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5. Climate shift and hominin evolution

a. Late Pliocene Climatic Shift

i. The South African australopithecine lineage

Resultat d'imatges de pliocene pleistoceneThe progressive climatic cooling initiated during the Middle Miocene (Miocene Climatic Optimum that gave rise to the Eurasian Hominoidea) continued during the Pliocene, though with a short warm period at around 3.5-4 Ma when average global temperature were 3ºC higher than today. This climatic plateau finished when the Panama istmus closed 2.8 Ma, joining the North and South American continents, which interrupted the oceanic currents between the Atlantic and Pacific oceans and established the current cold circulations around the Antarctic continent. Glaciers appeared in North America and Europe. In western Africa the forest was progressively substituted by more open environments. The savanna environments predominated in the Rift Valley and all the species in the area had to adapt to the new prevailing conditions. The vegetation changed. C3 plants, adapted to humid environments, were substituted by C4 plants, adapted to arid environments; grazer animals (eating leaves and fruits) either disappeared of adapted to browse grasses and sedges. The hominines were also affected by this climatic change. The australopithecines disappeared in East Africa, migrating to South Africa, and new taxa, more adapted to the new environmental conditions, substituted them: the genera Paranthropus and Homo.

Australopithecus africanus

Three main sites include fossil specimens of this taxon: Swartkrans, Sterkfontein and Taung, dating from the Late Miocene to the Early Pleistocene (between 4-3.5 and 2.3 Ma). This species is likely a descendant from Australopithecus afarensis that migrated to South Africa as a consequence of the climatic shift, which arrived somewhat later to the South of the continent. A. africanus was similar in many traits to A. afarensis, a bipedal hominin with arms slightly longer than the legs (a physical trait also found in chimpanzees). It has slightly human-like, advanced cranial features (seen in the crania of Mrs. Ples and STS 71), but also presents primitive features including ape-like, curved fingers adapted to tree climbing. Some researchers believe that A. africanus evolved into Paranthropus robustus. Both P. robustus and A. africanus crania seem very alike despite the more heavily built (robust) features of P. robustus, which are adaptations for heavy chewing. However, A. africanus had a cranium which quite closely resembled that of a modern chimpanzee (both brains measure about 400 cc to 500 cc) and a pelvis that would enable more efficient bipedalism than that of A. afarensis. Recent studies suggest that hand bones in A. africanus indicate a “human-like trabecular bone pattern in the metacarpals consistent with forceful opposition of the thumb and fingers typically adopted during tool use”. Such a morphology would support an earlier time for making and using tools than previously had been thought likely. Evidence of human-like sexual dimorphism in the lumbar spine has recently been described in A. africanus. This character of morphology has been seen as an evolutionary adaptation by female bipeds in order to more efficiently bear load on the lumbar column during pregnancy, an adaptation that non-bipedal primates do not need.

The Little Foot specimen has been dated to about 3.7 million years old, which gives support to the claim 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. The Makapansgat fossils have been dated to between 3.0 and 2.6 Ma. Those at Sterkfontein 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.

Australopithecus sediba (Malapa, South Africa – 2 Ma)

Australopithecus sediba is a species of Australopithecus of the early Pleistocene, identified based on fossil remains dated to about 2 Ma. The species is known from six skeletons discovered in the Malapa Fossil Site at the Cradle of Humankind World Heritage Site in South Africa, including a juvenile male (MH1 also called “Karabo”, the holotype), an adult female (MH2, the paratype), an adult male, and three infants. The fossils were found together at the bottom of the Malapa Cave and have been dated to between 1.980 and 1.977 million years ago. Over 220 fragments from the species have been recovered to date. The partial skeletons were initially described in two papers in the journal Science by American and South African palaeo-anthropologist Lee R. Berger from the University of the Witwatersrand, Johannesburg, and colleagues as a newly discovered species of early human ancestor called Australopithecus sediba (“sediba” meaning “natural spring” or “well” in the Sotho language). MH1 is disarticulated and 34% complete while MH2 is 45.6% complete and exhibits partial articulation. Australopithecus sediba may have lived in savannas but ate fruit and other foods from the forest (behavior similar to modern-day savanna chimpanzees). The conditions in which the individuals were buried and fossilized were extraordinary, permitting the extraction of plant phytoliths from dental plaque.

