Ardipithecus ramidus

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Ardipithecus ramidus
Temporal range: Zanclean 4.4 Ma
A skull
A. ramidus at the Museo Nacional de Ciencias Naturales
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Genus: Ardipithecus
Species:
A. ramidus
Binomial name
Ardipithecus ramidus
(White, Suwa & Asfaw, 1994)

Ardipithecus ramidus is a species of australopithecine from the Afar region of Ethiopia 4.4 million years ago (mya). The facial anatomy suggests that A. ramidus males were less aggressive than those of modern chimps, implying increased parental care and monogamy. It was capable of both suspensory behavior in the trees and walking bipedally, whereas modern apes are specialized for either or; consequently, it was less adapted in either habitat compared to modern specialized counterparts. It has also been suggested that it was among the earliest of human ancestors to use some proto-language, possibly capable of vocalizing at the same level as a human infant.

Taxonomy[edit]

Map showing discovery locations of various australopithecines

The first remains were described in 1994 by American anthropologist Tim D. White, Japanese paleoanthropologist Gen Suwa, and Ethiopian paleontologist Berhane Asfaw. The holotype specimen, ARA-VP-6/1, comprised an associated set of 10 teeth; and there were 16 other paratypes identified, preserving also skull and arm fragments. These were unearthed in the 4.4 million year (Ma) deposits of the Afar region in Aramis, Ethiopia from 1992 to 1993. They classified it as Australopithecus ramidus, the species name deriving from the Afar language ramid "root". In 1995, they made a corrigendum, recommending the species be split off into a separate genus from Australopithecus.[1] Ardipithecus stems from Afar ardi "ground" and Ancient Greek pithecus "ape".

About 45% of the total skeleton is known. Between 1999 and 2003, a team led by Sileshi Semaw from the Stone Age Institute discovered bones and teeth of nine A. ramidus individuals at As Duma in the Gona Western Margin of Ethiopia's Afar Region.[2] The fossils were dated to between 4.35 and 4.45 million years old.[3]

A. kadabba was considered to be a subspecies of A. ramidus until 2004.[4]

In 2009, White and colleagues reaffirmed the position of Ardipithecus as a hominin more closely related to modern humans than to chimps based on dental similarity, a short base of the skull, and adaptations to bipedality.[5] In 2011, primatologist Esteban Sarmiento said that there is not enough evidence to assign Ardipithecus to Hominini,[6] but its closer affinities to Homo than chimps have been reaffirmed in following years.[7][8]

Evolutionary tree according to a 2019 study:[9]

Hominini

Chimpanzee

Sahelanthropus

Ardipithecus

A. anamensis

A. afarensis

Paranthropus

P. aethiopicus

P. boisei

P. robustus

A. africanus

A. garhi

H. floresiensis

A. sediba

H. habilis

Other Homo

Description[edit]

"Ardi", the most complete Ardipithecus specimen

Ardipithecus ramidus had a small brain, measuring between 300–350 cc (18–21 cu in). This is slightly smaller than a modern bonobo or female chimp brain, but much smaller than the brain of australopithecines like Lucy (~400 to 550 cm3), and roughly 20% the size of the modern human brain. Like chimps, the A. ramidus face was much more pronounced (prognathic) than modern humans.[10] One of the larger specimens from Aramis, ARA-VP-6/500, was estimated to have stood 124 cm (4 ft 1 in) and weighed 51 kg (112 lb) based on comparisons with large-bodied female apes. Another specimen, A.L. 288-1, was estimated to have weighed 26 kg (57 lb). Assuming subsistence was primarily sourced from climbing in trees, A. ramidus may not have exceeded 35–60 kg (77–132 lb).[11]

The size of the upper canine tooth in A. ramidus males was not distinctly different from that of females, in contrast to the sexual dimorphism observed in chimps where males have significantly larger and sharper upper canine teeth than females.[12]

A. ramidus feet are better suited for walking than chimps. However, like most great apes, but unlike all previously recognized hominins, it had a grasping big toe adapted for locomotion in the trees (an arboreal lifestyle), though it was likely not as specialized for grasping as it is in modern great apes.[13][5] Its tibial and tarsal lengths indicate a leaping ability similar to bonobos.[6] It lacks any characters suggestive of specialized suspension, vertical climbing, or knuckle walking; and it seems to have used a method of locomotion unlike any modern great ape, which combined arboreal palm walking clambering and a form of bipedality more primitive than Australopithecus. The chimpanzee–human last common ancestor may have used a similar method of locomotion.[14][5]

