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4. The first hominines

a. Adaptations of the hominines

Resultado de imagen de lukeino formation

The sites were fossil specimens are found in Africa are usually termed Localities. Each one represents a different time period that has been dated using either relative or absolute dating tecniques. The fossil remains within each locality are assigned to the hominin lineage according to a specific set of anatomical traits. In particular, those indicative of the hominoidea, such as a bunodont dentition and an orthograd locomotion (which significantly differ from the bilophodontia and pronogrady of the cercopithecoidea). Since the divergence of the panini clade from that of the hominini took place at around 6 to 4 Ma, our direct ancestor needs to be characterized by a distinct feature that discerns it from those of the chimpanzee. No clear fossil ancestrals to the chimpanzee have been yet discovered, in part because they would probably resemble those that originated our own lineage. Traditionally, the main adaptation that has been assumed to be indicative of a hominin fossil specimen is bipedality. The ancestors of the chimpanzee might be expected to resemble those of the hominines except in their locomotion. The anatomical adaptations of the present day chimpanzee are nowadays considered as derived traits, not necessarily present in their ancestors. Given the scarcity of fossils between 8 and 5 Ma, it is difficult to determine wether a fossil specimen from that time period is a hominin or not. Thus, it is generally accepted that the first hominines should show some sort of bipedal locomotion, despite primitive. Other traits, such as the dentition, are assumed to be less informative. However, the evidence seems to support the hypothesis that the reduction of the size of the canines, especially that of the males, was among the traits that actually characterize the first hominini, whereas, a trend to bipedal locomotion could have been a homoplastic trait that appeared several times in the evolution of the hominoidea, as also suggested by the Miocene genus Oreopithecus from Italy. Having this in mind, let us consider the evidence, in chronological order, and what it tells us about the origin of the hominines.

Sahelanthropus tchadensis

Djura Desert (Chad)

Existing fossils ascribed to the species Sahelanthropus tchadensis include a relatively small cranium named Toumaï (“hope of life” in the local Daza language of Chad in central Africa), five pieces of jaw, and some teeth, making up a head that has a mixture of derived and primitive features. The braincase, being only 320 cm³ to 380 cm³ in volume, is similar to that of extant chimpanzees. The teeth, brow ridges, and facial structure differ markedly from those found in Homo sapiens. Cranial features show a flatter face, u-shaped dental arcade, small canines, an anterior foramen magnum, and heavy brow ridges. No postcranial remains have been recovered. The skull suffered a large amount of distortion during the time of fossilisation and discovery, as the cranium is dorsoventrally flattened, and the right side is depressed.

Resultado de imagen de sahelanthropus tchadensis

Sahelanthropus tchadensis

Sahelanthropus tchadensis may have walked on two legs. However, because no postcranial remains (i.e., bones below the skull) have been discovered, it is not known definitively whether Sahelanthropus was indeed bipedal, although claims for an anteriorly placed foramen magnum. Upon examination of the foramen magnum in the primary study, the lead author speculated that a bipedal gait “would not be unreasonable” based on basicranial morphology similar to more recent hominins. Some palaeontologists have disputed this interpretation adn suggest that Sahelpithecus would be a more appropriate name. They also state that the basicranium, as well as dentition and facial features, do not represent adaptations unique to the hominin clade, nor indicative of bipedalism; and that its canine wear is similar to that of other Miocene apes. The bones were found far from most previous hominin fossil finds, which are from Eastern and Southern Africa. However, an Australopithecus bahrelghazali mandible was also found in Chad by Mamelbaye Tomalta, Najia and Alain Beauvilain, Michel Brunet and Aladji H.E. Moutaye as early as 1995. With the sexual dimorphism known to have existed in early hominins, the difference between Ardipithecus and Sahelanthropus may not be large enough to warrant a separate species for the latter.