Because of the wide range of mosaic features exhibited in both cranial and post-cranial morphology, A. sediba may be a transitional species between the southern African A. africanus (the Taung Child or Mrs. Ples) and either Homo habilis or even the later H. erectus (Turkana boy). The cranial capacity of MH1, which has been estimated to be at 95% of adult capacity (420 cm3), is at the higher end of the range for A. africanus and far from the lower range of early Homo (631 cm3), but the mandible and tooth size are quite gracile and similar to what one would expect to find in H. erectus; so similar are these features that, if found in isolation without other skeletal remains, they could be classified as Homo based on tooth and mandible size. However, the cusp spacing is more like Australopithecus. Regardless of whether Australopithecus sediba is a direct ancestor of early Homo or not, our understanding of the range of variation in early hominins has been greatly increased with the finding of these new specimens.

A. sediba compared to its ancestor species A. africanus on the whole is described as more derived towards Homo than A. garhi, especially showing a number of synapomorphies taken to anticipate the reorganization of the pelvis in H. erectus, associated with “more energetically efficient walking and running”. The femur and tibia are fragmentary, but the foot combines an advanced anklebone combined with a primitive heel. Its cranial capacity is estimated at around 420–450 cm3 (26–27 cu in), about one-third of that of modern humans (1,200 cm3).

MH1 (left), Lucy (centre) and MH2 (right) Fossil remains described in 2010, superimposed on a generalized australopithecine background. A. sediba skeletons approximately 1.3 meters tall.

A. sediba had a surprisingly modern hand, whose precision grip suggests it might have been another tool-making Australopithecus. Evidence of the precision gripping and stone tool production can be seen from Homo-like features such as having a long thumb and short fingers. The nearly complete wrist and hand of an adult female from Malapa, South Africa presents Australopithecus-like features, such as a strong flexor apparatus associated with arboreal locomotion. A well-preserved and articulated ankle of A. sediba is mostly humanlike in form and function and possesses some evidence for a humanlike arch and Achilles tendon. However, A. sediba is apelike in possessing a more gracile calcaneal body and a more robust medial malleolus than expected. This shows that A. sediba may have practiced a unique combination of bipedalism and arboreality. As opposed to the authors of the initial description, who interpreted both fossils as a possible transitional species between Australopithecus and Homo, other palaeoanthropologists are reluctant to do so. In an accompanying news article published with the initial descriptions in 2010, detractors of the idea that A. sediba might be ancestral to the genus Homo (e.g. Tim White and Ron Clarke) suggest that the fossils could be a late southern African branch of Australopithecus, co-existing with already existing members of the Homo genus. This interpretation is based on the observation that the lower jaw, discovered by Friedemann Schrenk, of a 2.5 million year old fossil attributed to H. rudolfensis is the oldest known fossil ascribed to the genus Homo. This specimen is thus presumed to be older than the Australopithecus sediba fossils. Additionally, the basing of the description of the species largely on the skeleton of a juvenile specimen has been subject to criticism, given that there is no certain way of saying to what extent adults would differ from juvenile specimens. More recent reviews appear to have accepted the specific status of A. sediba as an independent species from A. africanus and other early hominins. Moreover, the use of juvenile specimens as type specimens has been a common practice in palaeoanthropology: the type specimens of Homo habilis and Australopithecus africanus are themselves juvenile specimens.

References

  • Dirks, Paul HGM, et al. Geological setting and age of Australopithecus sediba from southern Africa. Science 328.5975 (2010): 205-208.
  • Pickering, Robyn, et al. Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo. Science 333.6048 (2011): 1421-1423.
  • Henry, Amanda G., et al. The diet of Australopithecus sediba. Nature (2012).

