Ardi

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The fossil "Ardi": Drawing based on the reconstruction drawing in Science

Ardi is a 4.4 million year old, largely preserved skeleton of an individual of the species Ardipithecus ramidus . The remains of the presumably female fossil were recovered between 1994 and 1996 in northeast Ethiopia in the Afar Triangle . The preservation of the bone finds proved to be extremely difficult and time-consuming due to their extreme fragility, so that their comprehensive interdisciplinary treatment was only made public in October 2009 in eleven specialist articles published at the same time.

The particular importance of the fossil is that decades-old hypotheses about human tribal history , according to which the ankle gait of chimpanzees and gorillas is an original characteristic , have been questioned.

The name of the genus Ardipithecus is partly derived from the Afar language (from "ardi" = soil), partly from Greek (from "πίθηκος", pronounced in ancient Greek as "píthēkos" = monkey); Ardipithecus consequently means “ground monkey”. The scientific name for the find, whose nickname was derived from the generic name, is ARA-VP-6/500.

Find history

The first two Ardi fossils - two fragments of metacarpal bones - were discovered by Yohannes Haile-Selassie on November 5, 1994 in Aramis , about 100 km south of Hadar and west of the Awash . They came to light on the surface of a slope made of still unconsolidated, i.e. silty clay. This site was only 54 meters north of the location where the type specimen ARA-VP-6/1 of Ardipithecus ramidus had been discovered ten months earlier . By carefully sieving the superficial fine soil , further fragments of hominine finger bones were recovered. A phalanx of toe as well as a fragment of a thigh bone and an almost completely preserved shin were then excavated in the resulting excavation cavity . As a result, in the following months a total of three square meters was removed square millimeter by square millimeter; more than a hundred other pieces of bone were secured in this way, including several sesame bones and fragments from the face .

The determination of the age of Ardi is extremely reliable, as volcanic material was deposited both immediately above and immediately below the fossilized layer and both volcanic layers are 4.4 million years old. Both Ardi and the other finds of Ardipithecus ramidus from Aramis come from a three to six meter thick, fine-grained sediment layer that was formed in a relatively short period of only 100 to a maximum of 10,000 years. Both the fine grain of the sediment layer and the condition of the fossil bones (the lack of traces of wear) indicate that they were not or only negligibly drifted by water.

Find description

Metatarsal bone of the big toe (cast)
Three clearly curved finger bones (casts)

Ardi's bones are poorly fossilized . They are described by their discoverers as cream-colored. The smaller bones of the hands and feet are largely undamaged, while the larger bones of the legs and arms are broken to varying degrees.

No signs of weathering or bite marks were found on the bones . This is a peculiarity, because almost all other bone finds from the same find horizon were gnawed by hyenas and other carnivores . Nevertheless, the bones of Ardi were scattered over a larger area and were in no case found in their natural arrangement - a condition which, according to external signs, must have already occurred before they were embedded in the ground; the researchers suspect that the remains of Ardi were trampled on by other animals.

preparation

Many bones turned out to be so soft that they would crumble if touched lightly. After a careful stress test with toothpicks or similar aids, the sediments surrounding the bones were first moistened in order to avoid damage due to drying out during the recovery of the finds. Curing agents were then applied several times; only then were the bones removed from the ground, packed and transported to Addis Ababa for further processing . After years of detailed work, all finds were finally cleaned and, with the help of digital reconstructions, put in their presumed natural position. The remains of Ardi are still fragile despite various hardening, which is why most of the research was carried out on plaster casts or computer-aided, with the help of computed tomography .

