Australopithecus africanus

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Australopithecus africanus
Temporal range: 3.3-2.1 
Ma
Late Pliocene - Early Pleistocene
Mrs. Ples, at the University of Zurich
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Genus: Australopithecus
Species:
A. africanus
Binomial name
Australopithecus africanus
Dart, 1925
Synonyms
  • Australopithecus transvaalensis
    Broom, 1936
  • Plesianthropus transvaalensis
    Broom, 1938
  • Australopithecus prometheus?
    Dart, 1949

Australopithecus africanus is an

hominin found. However, its closer relations to humans than to other apes would not become widely accepted until the middle of the century because most had believed humans evolved outside of Africa. It is unclear how A. africanus relates to other hominins, being variously placed as ancestral to Homo and Paranthropus, to just Paranthropus, or to just P. robustus. The specimen "Little Foot
" is the most completely preserved early hominin, with 90% of the skeleton intact, and the oldest South African australopith. However, it is controversially suggested that it and similar specimens be split off into "A. prometheus".

A. africanus brain volume was about 420–510 cc (26–31 cu in). Like other early hominins, the

biped, albeit less efficient at walking than humans. A. africanus also had several upper body traits in common with arboreal non-human apes. This is variously interpreted as either evidence of a partially or fully arboreal lifestyle, or as a non-functional vestige from a more apelike ancestor. The upper body of A. africanus is more apelike than that of the East African A. afarensis
.

A. africanus, unlike most other primates, seems to have exploited

patrifocal, with females more likely to leave the group than males. A. africanus lived in a gallery forest surrounded by more open grasslands or bushlands. South African australopithecine remains probably accumulated in caves due to predation by large carnivores (namely big cats), and the Taung child appears to have been killed by a bird of prey
. A. africanus probably went extinct due to major climatic variability and volatility and possibly competition with Homo and P. robustus.

Research history

Taung child from the collection of the University of the Witwatersrand

In 1924, Australian anatomist Professor

bipedal. Dart, after hastily freeing the fossil from its matrix, already in January 1925 named the specimen as a new genus and species: Australopithecus africanus. At this time, great apes were classified into the family Pongidae encompassing all non-human fossil apes, and Hominidae encompassing humans and ancestors. Dart felt the Taung child fit into neither, and erected the family "Homo-simiadæ" ("man-apes").[2]: 284–286  This family name was soon abandoned, and Dart proposed "Australopithecidae" in 1929. In 1933, South African palaeoanthropologist Robert Broom suggested moving A. africanus into Hominidae, which at the time contained only humans and their ancestors.[2]
: 285 

A. africanus was the first evidence that humans evolved in Africa, as

The Descent of Man. However, Dart's claim of the Taung child as the transitional stage between apes and humans was at odds with the-then popular model of human evolution which held that large brain size and humanlike characteristics had developed rather early on, and that large brain size evolved before bipedalism. Resultantly, A. africanus was generally cast aside as a member of the gorilla or chimpanzee lineages, most notably by Sir Arthur Keith. This view was perpetuated by Charles Dawson's 1912 hoax Piltdown Man hailing from Britain. Further, the discovery of the humanlike Peking Man (Homo erectus pekinensis) in China also seemed to place the origins of humankind outside of Africa. Humanlike characteristics of the Taung child were attributed to the specimen's juvenile status, meaning they would disappear with maturity. Nonetheless, Dart and Broom continued to argue that Australopithecus was far removed from chimpanzees, showing several physical and claiming some behavioural similarities with humans.[2]: 285–288  To this extent, Dart made note of the amalgamations of large mammal bone fragments in australopithecine-bearing caves which are now attributed to hyena activity.[3] However, Dart proposed that the bones were instead evidence of what he named the "osteodontokeratic culture" produced by australopithecine hunters, who manufactured weapons using the long bones, teeth, and horns of large hoofed prey:[4]

On this thesis man's predecessors differed from living apes in being confirmed killers: carnivorous creatures, that seized living quarries by violence, battered them to death, tore apart their broken bodies, dismembered them limb from limb, slaking their ravenous thirst with the hot blood of victims and greedily devouring livid writhing flesh.