 

Homo naledi (Gauteng, South Africa – 0.285 Ma)

Homo naledi is an extinct species of hominin, which anthropologists first described in 2015 and have assigned to the genus Homo. In 2013, fossil skeletons were found in the Gauteng province of South Africa, in the Rising Star Cave system, part of the Cradle of Humankind World Heritage Site about 50 km northwest of Johannesburg. Prior to dating, initial judgement based on archaic features of its anatomy favoured an age of roughly two million years old. In 2017, however, the fossils were dated to between 335,000 and 236,000 years ago, long after much larger-brained and more modern-looking hominins had appeared. The research team therefore thinks that H. naledi is not a direct ancestor of modern humans, although it is probably an offshoot within the genus Homo. The species is characterised by a body mass and stature similar to small-bodied human populations, a smaller endocranial volume similar to Australopithecus, and a skull shape similar to early Homo species. The skeletal anatomy presents ancestral features known from australopithecines with more recent features associated with later hominins. As of 10 September 2015, fossils of at least fifteen individuals, amounting to more than 1550 specimens, have been excavated from the cave. Newer findings (remains of at least three individuals: two adults and a child) in a second chamber, known as Lesedi (“light” in the Sotho-Tswana languages), were reported by Hawks et al. (2017). The word “naledi” means “star”. It, and the corresponding name Dinaledi Chamber (“chamber of stars”), were chosen to reference the Rising Star cave system where the fossils were found.

Paleoanthopologist Tim D. White thinks that, based on the published descriptions, the fossils belong to a primitive Homo erectus. Anthropologist Chris Stringer also stated that the fossils look most similar to the small-bodied examples of Homo erectus from Dmanisi in Georgia, which have been dated at ∼1.8 million years old. Lee Berger rejected the possibility of the fossils representing H. erectus.

References

  • Morphological affinities of Homo naledi with other Plio-Pleistocene hominins: a phenetic approach. Neves WA, Bernardo DV, Pantaleoni I. An Acad Bras Cienc 2017 Jul 24:0. doi: 10.1590/0001-3765201720160841.
  • Behavioral inferences from the high levels of dental chipping in Homo naledi.Towle I, Irish JD, De Groote I. Am J Phys Anthropol 2017 Sep; 164(1):184-192.
  • Homo naledi and Pleistocene hominin evolution in subequatorial Africa. Berger LR, Hawks J, Dirks PH, Elliott M, Roberts EM. Elife 2017 May 9; 6. pii: e24234. doi: 10.7554/eLife.24234.
  • The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa. Dirks PH, et al. Elife 2017 May 9; 6. pii: e24231. doi: 10.7554/eLife.24231.
  • New fossil remains of Homo naledi from the Lesedi Chamber, South Africa. Hawks J, et al. Elife 2017 May 9; 6. pii: e24232. doi: 10.7554/eLife.24232.
  • New opportunities rising. Thompson JC. Elife 2017 May 9; 6. pii: e26775. doi: 10.7554/eLife.26775.
  • The vertebrae and ribs of Homo naledi. Williams SA, et al. Hum Evol 2017 Mar; 104:136-154. doi: 10.1016/j.jhevol.2016.11.003.
  • The skull of Homo naledi. Laird MF, Schroeder L, Garvin HM, Scott JE, Dembo M, Radovčić D, Musiba CM, Ackermann RR, Schmid P, Hawks J, Berger LR, de Ruiter DJ. J Hum Evol 2017 Mar; 104:100-123. doi: 10.1016/j.jhevol.2016.09.009.
  • The thigh and leg of Homo naledi. Marchi D, Walker CS, Wei P, Holliday TW, Churchill SE, Berger LR, DeSilva JM. J Hum Evol 2017 Mar; 104:174-204. doi: 10.1016/j.jhevol.2016.09.005.
  • The upper limb of Homo naledi. Feuerriegel EM, Green DJ, Walker CS, Schmid P, Hawks J, Berger LR, Churchill SE. J Hum Evol 2017 Mar;104:155-173. doi: 10.1016/j.jhevol.2016.09.013.
  • Skull diversity in the Homo lineage and the relative position of Homo naledi. Schroeder L, et al. J Hum Evol 2017 Mar; 104:124-135. doi: 10.1016/j.jhevol.2016.09.014. 9.
  • The evolutionary relationships and age of Homo naledi: An assessment using dated Bayesian phylogenetic methods. Dembo M, et al. J Hum Evol 2016 Aug; 97:17-26. doi: 10.1016/j.jhevol.2016.04.008.
  • Early hominin diversity and the emergence of the genus Homo. Harcourt-Smith W. J Anthropol Sci. 2016 Jun 20;94:19-27.