The fossils were discovered in the Djurab Desert of Chad by a team of four led by a Frenchman, Alain Beauvilain, and three Chadians, Adoum Mahamat, Djimdoumalbaye Ahounta, and Gongdibé Fanoné, members of the Mission paleoanthropologique Franco-tchadienne led by Michel Brunet. All known material of Sahelanthropus was found between July 2001 and March 2002 at three sites: TM 247, TM 266, which yielded most of the material, including a cranium and a femur, and TM 292.

References

Orrorin tugenensis

The Lukeino Formation (Tugen Hills, Kenya)

The Lukeino Formation dates to 6.1 to 5.7 Ma at the sites of  Cheboit, Kapsomin, Kapcheberek, and Aragai (Tugen Hills, Kenya). A total of about 20 fossil remains have been described, including 2 mandibular fragments, a mandibular symphysis, several isolated teeth, and some femoral and humeral fragments. The skeletal material suggests that the specimen was a bipedal hominoidea, although showing clear adaptations to tree climbing, a combination of characteristics that have been termed facultative bipedalism. The remains have been attributed to the species Orrorin tugenensis, which means primigenial man in Tugen.

Imagen relacionada

Orrorin tugenensis

Orrorin had small teeth relative to its body size. Its dentition differs from that found in Australopithecus in that its cheek teeth are smaller and less elongated mesiodistally and from Ardipithecus in that its enamel is thicker. The dentition differs from both these species in the presence of a mesial groove on the upper canines. The canines are ape-like but reduced, like those found in Miocene apes and female chimpanzees. Orrorin had small post-canines and was microdont, like modern humans, whereas robust australopithecines were megadont. In the femur, the head is spherical and rotated anteriorly; the neck is elongated and oval in section and the lesser trochanter protrudes medially. While these suggest that Orrorin was bipedal, the rest of the postcranium indicates it climbed trees. While the proximal phalanx is curved, the distal pollical phalanx is of human proportions, which should probably be associated with grasping abilities useful for tree-climbing. If Orrorin proves to be a direct human ancestor, then australopithecines such as Australopithecus afarensis (“Lucy”) may be considered a side branch of the hominid family tree: Orrorin is both earlier, by almost 3 million years, and more similar to modern humans than is A. afarensis. The main similarity is that the Orrorin femur is morphologically closer to that of H. sapiens than is Lucy’s; there is, however, some debate over this point. Other fossils (leaves and many mammals) found in the Lukeino Formation show that Orrorin lived in a dry evergreen forest environment, not the savanna assumed by many theories of human evolution.

The team that found these fossils in 2000 was led by Brigitte Senut and Martin Pickford from the Musée National d’Histoire Naturelle (Paris).

References

 

b. The genus Ardipithecus

Ardipithecus kadabba & Ardipithecus ramidus

Afar Depression (Ethiopia)

Two fossil species have been described: A. kadabba, dated to approximately 5.6 million years ago (late Miocene) and A. ramidus, which lived about 4.4 million years ago during the early Pliocene. Anatomical and behavioral analysis showed that Ardipithecus could have been very similar to chimpanzees. The two species are generally considered chronospecies that share similarities in the shape of the canines. Their postcranial skeleton indicated arboreal capacities.

Ardipithecus ramidus

Ardipithecus ramidus is represented by 17 individuals, 16 of which were found together. Thier occipital foramina is advanced in the cranial base, which suggests that bipedal locomotion aroused in a forested environment, well before open savanna enviroments predominated. The anatomy of the postcranial skeleton shows significant climbing habilities, including curved finger bones and an oposing big toe. Ardipithecus ramidus had a small brain, measuring between 300 and 350 cm3. This is slightly smaller than a modern bonobo or female common chimpanzee brain, but much smaller than the brain of australopithecines like Lucy (~400 to 550 cm3) and roughly 20% the size of the modern Homo sapiens brain. Like common chimpanzees, A. ramidus showed a much more prognathic face than modern humans. The teeth of A. ramidus lacked the specialization of other apes, and suggest that it was a generalized omnivore and frugivore (fruit eater) with a diet that did not depend heavily on foliage, fibrous plant material (roots, tubers, etc.), or hard and abrasive food. The size of the upper canine tooth in A. ramidus males was not distinctly different from that of females. Their upper canines were less sharp than those of modern common chimpanzees in part because of this decreased upper canine size, as larger upper canines can be honed through wear against teeth in the lower mouth. The features of the upper canine in A. ramidus contrast with the sexual dimorphism observed in common chimpanzees, where males have significantly larger and sharper upper canine teeth than females. The less pronounced nature of the upper canine teeth in A. ramidus has been used to infer aspects of the social behaviour of the species, which suggests that the last common ancestor (LCA) and the other African apes were characterized by relatively little aggression between males and between groups. This is markedly different from social patterns in common chimpanzees, among which intermale and intergroup aggression are typically high. This condition compromises the living chimpanzee as a behavioral model for the ancestral hominin condition.