Mrs. Ples at the Ditsong National Museum of Natural History, Pretoria

Broom was one of the few scientists defending the close human affinities of Australopithecus africanus. In 1936, he was informed by two of Dart's students, Trevor R. Jones and G. Schepers, that human-like remains had been discovered in the Sterkfontein Cave quarries. On 9 August 1936, he asked G.W. Barlow to provide him with any finds. On 17 August 1936 he received an adult skull including a natural endocast, specimen Sts 60. However, Broom classified it as a new species, "A. transvaalensis",[5][6] and in 1938 moved it into a new genus as "Plesianthropus transvaalensis". He also discovered the robust australopithecine Paranthropus robustus, showing evidence of a wide diversity of Early Pleistocene "man-apes".[7] Before World War II, several more sites bore A. africanus fossils. A detailed monograph by Broom and palaeoanthropologist Gerrit Willem Hendrik Schepers in 1946 regarding these australopithecines from South Africa, as well as several papers by British palaeoanthropologist Sir Wilfrid Le Gros Clark, had turned around scientific opinion, garnering wide support for A. africanus' classification as a human ancestor.[2]: 289–290  In 1947, the most complete skull was discovered, STS 5 ("Mrs. Ples").[8] Wider acceptance of A. africanus prompted re-evaluation of Piltdown Man in 1953, revealing its falsehood.[2]: 290 

Little Foot skeleton in situ; view of the entire skeleton above and of the skull below

In 1949, Dart recommended splitting a presumed-female facial fragment from

Lee Rogers Berger and John D. Hawks considered "A. prometheus" a nomen nudum ("naked name"), and has not been properly described with diagnostic characteristics which separate it from A. africanus.[14] At the time, these remains were dated to 3.3 million years ago in the Late Pliocene. In 2019, Clarke and South African palaeoanthropologist Kathleen Kuman redated StW 573 to 3.67 million years ago, making it the oldest Australopithecus specimen from South Africa. They considered its antiquity further evidence of species distinction, drawing parallels with A. anamensis and A. afarensis from Middle Pliocene East Africa.[15] Little foot is the most complete early hominin skeleton ever recovered, with about 90% preserved.[16]

Locations of A. africanus discoveries

In addition to Taung, Sterkfontein, and Makapansgat, A. africanus was in 1992 discovered in

Gladysvale Cave. The latter three are in the Cradle of Humankind.[17] Many hominin specimens traditionally assigned to A. africanus have been recovered from Sterkfontein Member 4 (including Mrs. Ples and 2 partial skeletons), previously dated to 2.8 to 2.15 million years ago. But in 2022 a team including Clarke and Kuman used cosmogenic nuclide techniques to date Member 4 at 3.4 million years, which it says discredits the assumption that A. africanus descended from A. afarensis.[18] However, given the wide range of variation exhibited by these specimens, it is debated if all these elements can be confidently assigned to only A. africanus.[19][20]

At present, the classification of australopithecines is in disarray. Australopithecus is considered a grade taxon, whose members are united by their similar physiology rather than close relations with each other over other hominin genera. It is unclear how A. africanus relates to other hominins.[21] The discovery of Early Pleistocene Homo in Africa during the latter half of the 20th century placed humanity's origins on the continent and A. africanus as ancestral to Homo. The discovery of A. afarensis in 1978, at the time the oldest known hominin, prompted a hypothesis that A. africanus was ancestral to P. robustus, and A. afarensis was the last common ancestor between Homo and A. africanus/P. robustus.[22] It is also suggested that A. africanus is closely related to P. robustus but not to the other Paranthropus species in East Africa,[23] or that A. africanus is ancestral to all Paranthropus.[24] A. africanus has also been postulated to have been ancestral to A. sediba which also inhabited the Cradle of Humankind, perhaps contemporaneously. A. sediba is also postulated to have been ancestral to Homo, which if correct would indeed put A. africanus in an ancestral position to Homo.[25]

