] In 2016, a comparative study concluded that LB1 did not exhibit a sufficient number of Down syndrome characteristics to support a diagnoses.[60]

The most important and obvious identifying features of H. floresiensis are its small body and small cranial capacity. Brown and Morwood also identified a number of additional, less obvious features that might distinguish LB1 from modern H. sapiens, including the form of the teeth, the absence of a chin, and the lesser angle in the head of the humerus (upper arm bone). Each of these putative distinguishing features has been heavily scrutinized by the scientific community, with different research groups reaching differing conclusions as to whether these features support the original designation of a new species,[46] or whether they identify LB1 as a severely pathological H. sapiens.[44]

A 2015 study of the dental morphology of 40 teeth of H. floresiensis compared to 450 teeth of living and extinct human species, states that they had "primitive canine-premolar and advanced molar morphologies," which is unique among hominins.A 2015 study of the dental morphology of 40 teeth of H. floresiensis compared to 450 teeth of living and extinct human species, states that they had "primitive canine-premolar and advanced molar morphologies," which is unique among hominins.[30]

The discovery of additional partial skeletons[4] has verified the existence of some features found in LB1, such as the lack of a chin, but Jacob and other research teams argue that these features do not distinguish LB1 from local modern humans.[44] Lyras et al. have asserted, based on 3D-morphometrics, that the skull of LB1 differs significantly from all H. sapiens skulls, including those of small-bodied individuals and microcephalics, and is more similar to the skull of Homo erectus.[48] Ian Tattersall argues that the species is wrongly classified as Homo floresiensis as it is far too archaic to assign to the genus Homo.[61]

LB1's height is estimated to have been 1.06 m (3 ft 6 in). The height of a second skeleton, LB8, has been estimated at 1.09 m (3 ft 7 in) based on tibial length.[4] These estimates are outside the range of normal modern human height and considerably shorter than the average adult height of even the smallest modern humans, such as the Mbenga and Mbuti at 1.5 m (4 ft 11 in),[62] Twa, Semang at 1.37 m (4 ft 6 in) for adult women of the Malay Peninsula,[63] or the Andamanese at also 1.37 m (4 ft 6 in) for adult women.[64] LB1's body mass is estimated to have been 25 kg (55 lb). LB1 and LB8 are also somewhat smaller than the australopithecines from three million years ago, not previously thought to have expanded beyond Africa. Thus, LB1 and LB8 may be the shortest and smallest members of the extended human family discovered thus far.[65]

Their short stature was likely due to insular dwarfism, where size decreases as a response to fewer resources in an island ecosystem.

Their short stature was likely due to insular dwarfism, where size decreases as a response to fewer resources in an island ecosystem.[3][66] In 2006, Indonesian palaeoanthropologist Teuku Jacob and colleagues said that LB1 has a similar stature to the Rampasasa pygmies who inhabit the island, and that size can vary substantially in pygmy populations.[44] Of course, the Rampasasa pygmies are completely unrelated to H. floresiensis.[67]

Aside from smaller body size, the specimens seem to otherwise resemble H. erectus, a species known to have been living in Southeast Asia at times coincident with earlier finds purported to be of H. floresiensis.[4]

In addition to a small body size, H. floresiensis had a remarkably small brain size. LB1's brain is estimated to have had a volume of 380 cm3 (23 cu in), placing it at the range of chimpanzees or the extinct australopithecines.[3][40] LB1's brain size is half that of its presumed immediate ancestor, H. erectus (980 cm3 (60 cu in)).[40] The brain-to-body mass ratio of LB1 lies between that of H. erectus and the great apes.[45] Such a reduction is likely due to insular dwarfism, and a 2009 study found that the reduction in brain size of extinct pygmy hippopotamuses in Madagascar compared with their living relatives is proportionally greater than the reduction in body size, and similar to the reduction in brain size of H. floresiensis compared with H. erectus.[68]

Smaller size does not appear to have affected mental faculties, as Brodmann area 10 on the prefrontal cortex, which is associated with cognition, is about the same size as that of modern humans.[40] H. floresiensis is also associated with evidence for advanced behaviours, such as the use of fire, butchering, and stone tool manufacturing.[4][7]

Limbs

The angle of humeral torsion is much less than in modern humans.[3][4][7]

Smaller size does not appear to have affected mental faculties, as Brodmann area 10 on the prefrontal cortex, which is associated with cognition, is about the same size as that of modern humans.[40] H. floresiensis is also associated with evidence for advanced behaviours, such as the use of fire, butchering, and stone tool manufacturing.[4][7]

The angle of humeral torsion is much less than in modern humans.[3][4][7] The humeral head of modern humans is twisted between 145 and 165 degrees to the plane of the elbow joint, whereas it is 120 degrees in H. floresiensis. This may have provided an advantage when arm-swinging, and, in tandem with the unusual morphology of the shoulder girdle and short clavicle, would have displaced the shoulders slightly forward into an almost shrugging position. The shrugging position would have compensated for the lower range of motion in the arm, allowing for similar manoeuverability in the elbows as modern humans.[26] The wrist bones are similar to those of apes and Australopithecus, significantly different from those of modern humans, lacking features which evolved at least 800,000 years ago.[25]

The leg bones are thicker than those of modern humans.[3][4][3][4][7] The feet were unusually flat and long in relation with the rest of the body.[69] As a result, when walking, they would have had to have bent the knees further back than modern humans do. This caused a high-stepping gait and low walking speed.[70] The toes had an unusual shape and the big toe was very short.[71]

Because of a deep neighbouring strait, Flores remained isolated during the Wisconsin glaciation (the last glacial period), despite the low sea levels that united Sundaland. Therefore, the ancestors of H. floresiensis could only have reached the isolated island by water transport, perhaps arriving in bamboo rafts around one million years ago. Liang Bua Cave shows evidence of the use of fire for cooking, and bones with cut marks.

The cave also yielded a great quantity of stone artefacts, mainly lithic flakes. Points, perforators, blades, and microblades were associated with remains of the extinct elephant Stegodon, and were probably hafted into barbs to sink into the elephant. This indicates the inhabitants were targeting juvenile Stegodon. Similar artefacts are found at the Soa Basin 50 km (31 mi) south, associated with Stegodon and Komodo dragon remains, and are attributed to a likely ancestral population of H. erectus.[3][4][7]

Extinction