Ardipithecus ramidus was named in September 1994. The first fossil found was dated to 4.4 million years ago on the basis of its stratigraphic position between two volcanic strata: the basal Gaala Tuff Complex (G.A.T.C.) and the Daam Aatu Basaltic Tuff (D.A.B.T.). The name stems mostly from the Afar language, in which Ardi means “ground/floor” (borrowed from the Semitic root in either Amharic or Arabic) and ramid means “root”. The pithecus portion of the name is from the Greek word for ape.

References

Kapthurin and Kapsomin (Lukeino, Kenia)

The Rift Valley has been considered an obstacle to chimpanzee occupation. However, a fossil chimpanzee has been reported from the Kapthurin Formation, in Kenya, which may be indicative that representatives of Pan were present in the East African Rift Valley during the Middle Pleistocene (between 0.6 and 0.5 Ma), where they were contemporary with an extinct species of Homo. Habitats suitable for both hominins and chimpanzees were clearly present there during this period, and the Rift Valley might not have presented an impenetrable barrier to chimpanzee occupation (McBrearty & Jablonsk, 2005). Additionally, a large molar fragment has been discovered at Kapsomin (Lukeyno), dated to 5.9 Ma, that shows significant differences with Orrorin and some affinities with the extant gorilla. The molar has a large trigon and distal fovea, as well as tall and conic shape cusps. This specimen suggests that the split of the gorilla from the homininae lineage took place well before 6 Ma.

c. The australopithecines from East and South Africa

In 1924, at the Buxton quarry, close to the city of Taung, in South Africa, a relevant fossil specimen was unearthed. It showed a small fossilized brain with a gracile face, similar to the baboons found in the area. However, Raymond Dart pointed out that the great brain development seen in the specimen was not comparable to that of a 3-year old monkey, and thus concluded that it was ancestral to our lineage and that Africa was the cradle of the hominines. The Taung child showed a mixture of traits: human-like traits were the small canines, flat facial profile, and the postero-inferiorly placed sulcus lunatus. Chimpanzee-like traits were the large molar teeth and small cranial capacity (440 cm³). This was the first fossil specimen attributed to the Australopithecus africanus species. Since then, many specimens of the genus Australopithecus have been unearthed from East Africa (Kenya and Tanzania), Central Africa (Tchad) and South Africa, all dating to between 4.2 and 2 Ma. The main anatomical traits of the specimens assigned to the Australopithecus genus are: small to medium height (1.2-1.5 m), small brain case (range 390-450 cm³), slender long bones and absence of ectocranial structures (Sagital or nuchal crests), advanced position of the foramen magnum (indicative of bipedal locomotion), marked facial prognathism, absence of the honing complex (between the upper canine and the lower first premolar), lateralized illiac crest, longer forelimbs than hindlimbs (brachiation capability), bipedal locomotion in the legs and brachiation in the arms.