African
hominin timeline (in mya)
View references
H. sapiensH. nalediH. rhodesiensisH. ergasterAu. sedibaP. robustusP. boiseiH. rudolfensisH. habilisAu. garhiP. aethiopicusLD 350-1K. platyopsAu. bahrelghazaliAu. deyiremedaAu. africanusAu. afarensisAu. anamensisAr. ramidusAr. kadabba


Anatomy

Skull

Beijing Museum of Natural History

Based on 4 specimens, the A. africanus brain volume averaged about 420–510 cc (26–31 cu in). Based on this, neonatal brain size was estimated to have been 165.5–190 cc (10.10–11.59 cu in) using trends seen in adult and neonate brain size in modern primates. If correct, this would indicate that A. africanus was born with about 38% of its total brain size, which is more similar to non-human great apes at 40% than humans at 30%.

semicircular canals like non-human apes, as well as loose turns at the terminal end of the cochlea like humans. Such a mix may reflect habitual locomotion both in the trees and walking while upright because inner ear anatomy affects the vestibular system (sense of balance).[27]

A. africanus had a

eye sockets, and a sagittal crest.[13] A. africanus has a wide range of variation for skull features, which is typically attributed to moderate to high levels of sexual dimorphism in that males were more robust than females.[28]

Build

In 1992, American anthropologist Henry McHenry estimated an average weight (when assuming humanlike or apelike body proportions, respectively) of 40.8 or 52.8 kg (90 or 116 lb) for males based on five partial leg specimens, and 30.2 or 36.8 kg (67 or 81 lb) for females based on seven specimens.[29] In 2015, American anthropologist William L. Jungers and colleagues similarly reported an average weight (without attempting to distinguish males from females) of 30.7 kg (68 lb) with a range of 22.8–43.3 kg (50–95 lb) for weight based on 19 specimens.[30] Based on seven specimens, McHenry estimated that males, on average, grew to 138 cm (4 ft 6 in) tall and females 125 cm (4 ft 1 in).[31] In 2017, based on 24 specimens, anthropologist Manuel Will and colleagues estimated a height of 124.4 cm (4 ft 1 in) with a range of 110–142 cm (3 ft 7 in – 4 ft 8 in).[32] The elderly, probably female StW 573 was estimated to have stood about 130 cm (4 ft 3 in).[33]: 7 

Based on the A. afarensis skeleton

thoracic inlet unlike A. afarensis and humans. The clavicle is proportionally quite long, with a similar absolute length to that of modern humans.[33]
: 12 

Sts 14 sacrum

Like in modern women, L3–L5 curve outwards in specimen StS 14, whereas these are straighter in StW 431 as in modern men. This probably reflects reinforcement of the female spine to aid in walking upright while pregnant.

birth canal compared to neonate head size, and thus a non-rotational birth (unlike humans), though this is debated. When standing, the angle between the sacrum and the lumbar vertebrae was reconstructed to have been about 148.7°, which is much more similar to that of chimps (154.6°) than humans (118.3°). This would indicate A. africanus standing posture was not as erect as in humans.[37]

Limbs

The A. africanus hand and arm exhibit a mosaic anatomy, with some aspects more similar to humans and others to non-human apes. It is unclear if this means australopiths were still arboreal to a degree, or if these traits were simply inherited from the

precision grip and forceful opposition between the thumb and fingers. The adoption of such a grip is typically interpreted as an adaptation for tool making at the expense of efficient climbing and arboreal habitation.[39]

Australopithecus africanus pelvis and vertebrae at the American Museum of Natural History