The skull

Digital reconstruction of the skull fragments from "Ardi"

Although the skull of Ardi is heavily fragmented and deformed, the left half of the lower jaw , almost all teeth and significant parts of the face, the roof of the skull and the base of the skull have been preserved. With the help of computer-aided processes and with the inclusion of another fossil - ARA-VP-1/500 from the area of ​​the skull base - the Japanese paleoanthropologist Gen Suwa (University of Tokyo) was able to determine the appearance of the skull from 65 fragments as a “pre- Australopithecus- like morphology “Reconstruct. A separate analysis of the construction of the skull base ARA-VP-1/500 confirmed the morphological proximity to Australopithecus and Homo in early 2013 .

The reconstruction of the skull size resulted in a brain volume of 280 to 350 cubic centimeters, which in relation to the body size corresponds to a very small brain - comparable to that of today's chimpanzees ; Australopithecus is assigned a brain volume of 400 to 550 cubic centimeters. The skull of Ardi is similar to that of the larger and more powerful Sahelanthropus , for which a comparable internal skull volume has been calculated. Numerous features distinguish the skull of Ardi from the skulls of recent gorillas and chimpanzees, but also from the Australopithecus skulls.

The exact position of the large occipital hole could not be determined for Ardi due to a lack of fossil evidence, but it was reconstructed for the skull fragment ARA-VP-1/500. The reconstruction confirmed the assumption already expressed in the first description of the species that the occipital opening of Ardipithecus ramidus had already moved into a position under the head, similar to that of later hominini species, and its position therefore "fully" (English " squarely ”) from the position of the chimpanzees.

The upper jaw of Ardi - ARA-VP-6 / 500-115 - does not point particularly far forward, unlike in chimpanzees living today, so it only has a weak prognathism (“a superoinferiorly short face and weak prognathism compared with the common chimpanzee "). It resembles the upper jaw of Sahelanthropus , but is overall slightly smaller than this. The incisors are relatively small, from their position as well as from the position of the nostril it can be deduced that Ardi only had a slightly protruding snout . Together with other findings, this structure of the face is assessed by the authors as an original feature of the early hominini , i.e. similar to the last common ancestor of chimpanzees and hominini; They interpret the pronounced, protruding snout of the chimpanzee as a younger, derived characteristic . The lower jaw also resembles that of Sahelanthropus and that of Ardipithecus kadabba . The structure of both jaws and the preserved teeth do not indicate any specialization in food intake.

Hand and foot

The hand bones: drawing based on the original publication
The bones of the foot: drawing based on the original publication

So many bones have been handed down from Ardi's hands and feet that a very reliable reconstruction of both extremities is possible. Their accuracy is at best comparable with the much younger Fossil Little Foot from South Africa , since only two hand bones of Lucy are known.

The construction of Ardi's hands shows clear echoes of one of the earliest known representatives of the human race (Hominoidea), the more than four times as old proconsul . Ardi's hand was extremely flexible in the area of ​​the metacarpal joints, so that she was able to grip branches very well with her hands when moving horizontally. The other wrists were also much more flexible than those of the chimpanzees and gorillas living today and therefore differed greatly from their hands. These African great apes have relatively long palms and long fingers, which - unlike Ardipithecus - are particularly capable of pulling their bodies powerfully up into the higher regions of the trees. The result of the forces involved was that the joints between the fingers and the palm of the hand were only slightly mobile. This in turn results in the ankle gait on the ground ; Ardi has no evidence of a comparable mode of transport.

The foot of Ardi also resembles the foot of the macaques and gibbons more closely than the foot of the great African apes. So when Ardi is still a special bone available (Os peroneum) serving as an original , that is phylogenetically applies old feature; and the reconstructed tendon for the fibula -Muskel peroneus longus refers to a homology with these more distant relatives of the Hominini. The most prominent feature of the foot, however, is the wide spreadable big toe. With the help of his big toes, Ardi was able to hold on to the branches over which she climbed four-footed . This anatomical feature is not known from any living ape and is only fossilized by the Burtele foot recovered in the Ethiopian excavation area Woranso-Mille ; it gave Ardipithecus a gait that was unique among all known primates , because the big toes probably remained splayed even when walking upright.