The discovery of the Laetoli (Tanzania) footprints by Mary Leakey, dated to 3.6 Ma, has evidenced the bipedal locomotion of the australopithecines. There are 69 prints of at least 2 individuals (a third juvenile individual is still debated) that clearly show a big toe in line with the rest of the fingers (which differs with the fossil specimen of Little Foot) and the characteristical curvature of the foot as in modern humans. This derived anatomical traits in the footprints of Laetoli could be consistent with the derived morphology in the tibia of A. anamensis, rather than with the more primitive one in A. afarensis. This would imply that several evolving lineages of australopithecines could be present at the same time in Africa. The same case can be considered for Orrorin that shows a derived suit of bipedal traits in a much earlier date.

i. East African australopithecines

Australopithecus anamensis

It is the most ancient australopithecine species (4.2-3.9 Ma). It was found in the Kenyan Rift. The specimens found include dental, cranial and mandibular fragments, as well as postcranial remains (humerus and tibia). The cranial remains retain primitive traits already present in Ardipithecus, whereas the tibia shows highly derived traits (excavated proximal articular surfaces) that suggest a derived bipedal locomotion in line with Homo and differing from that of A. afarensis, its most likely descendant. The paleoecological reconstructions suggest forested environments (gallery forest) along rivers.

Australopithecus anamensis (or Praeanthropus anamensis) is a stem-human species that lived approximately four million years ago. Nearly one hundred fossil specimens are known from Kenya and Ethiopia, representing over 20 individuals. It is accepted that A. anamensis is ancestral to A. afarensis and continued an evolving lineage. Fossil evidence determines that the Australopithecus anamensis is the earliest hominin species in the Turkana Basin.

References

 

Australopithecus afarensis

The best known australopithecine species (3.6-3 Ma). It has been described in various sites, form Afar (Ethiopia) to Laetoli (Tanzania), 1,000 Km appart. It is a highly dimorphic species. Some specimens show primitive traits, such as the honing complex (C/P3) or a small diastema, while other do not. The species also shows a significant dregree of sexual dimorphism. Lucy (AL-288-1) is a well preserved (52 fragments, 40%) female skeleton that is significantly smaller and more gracile than AL-442, a large, complete male skull with robust tori and large canines. The most significant traits of A. afarensis are: strong facial prognathism, large bizigomatic breadth (robust temporalis muscles), marked prenasal sulcus (large canine roots), bell shape cranium (small brain case), strong nucal crest (robust nucal muscle), absence of sagital crest but highly positioned crotafit line (stron temporalis muscle), advanced foramen magnum and large mastoid processes (bipedal locomotion), primitive glenoid fossa and timpanic bone, U-shaped palate (long and strait maxillary branches), conical shape of the rib cage, tall and narrow ileon, not yet lateralized (not a fully developed bipedalism), tibial articular surfaces less excavated than in Ar. ramidus (less specialized bipedalism), curved hand and foot falanges (brachiation), More aligned toe than in Ardipithecus, longer arms than legs (humero-femoral index of 83.9; 74.2 in humans and over 100 in chimpanzee and gorilla), and short stature (1.1-1 m). Given all these traits, A. afarensis is a mosaic of both primitive and derived traits. It likely occupied forested environments with great climbing habilities and a limited bipedalism in more open areas. The paleoecological reconstruction shows both types of environments, more close in Hadar and more open in Laetoli.

Australopithecus afarensis (Southern ape from Afar) is an extinct hominin that lived between 3.9 and 2.9 million years ago. A. afarensis was slenderly built, like the younger Australopithecus africanus, and is thought to be closely related to the genus Homo, whether as a direct ancestor or a close relative of an unknown ancestor. Some researchers include A. afarensis in the genus Praeanthropus. Despite Laetoli being the type locality for A. afarensis, the most extensive remains assigned to the species are found in Hadar, Afar Region of Ethiopia. Other localities bearing A. afarensis remains include Omo, Maka, Fejej, and Belohdelie in Ethiopia, and Koobi Fora and Lothagam in Kenya.

References

Australopithecus bahrelghazali

Found in the Bahr el Ghazal, near Koro Toro (Chad), 2,500 Km West of the Great Rift. Dated to 3.5-3 Ma, it only includes a mandibular fragment (mandibular symphysis). Despite at present the area is a desert, it was a mosaic of gallery forest and savanna environments, similar to those described for A. afarensis in East Africa. Both chronologically and anatomically, this fossil specimen falls well within the variability of Australopithecus afarensis, which suggests that this species was probably highly dispersed away from the Rift Valley.