The upper pelvis (distance from the sacrum to the hip joint) is shorter than in any known ape. It is inferred to have had a long lumbar vertebral series, and lordosis (human curvature of the spine), which are adaptations for bipedality. However, the legs were not completely aligned with the torso (were anterolaterally displaced), and Ardipithecus may have relied more on its quadriceps than hamstrings for walking (the opposite is true for modern humans).[15] However, it lacked foot arches and had to adopt a flat-footed stance. These would have made it less efficient at walking and running than Australopithecus and Homo. It may not have employed a bipedal gait for very long time intervals.[8] It may have predominantly used palm walking on the ground,[16] Nonetheless, A. ramidus still had specialized adaptations for bipedality, such as a robust fibularis longus muscle used in pushing the foot off the ground while walking (plantarflexion).[13]

Paleobiology[edit]

The reduced canine size and reduced skull robustness in A. ramidus males (about the same size in males and females) is typically correlated with reduced male aggression, increased parental investment, and monogamy.[5] They likely lived in a society similar to bonobos and atelines.[12] A 2015 study argued that this was due to a process of self domestication. Because a similar process is thought to have occurred with the comparatively docile bonobos from more aggressive chimps, the study said that A. ramidus society may have seen an increase in maternal care and female mate selection compared to its ancestors.[17]

They also claimed that these changes to the skull increased vocal ability, pushing the origins of language back from the origins of the genus Homo to 4.5 mya. They argued that self domestication was aided by the development of vocalization, living in a pro-social society. They conceded that chimps and A. ramidus likely had the same vocal capabilities, but argued that A. ramidus made use of more complex vocalizations, and vocalized at the same level as a human infant. They also noted that the base of the skull stopped growing with the brain by the end of juvenility, whereas in chimps it continues growing with the rest of the body into adulthood; and considered this evidence of a switch from a gross skeletal anatomy trajectory to a neurological development trajectory due to selective pressure for socio-neural behavioral adaptations.[17]

The teeth of A. ramidus indicate that it was likely a generalized omnivore and fruit eater which predominantly consumed C3 plants in woodlands or gallery forests. The teeth lacked adaptations for abrasive foods.[12][5][6]

Paleoecology[edit]

Half of the large mammal species associated with A. ramidus at Aramis are spiral-horned antelope and colobine monkeys (namely Kuseracolobus and Pliopapio). There are a few specimens of primitive white and black rhino species, and elephants, giraffes, and hippo specimens are less abundant. These animals indicate that Aramis ranged from wooded grasslands to forests, but A. ramidus likely preferred the closed habitats,[18] specifically riverine areas as thicker forests may have clung to such water sources.[19] There were exceedingly high rates of scavenging, indicating a highly competitive environment somewhat like Ngorongoro Crater. Predators of the area were the hyenas Ikelohyaena abronia and Crocuta dietrichi, the bear Agriotherium, the dog Eucyon, and crocodiles.[20] Bayberry, hackberry, and palm trees appear to have been common at the time from Aramis to the Gulf of Aden; and botanical evidence suggests a cool, humid climate.[21]

See also[edit]

References[edit]