ii. The East African paranthropine lineage

The so called “robust australopithecines” derive from the 2.7-2.3 Ma old fossil species Paranthropus aethiopicus, whose best preserved specimen is WT-17000, from West Turkana (Kenya, 2.5 Ma). This and other remains assigned to the same species come from the Shungura Formation (Omo River, Ethiopia), Nachukui Formation (West Turkana, Kenya), and Ndolanya Beds (Laetoli, Tanzania). Between 2.4 and 1.8 Ma two other species appeared, most likely derived from P. aethiopicus: P. boisei in East Africa and P. robustus in South Africa. The three taxa share derived features not present in the australopithecines, such as a robust cranium, with nuchal and sagittal crests, and large teeth that are indicative of a powerful bite. Paranthropus boisei was contemporary to Homo habilis in East Africa. These two genera represent distinct adaptations to the climatic shift in Africa at around 2 Ma.

Paranthropus aethiopicus (West Turkana, Kenya – 2.5 Ma)

Resultat d'imatges de aethiopicus

KNM WT 17000

The paranthropines share many characteristics of the cranium and mandible, suggesting a shared evolutionary development. Paranthropus aethiopicus has notable features that differ from the other paranthropines, including a larger zygomatic arch, extended ramus of the mandible, and a more prognathic face. The skull is dated to 2.5 million years ago, older than the other two paranthropine forms (P. aethiopicus lived between 2.7 and 2.5 Ma). The features are quite primitive and share many traits with Australopithecus afarensis; thus P. aethiopicus is likely to be a direct descendant. With its face being as prognathic (projecting) as A. afarensis, its brain size was also quite small (410 cm³). P. aethiopicus was first proposed in 1967 to describe a toothless partial mandible (Omo 18) found in Ethiopia. Lower jaw and teeth fragments have also been uncovered. P. aethiopicus had a large sagittal crest and zygomatic arch adapted for heavy chewing (as in gorilla skulls). Not much is known about this species since the best evidence comes from the “Black Skull” and the jaw. There is not enough material to make an assessment of how tall they were, but they may have been as tall as Australopithecus afarensis. Paranthropus aethiopicus is considered a megadont archaic hominin (the anterior dentition is also large, which differs from the later paranthropines). The initial discovery was a toothless adult mandible in the Shungura formation of the Omo region of Ethiopia in 1967 (Omo 18.18). The ash layers above and below the fossils give an approximate date of 2.3-2.5 Ma. There is only one mostly complete skull for this hominin, so it’s hard to make inferences about physical characteristics.

References

  • Melanie A. McCollum (1999) The Robust Australopithecine Face: A Morphogenetic Perspective.” Science 284(5412), 301-305.
  • Wood, Bernard and Nicholas Lonergan (2008) The Hominin Fossil Record: Taxa, Grades and Clades. Journal of Anatomy 212(4): 354-376.