Pelvis and upright gait

Although the arrangement of the bones of the foot and hands suggests that Ardi moved on all fours in the branches of the trees and that her big toes were designed as grasping organs, the reconstruction of the bones of the pelvis shows that she could walk upright on the ground . For example, the approach is the buttock muscles in comparison to older fossil human-like thanks to a developed ilium for the upright posture stabilizing positioned so low that Ardi could proceed without pivoting every step of the body from one side to the other. In contrast to Australopithecus afarensis , which already had all the essential adaptations for a constant upright gait, Ardi's pelvic girdle is still described by the researchers as a “mosaic of features” that enabled her to climb as well as to run quickly.

The few bone fragments preserved from the foot have also been described as "a mixture of retained primitive properties and features that are adapted to a habitual bipedal".

Gender determination, body weight and height

Of the total of 21 preserved canines of Ardipithecus ramidus , the canines of Ardi are among the smallest: the upper jaw canine is the second smallest of 13, the lower canine is the second smallest of eight preserved canines. From this, the researchers deduce that Ardi was female. This interpretation is supported by a small fragment of the bulge above the eye (torus supraorbitalis) , the thickness of which is also in the lowest area.

Comparative measurements in primates living today have shown that the size of the head bone and the talus correlates very well with the body weight of the primate . Thanks to this correlation, a body weight of 51 kg could be derived for Ardi. Because Ardi was one of the larger individuals of its kind, a weight of around 50 kg could also be considered typical of the entire species. The reconstruction of the height of Ardi, including other fossils, resulted in an estimate of 117 to 124 cm.

Scientific importance

The hand bones of Ardi refuted the assumption that the ankle gait (here in the gorilla ) preceded the bipedia of the hominini

From the period before the appearance of Australopithecus anamensis , only a few finds of fossil hominini have so far been known. These finds are also - like Orrorin - so fragmentary or - like the skull of Sahelanthropus - so deformed that their position in the genealogical tree of the great apes is controversial and therefore unclear. The lack of meaningful fossil evidence had the consequence, among other things, that the hypotheses on the evolution of the physique of the early hominini were derived primarily from the appearance of the closest human relatives, the physique of the chimpanzees. These models, for example, interpreted the hominini's upright gait in the sense of a homology as a further development of a supposedly original ankle gait, i.e. as an intermediate stage in the transition from a primarily tree-dwelling way of life to permanent residence in the open savannah that has been preserved in chimpanzees and gorillas . Ardi's hand skeleton now provided evidence that the early hominini had relatively original grasping hands and - furthermore - that the hands of modern humans are “more primitive ” (less deviating from their original state) than those of chimpanzees and gorillas. In addition, Ardi's foot and pelvic girdle show that Ardipithecus ramidus was already able to walk upright when the individuals of this species were still regular tree dwellers with regard to numerous other anatomical features. The physique of Ardi therefore also falsifies the historical savannah hypothesis , according to which the upright gait developed in four-footed savannah dwellers.

The editors of the journal Science declared the description of Ardi and her environment to be the most important scientific publication of 2009, as this find was “a major figure in the history of human evolution”, on a par with the discovery of the first Neanderthal man , the “ child of Taung "And the skeleton of" Lucy ". In the journal Nature it was said that the fossil "allows for the first time a comprehensive look at the biology of a species in temporal proximity to the last common ancestor of humans and the other great apes "; it contributes to the fact that the previous description of the early evolution of the hominini must be “rewritten”.