The leg bones clearly show that A. africanus habitually engaged in bipedal locomotion, though some aspects of the

adducted big toe (it was not dextrous) like humans, A. africanus likely did not push off with the big toe, using the side of the foot instead. StW 573 is the oldest hominin specimen with an adducted big toe. The specimen StW 355 is the most curved proximal foot phalanx bone of any known hominin, more similar to that of orangutans and siamangs.[43]

The arms of StW 573 were about 53.4 cm (1 ft 9 in), and her legs 61.5 cm (2 ft 0 in). This means the arm was 86.9% the length of the leg. She is the first and only early hominin specimen to definitively show that the arms were almost all long as the legs. Nonetheless, these proportion are more similar to humans than non-human apes, with humans at 64.5–78%, chimpanzees about 100%, gorillas 100–125%, and orangutans 135–150.9%.[33]: 17–18 

Palaeobiology

Diet

In 1954, Robinson proposed that A. africanus was a generalist omnivore whereas P. robustus was a specialised herbivore; and in 1981, American palaeoanthropologist Frederick E. Grine suggested that P. robustus specialised on hard foods such as nuts whereas A. africanus on softer foods such as fruits and leaves. Based on carbon isotope analyses, A. africanus had a highly variable diet which included a notable amount of C4 savanna plants such as grasses, seeds, rhizomes, underground storage organs, or perhaps grass-eating invertebrates (such as locusts or termites), grazing mammals, or insectivores or carnivores. Most primates do not eat C4 plants.[44][45] A. africanus facial anatomy seems to suggest adaptations for producing high stress on the premolars, useful for eating small, hard objects such as seeds and nuts that need to be cracked open by the teeth, or for processing a large quantity of food at one time. However, like for P. robustus, microwear analysis on the cheek teeth indicate small, hard foods were infrequently eaten, probably as fall back foods during leaner times.[46] Still, A. africanus, like chimps, may have required hammerstones to crack open nuts (such as marula nuts), though A. africanus is not associated with any tools.[44]

A. africanus conspicuously lacks evidence of dental

acid erosion, which indicates this individual was regularly biting into acidic foods such as citrus. Tubers could have caused the same damage if some chewing was done by the front teeth.[49]

Barium continually deposits onto A. africanus teeth until about 6–9 months of development, and then decreases until about 12 months. Because the barium was most likely sourced from breast milk, this probably reflects the weaning age. This is comparable to the human weaning age. Following this initial period, barium deposits stall and then restart cyclically every year for several years. In the first molar specimen StS 28 (from Sterkfontein), this occurred every 6–9 months, and in the lower canine specimen StS 51 every 4–6 months, and this carried on until 4–5 years of development. Lithium and strontium also deposit cyclically. Cyclical barium, lithium, and strontium bands occur in modern primates—for example, wild orangutans up to 9 years of age—which is caused by seasonal famine when a child has to rely on nursing to sustain themselves and less desirable fallback foods. However, it is unclear if this can be extended to A. africanus.[50]

Society

The group dynamics of australopithecines is difficult to predict with any degree of accuracy. A 2011

patrilocal). This is similar to the dispersal patterns of modern-day hominins which have a multi-male kinship-based society, as opposed to the harem society of gorillas and other primates. However, the small canines of males compared to those of females would seem to suggest a much lower degree of male–male aggression than non-human hominins. Males did not seem to have ventured very far from the valley, which could either indicate small home ranges, or that they preferred dolomitic landscapes due to perhaps cave abundance or factors related to vegetation growth.[51]

Pathology

In a sample of ten A. africanus specimens, seven exhibited mild to moderate alveolar bone loss resulting from periodontal disease (the wearing away of the bone which supports the teeth due to gum disease).[52] The juvenile specimen STS 24a was diagnosed with an extreme case of periodontal disease on the right side of the mouth, which caused pathological bone growth around the affected site, and movement of the first two right molars during cyclical periods of bacterial infection and resultant inflammation. Similarly, the individual appears to have preferred to chew using the left side of the jaw. The periodontal disease would have severely hindered chewing, particularly in the last year of life, and the individual potentially may have relied on group members to survive for as long as it did.[53]