Paranthropus boisei (East Rudolf, Kenya – 1.7 Ma) § Peninj (1.5 Ma) & Olduvai (1.8-1.2 Ma), Tanzania

Resultat d'imatges de paranthropus boisei

Paranthropus boisei OH 5

The first Paranthropus boisei specimen was found at BK, Lower Bed II, Olduvai Gorge, in 1955: OH3 (Olduvai Hominid 3), showing decidual canines and molars. The cranial morphology of the species was described after Mary and Louis Leakey’s discovery of the specimen OH 5 in July 1959, dated to 1.8 Ma and known as Dear boy (Nutcracker). It is an adult skull with 530 cm³. No mandibles were found at Olduvai but in 1964 in Lake Natron an adult mandible (NMT-W64-160, Peninj 1) was found, with robust traits similar to those of the OH 5 skull. Fossil specimens from Paranthropus boisei have been discovered at various sites: West Turkana (Kenya, 2.3-1.6 Ma), Chiwondo Beds (Malawi, 2.3 Ma), Omo (Ethiopia, 2.3-1.2 Ma), Koobi Fora (Kenya, 2.2-1.4 Ma), Olduvai (Tanzania, 1.8-1.2 Ma), Peninj (Tanzania, 1.7 Ma), Chesowanja (Kenya, 1.5 Ma), and Konso (Ethiopia, 1.4 Ma). The most significant anatomical traits of P. boisei include: well developed nuchal and sagittal crests, reduced facial prognathism compared to P. aethiopicus, robust vertical face (better bitting strength), broad zygomatic arches, strong masseter and temporalis muscles, very large and robust premolars and molars, reduced incisors and canines (which differs from P. aethiopicus), molarized premolars (dental size increases significantly between 2.3 and 2 Ma), thick enamel in molar teeth. The lower second premolar (LP4) shows 3 roots in a high percentage of specimens (which constitutes an apomorphic trait of the species), the brain is still small (but somewhat larger than in the australopithecines), and includes the Brocca and Wernicke areas (facultative communication capabilities). Some representative fossil sites and specimens of this species are:

  • OH 5 Zinjanthropus, “Zinj” or “Nutcracker Man”, was the first P. boisei specimen found by Mary Leakey at Olduvai Gorge, Tanzania.
  • KNM ER 406 is a small partial cranium discovered by Richard Leakey and H. Mutua in 1969, found at Koobi Fora, Kenya, which displays large zygomatic arches and a cranial capacity of 510 cm³ (c. 1.7 ma).
  • Peninj Mandible is a well-preserved jaw, found by Kamoya Kimeu in the Lake Natron region, near the Peninj River in Tanzania (c. 1.5 ma).[citation needed]

References

  • Wood, Bernard; Constantino, Paul (2007). Paranthropus boisei: Fifty years of evidence and analysis”. American Journal of Physical Anthropology 134 (Suppl 45): 106–132.
  • Leakey, Louis SB. “A new fossil skull from Olduvai.” Nature 184.4685 (1959): 491-493.
  • Fleischer, R. L., et al. “Fission-Track Dating of Bed I, Olduvai Gorge.” Science 148.3666 (1965): 72-74.
  • Leakey, Mary Douglas. Olduvai gorge. Eds. Phillip V. Tobias, and L. S. B. Leakey. Vol. 2. Cambridge University Press, 1967.
  • Blumenschine, Robert J., Ian G. Stanistreet, and Fidelis T. Masao. “Olduvai Gorge and the Olduvai Landscape Paleoanthropology Project.” Journal of Human Evolution 63.2 (2012): 247-250.

iii. The South African paranthropines

Paranthropus robustus (South Africa, 2.25-1 Ma)