References

Australopithecus garhi

It was found in Ethiopia, dating 2.4-2.1 Ma. The fossil remains include skull fragmens, teeth and long bones. It has been found in association with cut-marked animal bones, which may imply the use of lithic industry. Its chronological position makes it a candidate for being ancestral to Homo in East Africa, but the cranium shows primitive (australopithecine like) features, not transitional to humans. In addition, reliable Homo fossil specimens have been found in East Africa predating the dates for A. garhi.

Australopithecus garhi is a 2.5-million-year-old gracile australopithecine species whose fossils were discovered in 1996 by a research team led by Ethiopian paleontologist Berhane Asfaw and Tim White, an American paleontologist. The hominin remains are believed to be a human ancestor species and possibly the direct ancestor to the human genus, Homo.

References

  • Asfaw, B; White, T; Lovejoy, O; Latimer, B; Simpson, S; Suwa, G (1999). “Australopithecus garhi: a new species of early hominid from Ethiopia”. Science. 284 (5414): 629–35. doi:10.1126/science.284.5414.629.
  • De Heinzelin, J; Clark, JD; White, T; Hart, W; Renne, P; Woldegabriel, G; Beyene, Y; Vrba, E (1999). “Environment and behavior of 2.5-million-year-old Bouri hominids”. Science. 284 (5414): 625–9. doi:10.1126/science.284.5414.625.

Kenyanthropus platyops

It was found by Meave Leakey in 1998 West of the Turkana Lake in Kenya, and was described in 2001 as a direct ancestor of the Homo genus for its reduced facial prognathism and teeth size compared to Australopithecus. It has been compared to the KNM-ER 1460 Homo rudolfensis specimen dated in 1.8 Ma. The Kenyanthropus fossil has been dated to 3.5 Ma, which is coincident with A. afarensis. Its resemblance to Homo is still debated.

Leakey (2001) proposes that the fossil represents an entirely new hominin species and genus, while others classify it as a separate species of Australopithecus, Australopithecus platyops, and yet others interpret it as an individual of Australopithecus afarensis.

References

  • Kenyanthropus platyops
  • Lieberman, D. E. (2001). Another face in our family tree, Nature 410: 419-420.
  • Leakey, Meave G.; et al. (2001). “New hominin genus from eastern Africa shows diverse middle Pliocene lineages”. Nature. 410 (6827): 433–440. doi:10.1038/35068500.

ii. South African australopithecines

Australopithecus africanus

Fossil remains of this species have been found in Taung, Makapansgat, Sterkfontein, and Gadysvale, with a chronology between 3.5 and 2.3 Ma, though could date back to 4 Ma. The fossil remains are well preserved and show some derived traits compared to A. afarensis, its more likely ancestor. Their main anatomical traits are: short stature (1.29 m and 53 Kg, as suggested by a fairly well preserved skeleton, Sts 14), femoral index between 76.9 and 96.8 (in general longer arms than legs), spinal curvatures as in humans but small bodies of the lumbar vertebrae (not fully developed bipedalism), femoral trocanters unlike humans (less bipedal), small brain case (445 cm³), small Broca’s area present, reduced prognathism compared to A. afarensis, absence of nuchal crests, glenoid cavity unlike humans, rounded and tall neurocranium, lack of diastema, and 90º bucco-lingual axis of the lower P3 as in humans (45º in A. afarensis). The best preserved specimens from Sterkfontein are Sts 5, Sts 17, and Sts 71. Also from Sterkfontein is the specimen StW 553, a complete skeleton associated to the Little Foot specimen. A date of 4.0 Ma has been attributed to this remains. The ecological reconstruction is similar to the environments inhabited by A. afarensis, with a significant impact of the climatic shift between 2.5 and 2 Ma. The suit of derived traits of A. africanus has been considered an indication that this species was ancestral to the Homo lineage. The recent discoveries of Australopithecus sediba and Homo naledi, also from South Africa, have emphasized this view to the extent that many researchers suggest that A. africanus was the direct ancestor of the first humans.