Web links

Commons : Ardipithecus  - collection of images, videos and audio files

Individual evidence

  1. ↑ Original drawing from Science
  2. ↑ sciencemag.org ( Memento from November 2, 2009 in the Internet Archive ): Cover picture of the journal Science (Volume 326, No. 5949) from October 2, 2009 with a complete overview of the bones found by Ardi.
  3. ↑ The eleven studies appeared in Science , Volume 326, No. 5949, October 2, 2009, pp. 60-106; an introduction can be found in the same booklet on pages 36–43
  4. ↑ a b Tim D. White et al .: Ardipithecus ramidus and the Paleobiology of Early Hominids. In: Science. Volume 326, No. 5949, 2009, pp. 64, 75-86, doi : 10.1126 / science.1175802
  5. ↑ Giday Wolde-Gabriel et al .: The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus. In: Science. Volume 326, No. 5949, 2009, pp. 65, 65e1–65e5, doi : 10.1126 / science.1175817
  6. ^ Antoine Louchart et al .: Taphonomic, Avian, and Small-Vertebrate Indicators of Ardipithecus ramidus Habitat. In: Science. Volume 326, No. 5949, 2009, p. 66, doi : 10.1126 / science.1175823
  7. ↑ a b Gen Suwa et al .: The Ardipithecus ramidus Skull and Its Implications for Hominid Origins. In: Science. Volume 326, 2009, No. 5949, p. 68, 68e1–68e7, doi : 10.1126 / science.1175825
    literally says: “Anatomical comparisons and micro – computed tomography – based analysis of this and other remains reveal pre- Australopithecus hominid craniofacial morphology and structure. "
  8. ^ William H. Kimbel et al .: Ardipithecus ramidus and the evolution of the human cranial base. In: PNAS. Volume 111, No. 3, 2014, pp. 948-953, doi: 10.1073 / pnas.1322639111
    'Ardi' skull reveals links to human lineage. eurekalert.org from January 6, 2014
  9. ↑ Tim White, Gen Suwa, Berhane Asfaw : Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia. In: Nature . Volume 371, No. 6495, 1994, pp. 306-312; doi : 10.1038 / 371306a0
  10. ^ Gen Suwa et al .: The Ardipithecus ramidus Skull and Its Implications for Hominid Origins. In: Science. Volume 326, 2009, No. 5949, Section The Ar. ramidus face and vault: basic morphology.
  11. ↑ "This reflects a less prognathic face compared with Pan and probably represents the primitive condition for both hominids and African apes." Gen Suwa et al .: The Ardipithecus ramidus Skull and Its Implications for Hominid Origins, pp. 68e3
  12. ↑ C. Owen Lovejoy et al .: Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive. In: Science. Volume 326, No. 5949, 2009, pp. 70, 70e1–70e8, doi : 10.1126 / science.1175827
  13. ↑ C. Owen Lovejoy et al .: Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus. In: Science. Volume 326, 2009, No. 5949, pp. 72, 72e1-72e8, doi : 10.1126 / science.1175832
  14. ↑ C. Owen Lovejoy et al .: The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. In: Science. Volume 326, No. 5949, 2009, pp. 71, 71e1-71e6, doi : 10.1126 / science.1175831
  15. ^ Gen Suwa et al .: Paleobiological Implications of the Ardipithecus ramidus Dentition. In: Science. Volume 326, No. 5949, 2009, p. 96, doi : 10.1126 / science.1175824
  16. ↑ C. Owen Lovejoy et al .: The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes. In: Science. Volume 326, No. 5949, 2009, pp. 73, 100-106, doi : 10.1126 / science.1175833
  17. ↑ C. Owen Lovejoy: Reexamining Human Origins in Light of Ardipithecus ramidus. In: Science. Volume 326, No. 5949, 2009, pp. 74, 74e1-74e8, doi : 10.1126 / science.1175834
  18. ↑ Ann Gibbons: Breakthrough of the Year: Ardipithecus ramidus. In: Science. Volume 326, December 18, 2009, pp. 1598–1599, doi : 10.1126 / science.326.5960.1598-a
  19. ↑ Rex Dalton: Fossil rewrites early human evolution. In: Nature. Volume 461, 2009, p. 705, doi : 10.1038 / 461705a
This article was added to the list of excellent articles on November 13, 2012 in this version .