Sahelanthropus

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Sahelanthropus
Skull of Sahelanthropus tchadensis (replica)

Skull of Sahelanthropus tchadensis ( replica )

Temporal occurrence
late Miocene
6–7 million years
Locations
Systematics
Human (Hominoidea)
Apes (Hominidae)
Homininae
Hominini
Sahelanthropus
Scientific name
Sahelanthropus
Brunet et al. , 2002
species
  • Sahelanthropus tchadensis
Site in Africa
Facial reconstruction
( National Museum of Natural History , USA)
Different views of the skull.

Sahelanthropus is an extinct genus of great apes , which in the late Miocene in Central Africa occurred. Fossil remains of this human ,dated to an age of 6 to 7 million years, have been discovered in Chad . The classification of Sahelanthropus in the human family tree is controversial.

Naming

Sahelanthropus is an artificial word . The name of the genus is derived from the location of the fossils in the sub-Saharan Sahel zone and from ancient Greek ἄνθρωπος anthropos , German ' man ' . The epithet of the only scientifically described species so far , Sahelanthropus tchadensis , refers to Chad. Sahelanthropus tchadensis therefore means “Sahel man from Chad”.

The type specimen of Sahelanthropus was also given the name Toumaï by its discoverers , which roughly means "courage to live" or "hope for life" in German. In the Dazaga language, this is a name for children who are born just before the dry season begins .

Initial description

Holotype of the genus and at the same time the type species Sahelanthropus tchadensis is an almost complete brain skull with partially preserved dentition and the inventory designation TM 266-01-060-1. In addition, the fragments (individual teeth and groups of teeth from the upper and lower jaws) from another five individuals were used in the first description . These finds were excavated between July 2001 and February 2002 from the approximately 5000 square meter excavation site mentioned in TM 266 in the Djurab desert in northern Chad . The type specimen was discovered by Ahounta Djimdoumalbaye from the Center National d'Appui à la Recherche on July 19, 2001.

The skull of the type specimen was almost completely flattened, so that its original volume and the exact arrangement of the numerous bone fragments had to be reconstructed by the group around Michel Brunet and Patrick Vignaud from the Université de Poitiers . In the first description, a presumed internal volume of 320 to 380 cubic centimeters was shown, which corresponds approximately to that of a chimpanzee living today . In a comment on the first description, the paleoanthropologist Bernard Wood of George Washington University described the type specimen as a "remarkable mosaic shape ": "From behind it looks like a chimpanzee, while from the front it could look like 1.75 million year old Australopithecus ."

After the scientific processing, all finds were handed over to the Département de Conservation des Collections, Center National d'Appui à la Recherche in N'Djamena for permanent safekeeping.

Relationship to humans

In the first description of genus and type species, the approximately chimpanzee / bonobo- sized individuals of Sahelanthropus were identified as the ancestors of humans: " Sahelanthropus is the oldest and most primitive known member of the hominid clade , to be located near the branch of the chimpanzees." in this context that the authors use the term “hominid” expressly for all taxa “that are closer to humans than to chimpanzees”, i. H. in the sense of hominin . This classification was justified by the fact that the fossil has a number of "derived" (that is, evolutionarily advanced) features, including small, anterior canine teeth and directly adjoining molars with a medium- thick enamel layer . The lower face (basicranium) and the relatively flat expression of the facial bones as well as the missing diastema in the area of ​​the canines are "similar to the later hominids including Kenyanthropus and Homo ." Other features, however, are still "original", for example the very small brain volume and diverse other anatomical features, which is why the species should be phylogenetically “close to the last common ancestor of humans and chimpanzees”.

Three years later, the French-Chadian research group confirmed its classification of Sahelanthropus on the basis of other fossils from the same site, including a well-preserved canine tooth and an equally well-preserved lower jaw. At the same time, a virtual reconstruction of the holotype supported the interpretations of its discoverers. An international team led by the Swiss anthropologist and expert in computer-aided paleontology, Christoph Zollikofer ( University of Zurich ), had scanned the fragments of the holotype with the help of a computer tomograph, restored the deformed bone fragments to their natural state and finally adapted them to one another. After comparing them with other fossils and with the skulls of primates living today, they summarized their study in the journal Nature as follows: “The reconstruction confirms that S. tchadensis is a hominid and not particularly close to the great African great apes.” Based on the reconstruction of the Heads, the brain volume was limited to 360 to 370 cubic centimeters and the assumption was expressed that the hominid may have already moved on two legs.