Recent analysis of the Little Foot specimen dated it to about 3.7 million years old, which gives support to the claim by Ronald Clarke that it is of the previously unknown species with characters similar to Paranthropus robustus and named Australopithecus prometheus. Instead, the specimen has usually been lumped into A. africanus by most scholars. Earlier dating had placed it between 3.0 and 2.0 ma based on combined analyses of palaeomagnetism, electron spin resonance, and faunal dating. The Makapansgat fossils have been dated to between 3.0 and 2.6 ma. Those at Sterkfontein currently are dated to between 2.6 and 2.0 ma with the Mrs Ples fossil dating to around 2.0 million years. And Gladysvale fossils were dated between about 2.4 and 2.0 ma. The age of the Taung child remains more difficult to determine and is the focus of a current dating project.

References

  • Clarke, R.J., (2008) “Latest information on Sterkfontein’s Australopithecus skeleton and a new look at Australopithecus”; South African Journal of Science, 104(11 & 12), Nov / Dec, 2008, 443–449. See also “Who was Little Foot?” The Witness, 20 March 2009.
  • Bower, Bruce, (2013) “Notorious Bones“; Science News, August 10, 2013, Vol. 184, No. 3, p. 26.
  • Gardner., Elizabeth K.; Purdue University (April 1, 2015). “New instrument dates old skeleton before ‘Lucy’; ‘Little Foot’ 3.67 million years old”. Science Daily. Retrieved April 3, 2015.
  • Skinner MM; Stephens, NB; Tsegai, ZJ; Foote, AC; Nguyen, NH; Gross, T; Pahr, DH; Hublin, JJ; Kivell, TL; et al. (2015). “Human-like hand use in Australopithecus africanus. Science. 347 (6220): 395–399. doi:10.1126/science.1261735.
  • Herries, A.I.R., Shaw, J. 2011. Palaeomagnetic analysis of the Sterkfontein palaeocave deposits; age implications for the hominin fossils and stone tool industries. J Hum Evol 60, 523-539.
  • Herries, A.I.R.., Hopley, P., Adams, J., Curnoe, D., Maslin, M. 2010. Geochronology and palaeoenvironments of the South African early hominin bearing sites: a reply to ‘Wrangham et al., 2009: Shallow-Water Habitats as Sources of Fallback Foods for Hominins’ Am J Phys Anthropol 143, 640–646.

The gracile australopithecines are clearly distinct from the so called robust australopithecines, which should be more properly refered to as paranthropines. The debate is centered in wether the anatomical traits of the paranthropines, reflecting strong adaptations to hash climatic conditions and a shift in habitat and food exploitation strategies, are sufficient to support a change at the genus level, from Australopithecus to Paranthropus. Many researchers still think that the original Australopithecus genus should be maintained for the robust representatives. However, most scholars use Paranthropus for its significance in pointing out the differential ecological and anatomical differences existing between the two groups. Whatever the case, the term Paranthropus is firmly gaining adepts and it clearly emphasizes the particular evolutionary trends of the robust clade.

References

  • Conroy GC (2005) Reconstructing human origins. A mothern synthesis. WW Norton Co, New York.
  • Coppens Y (1999) Le genou de Lucy. Ed. Odike Jacob. Paris
  • Coppens Y & Picq P (Eds.) Los orígenes de la humanidad. Espasa Calpe, Madrid 2004.
  • Gibbons A (2004) Oldest human femur wades into controversy. Science 305, 1885.
  • Klein R (1999) The human carreer. The University of Chicago Press.
  • McBrearty S & Jablonsky NG (2005) First fossil chimpanzee. Nature 437, 105-108.
  • Patridge TG et al (2003) Lower Pliocene Hominid remains from Sterkfontein. Science 300, 607-612.
  • Tobias PV (2000) The fossil hominids. In: The Cenozoic of Southern Africa. Oxford University Press.
  • Wolpoff M (1999) Paleoanthropology. McGraw Hill, New York.

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