  1. ^ White, T. D.; Suwa, G.; Asfaw, B. (1994). "Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia" (PDF). Nature. 371 (6495): 306–312. Bibcode:1994Natur.371..306W. doi:10.1038/371306a0. PMID 8090200. Archived from the original (PDF) on 2013-04-13.
  2. ^ "New Fossil Hominids of Ardipithecus ramidus from Gona, Afar, Ethiopia". Archived from the original on 2008-06-24. Retrieved 2009-01-30.
  3. ^ Indiana University News Release. "Anthropologists find 4.5 million-year-old hominid fossils in Ethiopia". Archived from the original on 14 February 2009. Retrieved 2009-01-30.
  4. ^ Haile-Selassie, Yohannes; Suwa, Gen; White, Tim D. (2004). "Late Miocene Teeth from Middle Awash, Ethiopia, and Early Hominid Dental Evolution". Science. 303 (5663): 1503–1505. Bibcode:2004Sci...303.1503H. doi:10.1126/science.1092978. PMID 15001775.
  5. ^ a b c d e White, Tim D.; Asfaw, Berhane; Beyene, Yonas; Haile-Selassie, Yohannes; Lovejoy, C. Owen; Suwa, Gen; WoldeGabriel, Giday (2009). "Ardipithecus ramidus and the Paleobiology of Early Hominids". Science. 326 (5949): 75–86. Bibcode:2009Sci...326...64W. doi:10.1126/science.1175802. PMID 19810190.
  6. ^ a b c Sarmiento, E. E.; Meldrum, D. J. (2011). "Behavioral and phylogenetic implications of a narrow allometric study of Ardipithecus ramidus". HOMO. 62 (2): 75–108. doi:10.1016/j.jchb.2011.01.003.
  7. ^ Kimbel, W. H.; Suwa, G.; Asfaw, B.; Rak, Y.; White, T. D. (2014). "Ardipithecus ramidus and the evolution of the human cranial base". Proceedings of the National Academy of Sciences. 111 (3): 948–953. doi:10.1073/pnas.1322639111. PMC 3903226. PMID 24395771.
  8. ^ a b White, T. D.; Lovejoy, C. O.; Asfaw, B.; Carlson, J. P.; Suwa, G. (2015). "Neither chimpanzee nor human, Ardipithecus reveals the surprising ancestry of both". Proceedings of the National Academy of Sciences. 112 (16): 4877–4884. doi:10.1073/pnas.1403659111. PMC 4413341. PMID 25901308.
  9. ^ Parins-Fukuchi, C.; Greiner, E.; MacLatchy, L. M.; Fisher, D. C. (2019). "Phylogeny, ancestors and anagenesis in the hominin fossil record" (PDF). Paleobiology. 45 (2): 378–393. doi:10.1017/pab.2019.12.
  10. ^ Suwa, G; Asfaw, B.; Kono, R. T.; Kubo, D.; Lovejoy, C. O.; White, T. D.; et al. (2 October 2009). "The Ardipithecus ramidus skull and its implications for hominid origins" (PDF). Science. 326 (5949): 68, 68e1–68e7. Bibcode:2009Sci...326...68S. doi:10.1126/science.1175825. PMID 19810194.
  11. ^ Lovejoy, C. O.; Suwa, G.; Simpson, S. W.; Matternes, J. H.; White, T. D. (2009). "The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes". Science. 326 (5949): 73–106. doi:10.1126/science.1175833.
  12. ^ a b c Suwa, G; Kono, R. T.; Simpson, S. W.; Asfaw, B.; Lovejoy, C. O.; White, T. D.; et al. (2 October 2009). "Paleobiological implications of the Ardipithecus ramidus dentition" (PDF). Science. 326 (5949): 69, 94–99. Bibcode:2009Sci...326...94S. doi:10.1126/science.1175824. PMID 19810195.
  13. ^ a b Lovejoy, C. O.; Latimar, B.; Suwa, G.; Asfaw, B.; White, T. D. (2011). "Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus". Science. 326 (5949): 72-72e8. doi:10.1126/science.1175832. PMID 19810198.
  14. ^ Lovejoy, C. O.; Simpson, S. W.; White, T. D.; Asfaw, B.; Suwa, G. "Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive". Science. 326 (5949): 70–70e8. doi:10.1126/science.1175827.
  15. ^ Lovejoy, C. O.; Suwa, G.; Spurlock, L.; Asfaw, B.; White, T. D. (2009). "The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking". Science. 326 (5949): 71–71e6. doi:10.1126/science.1175831.
  16. ^ Prang, T. C. (2019). "The African ape-like foot of Ardipithecus ramidus and its implications for the origin of bipedalism". eLife. 8: e44433. doi:10.7554/eLife.44433. PMC 6491036. PMID 31038121.
  17. ^ a b Clark, Gary; Henneberg, Maciej (2015). "The life history of Ardipithecus ramidus: A heterochronic model of sexual and social maturation". Anthropological Review. 78 (2): 109–132. doi:10.1515/anre-2015-0009.
  18. ^ White, T. D.; et al. (2009). "Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus" (PDF). Science. 326 (5949): 67–93. doi:10.1126/science.1175822.
  19. ^ Gani, M. R.; Gani, N. D. (2011). "River-margin habitat of Ardipithecus ramidus at Aramis, Ethiopia 4.4 million years ago". Nature Communications. 2: 602. doi:10.1038/ncomms1610. PMID 22186898.
  20. ^ Louchart, A.; Wesselman, H.; Blumenschine, R. J.; et al. (2009). "Taphonomic, Avian, and Small-Vertebrate Indicators of Ardipithecus ramidus Habitat". Science. 326 (5949): 66–66e4. doi:10.1126/science.1175823.
  21. ^ WoldeGabriel, G.; Ambrose, S. H.; Barboni, D.; et al. (2009). "The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus". Science. 326 (5949): 65–65e5. doi:10.1126/science.1175817.

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