In 1992, anthropologists Geoffrey Raymond Fisk and Gabriele Macho interpreted the left ankle bone Stw 363 as bearing evidence of a healed calcaneal fracture on the heel bone (which was not preserved), which they believed resulted from a fall from a tree. If correct, then the individual was able to survive for a long time despite losing a great deal of function in the left leg. However, they also noted that similar damage could potentially have also been inflicted by calcite deposition and crystallisation during the fossilisation process. Calcaneal fractures have been recorded in humans, and are present quite often in arboreal primates.[54]

Palaeoecology

eye sockets

South African australopithecines appear to lived in an area with a wide range of habitats. At Sterkfontein,

Anastrabe integerrima was also found, which today only grows on the wetter South African coastline. This could indicate the Cradle of Humankind received more rainfall in the Plio-Pleistocene. In total, the Cradle of Humankind may have featured gallery forests surrounded by grasslands.[55] Taung also appears to have featured a wet, closed environment.[56]
Australopithecines and early Homo likely preferred cooler conditions than later Homo, as there are no australopithecine sites that were below 1,000 m (3,300 ft) in elevation at the time of deposition. This would mean that, like chimps, they often inhabited areas with an average diurnal temperature of 25 °C (77 °F), dropping to 10 or 5 °C (50 or 41 °F) at night.[57]

In 1983, studying P. robustus remains, South African palaeontologist

hunting hyenas and jackals may have also played a role.[59] Scratches, gouges, and puncture marks on the Taung child similar to those inflicted by modern crowned eagles indicate this individual was killed by a bird of prey.[60][61]

Around 2.07 million years ago, just before the arrival of P. robustus and H. erectus, A. africanus became extinct in the Cradle of Humankind. It is possible that South Africa was a refuge for Australopithecus until the beginning of major climatic variability and volatility, and, perhaps, competition with Homo and Paranthropus.[62]

See also

References

  1. ^ "Australopithecus africanus". humanorigins.si.edu. Smithsonian Institution.
  2. ^
    JSTOR 3034503
    .
  3. .
  4. ^ Dart, R. A. (1953). "The Predatory Transition from Ape to Man". International Anthropological and Linguistic Review. 4 (4).
  5. ^ Broom, R. 1936. "New Fossil Anthropoid Skull from South Africa". Nature 138: 486–488
  6. ^ Robert Broom, 1937, "On some new Pleistocene mammals from limestone caves of the Transvaal", South African Journal of Science 33: 750–768
  7. .
  8. .
  9. .
  10. .
  11. .
  12. . Kirtlandia. 28: 1–14.
  13. ^ .
  14. .
  15. .
  16. ^ .
  17. .
  18. .
  19. .
  20. ^ Moggi-Cecchi, J. (2003). "The elusive 'second species' in Sterkfontein Member 4: The dental metrical evidence". South African Journal of Science. 99 (5): 268–270.
  21. ^ McNulty, K. P. (2016). "Hominin Taxonomy and Phylogeny: What's In A Name?". Nature Education Knowledge. 7 (1): 2.
  22. PMID 104384
    .
  23. .
  24. .
  25. .
  26. .
  27. .
  28. .
  29. .
  30. .
  31. .
  32. .
  33. ^ .
  34. .
  35. .
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  38. .
  39. .
  40. .
  41. .
  42. .
  43. .
  44. ^ .
  45. .
  46. .
  47. .
  48. .
  49. .
  50. .
  51. .
  52. .
  53. .
  54. .
  55. ^ Bamford, M. (1999). "Pliocene Fossil Woods from an Early Hominid Cave Deposit, Sterkfontein, South Africa". South African Journal of Science. 95 (5): 231–237.
  56. S2CID 128689788
    .
  57. .
  58. .
  59. .
  60. .
  61. . South African Journal of Science. 103 (11–12): 496–498.
  62. .

Further reading

External links