Resultat d'imatges de drimolen

Paranthropus robustus DNH 7 Drimolen

Paranthropus robustus is an early hominin, discovered in Southern Africa in 1938. Particularly regarding cranial features, the development of P. robustus seemed to be in the direction of a “heavy-chewing complex”. On account of the definitive traits associated with this “robust” line of australopithecines, anthropologist Robert Broom established the genus Paranthropus and placed this species in it. Paranthropus robustus is generally dated to have lived between 2.0 and 1.2 Ma. It had large jaws and jaw muscles with the accompanying sagittal crest, and post-canine teeth that were adapted to serve in the dry environment they lived in. Typical of the robust forms, P. robustus had a head with a massive built jaw and teeth, the sagittal crest acts as an anchor for large chewing muscles. The DNH 7 skull of Paranthropus robustus, “Eurydice”, was discovered in 1994 at the Drimolen Cave in Southern Africa by Andre Keyser, and is dated to 2.3 Ma, belonging to a female, the teeth were larger and thicker than any gracile australopithecine. The males may have stood only 1.2 m tall and weighed 54 kg while females stood just under 1 meter tall and weighed only 40 kg, indicating a large sexual dimorphism. The teeth found on P. robustus are almost as large as those of P. boisei, the average brain size of P. robustus is 410-530 cm³. Several sites in South Africa contain remains of this species:

  • Kromdraai B (KB) (Gauteng), 2-1,5 Ma and Kromdraai E (KE).
  • Sterkfontein Member 5, 2.18 Ma.
  • Drimolen, 2-1.5 Ma.
  • Swartkrans, Member 1 (2.25-1.7 Ma), Member 2 (1.7 Ma) & Member 3 (1 Ma).

References

  • Wood, B. & Strait, D. (2004). Patterns of resource use in early Homo and Paranthropus. Journal of Human Evolution 46 (2), 119–162.
  • Broom, Robert, and John Talbot Robinson. “Man contemporaneous with the Sawartkrans ape‐man.” American journal of physical anthropology 8.2 (1950): 151-156.
  • Robinson, John Talbot. “Further remarks on the relationship between “Meganthropus” and australopithecines.” American journal of physical anthropology 13.3 (1955): 429-445.
  • Sillen, Andrew. “Strontium-calcium ratios (Sr/Ca) of Australopithecus robustus and associated fauna from Swartkrans.” Journal of Human Evolution 23.6 (1992): 495-516.
  • Lee-Thorp, Julia A., Nikolaas J. van der Merwe, and C. K. Brain. “Diet of Australopithecus robustus at Swartkrans from stable carbon isotopic analysis.” Journal of Human Evolution 27.4 (1994): 361-372.
  • Keyser, André W. “The Drimolen skull: the most complete australopithecine cranium and mandible to date.” South African Journal of Science 96.4 (2000): 189-192.
  • Blackwell, Lucinda, and Francesco d’Errico. “Early hominid bone tools from Drimolen, South Africa.” Journal of Archaeological Science 35.11 (2008): 2880-2894.
  • Moggi-Cecchi, Jacopo, et al. “Early hominin dental remains from the Plio-Pleistocene site of Drimolen, South Africa.” Journal of Human Evolution 58.5 (2010): 374-405.

b. Ecology and adaptation

Resultat d'imatges de paranthropus dietAs seen in the previous chapters, Ardipithecus ramidus lived in forested environments. Bipedalism, thus, evolved is closed environments, rather than in the open savannas. The final cause of bipedalism should be found in 4 main aspects of hominin adaptation; improvement in food adquisition, avoidance of predators, reproductive success and energetic balance. In chimpanzees, over 80% of bipedal activity was related to food procurement in forested areas, while only 4% of the bipedal activity was for locomotion. This facultative bipedal locomotion of the chimpanzees gradually evolved till the Laetoli footprints (3.6 Ma) that indicate an efficient locomotor bipedal behaviour. The arboreal bipedalism of Australopithecus afarensis was likely a balance between the search for safety in the trees and for food on the ground. Their diet might have included suculent fruits, similar to the chimpanzee, perhaps also on a seasonal basis. Despite the marked sexual dimporphism, suggestive of a dominance polyginic behaviour, the canines gradually reduced until the honing complex disappeared. A. afarensis occupied various types of environments, from gallery forest to open savannas. The open savanna chimpanzee, model of social behaviour would suit this species, with a reduced polyginic structure with exogamy of females. The progressive cooling of the climatic conditions along the Pleistocene forced the hominines to exploit more open environments. The paranthropines represent an especialized lineage that diverged to adapt to open environments by consuming tough and hard foodstuffs using their large postcanine dentition and strong masticatory muscles. Despite alternative hypotheses, it is likely that the robust paranthropines first appeared in East Africa, with Paranthropus aethiopicus being the ancestor of both the later East and South African Paranthropus species. Both species (P. boisei and P. robustus) share many anatomical traits and suggest similar dietary adaptations. However, isotopic and microwear studies have shown that the two species had very distinct dietary signals, more C4 oriented for P. boisei, that was the first hominin to exploit open environments. Isotopic data shows that P. boisei had a diet mainly based on C4 resources (75%), which contradicts the assumption that it consumed a diet similar to modern chimpanzees. However, occlusal microwear research (Ungar et al., 2011) confirmed the absence of microwear patterns indicative of hard foods consumption. Gabiele A. Macho (2014) suggested that it could have a diet similar to that of the young baboons living in similar areas, which consume Cyperus esculentus (a hard food) and other fruits and invertebrates.

c. Plio-Pleistocene hominin phylogeny

Resultat d'imatges de hominin phylogenyThe phylogenetic relationships among the Plio-Pleistocene hominins is not yet fully stablished. If we let aside the hominin lineage leading to the genus Homo, the species Sahelanthropus (for showing derived traits in the supraorbital torus) and Orrorin (for showing an especialized bipedism compared to later hominines), Ardipithecus is a likely candidate for a basal hominin. Its anatomy clearly shows that such candidate did not likely showed traits similar to the modern chimpanzee. Anatomically, the chimpanzee is noy a model to explain the adaptations of our lineage. Our most remote ancestor was likely an arboreal, climbing form from which more derived bipedals evolved gradually. Australopithecus anamensis shows somewhat derived bipedal adaptations in the tibial facets, but dentally is a good candidate because it shows clear signs of enamel thickening (not yet present in Ardipithecus), and therefore is a precursor of Australopithecus afarensis. This species shows great anatomical diversity, as well as marked sexual dimorphism. Climatic conditions between 3 and 2 Ma determined the diversification, both geographically and anatomically, of the hominines, giving rise to the South African australopithecines and the East and South African paranthropines. This diversification can be seen in the various species that have been described: A. bahrelghazali, A. garhi, A. africanusA. sediba, P. aethiopicus, P. boisei, and P. robustus. The origin of the genus Homo is also characterized by great diversity, as will show the next chapter. The derived cranial traits of  A.  africanus have been used to suggest that this species could be ancestral to Homo. However, the most ancient Homo specimens do not yet show these derived traits. Instead, A. africanus is likely ancestral to A. sediba, with whom shares many derived traits, while others are exclusive of A. sediba. The initial reports of Homo naledi dated this species up to 3 Ma, but its actual date (around 350,000 years) separates if from A. sediba. Despite A. sediba shows some derived traits in line with later Homo specimens, it is still unclear that A. sediba, or even A. africanus, is the ancestor of Homo. However, East African candidates for being ancestral to Homo are scarce. Australopithecus garhi shows australopithecine anatomical traits and the evidence of use of lithic industries is not exclusive of this fossil australopithecines (eventually Australopithecus will be accepted as a tool making species). Still an anatomical transitional between Australopithecus and Homo is lacking.

References

  • De Menocal P (1995) Plio-Pleistocene African climate. Science 270, 53-59.
  • Ruff C (1001) Climate and body shape in hominid evolution. J Hum Evol 21, 81-105.
  • Vrba ES et al. (1996) Paleoclimate and evolution with emphasis on human origins. Yale University Press.

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