This statement was based primarily on a virtual reconstruction of the position of the foramen magnum (occipital hole), the point on the skull where the spinal cord passes through a large opening into the spine. However, this skull region is particularly badly deformed in the holotype. As early as 2004, David Begun had therefore warned against drawing too far-reaching conclusions in Science , but at the same time confirmed that Sahelanthropus could be placed on the basis of the human family tree due to a mosaic-like combination of "original" and "derived" features. Also in 2004, Tim White had certified the canines of Sahelanthropus that there were no traces of abrasion on them, neither in the lower jaw nor in the upper jaw . and Sahelanthropus therefore placed in a development line towards the Australopithecines. Such grinding marks are known from all older finds. They arise when the canine teeth rub against each other when biting and thus constantly sharpen their tips. In order to be able to perform this honing , the dentition has a gap next to each canine ( diastema ) into which the canine of the antagonistic jaw fits.

The numerous bone finds from the area below the skull TM 266-01-060-1 - including a thigh bone that could provide information about the posture when running - have not been scientifically described (as of 2018).

Habitat

The environment of Sahelanthropus could be explored relatively precisely - but mainly indirectly - on the basis of the fossil finds of other animal and plant species. In the immediate vicinity of his bones, the remains of more than ten species of freshwater fish , amphibians , crocodiles and turtles were recovered, but also some bones from other primates and numerous bones from rodents , elephants , giraffes , horses , pigs and horned bearers . Fossil hyenas have been excavated as the most common predators (both in terms of the number of individuals found and the number of species) . Almost all fish found have related species in today's Lake Chad , some of the fossil fish were longer than one meter; Bird species were absent from the fossilized layers.

The sediments in which Sahelanthropus and the fossil animals of the same age were embedded indicated, on the one hand, a sandy dune landscape and, on the other hand, shallow waters, the area of ​​which could expand considerably after heavy rainfall. However, the existence of large fish is also evidence that the lake landscape must have included permanent, deeper bodies of water. Many of the other animal species found, as well as numerous plant fossils, show that the bodies of water were surrounded by gallery forests and adjoining savannas . According to the findings, Sahelanthropus lived in a thinly forested landscape that was probably similar to the immediate vicinity of today's Lake Chad. These ecological findings contributed to the fact that the so-called savanna hypothesis - an attempt to explain the origin of the upright gait in the hominini from life in a savannah landscape - was refuted.

Dating

Due to the geological conditions, the age of the site could neither be determined absolutely with the help of potassium-argon dating nor with the help of aluminum-magnesium dating . With the help of biostratigraphic methods, an age of six to seven million years was initially determined, in later publications seven million years were repeatedly given. Against the dating of the finds to the late Miocene, a member of the research team of the first description objected that the fossils had not been discovered in situ , that a shift due to geological or other processes cannot be ruled out.

Controversy: “Sahelpithecus” instead of Sahelanthropus ?

Michel Brunet and his assistant Ahounta Djimdoumalbaye with the skull of Sahelanthropus

Just six months after the first description, a US-American-French group of four researchers around Milford H. Wolpoff spoke up and criticized the assignment of the finds to the ancestors of man as incorrect under the heading "Sahelanthropus or 'Sahelpithecus'?" Instead, they assigned the fossils - apart from the line of development leading to humans - to the great apes (preferably the gorillas ). These animals would have lived in the same habitat as the australopithecines later. The "modern" appearing bone features of Sahelanthropus are independent parallel developments to those of the ancestors of humans due to similar habitats. In a detailed reply, Michel Brunet immediately rejected the rejection of his assignment of the fossil finds and criticized the critics for not having mentioned any features in the bone structure that would suggest a closer relationship between Sahelanthropus and the gorillas (see also comparison: humans and great apes ).

After other experts in the journal Science - including the co-author of Lucy's first description , Tim White - supported Brunet's interpretation in 2004 and the creators of a virtual reconstruction also left no doubt about Brunet's interpretation in 2005, Milford H's group renewed again in 2006 Wolpoff their criticism. They admitted that Sahelanthropus had an "enigmatic" (enigmatic) combination of characteristics of the Australopithecinen and the genus Homo . Precisely because of this much too early similarity to the representatives of the genus Homo , which appeared millions of years later, they exclude a close relationship to modern humans. Instead, two of the five authors reassign Sahelanthropus to the gorillas. The three other authors also consider a proximity to the chimpanzees possible, but also do not rule out that the species could have belonged to a later extinct branch of the great apes. They also state that, in contrast to the authors of the first description and the virtual reconstruction, they discovered traces of wear on the canine flanks, which indicated a “honing” typical of monkeys. Wolpoff estimates the age of the type specimen at eleven years, the gender cannot be determined, and a permanently upright gait is also denied.

First descriptor Michel Brunet, on the other hand, continues to stick to his assignment of Sahelanthropus to the basis of the family tree of modern humans. In August 2007, on the occasion of the first description of Chororapithecus in New Scientist , he was quoted as saying that this finding of a 10 million year old gorilla ancestor also assigned the 7 million year old Sahelanthropus to the direct ancestors of humans instead of those of the gorillas make plausible.

In February 2011, Bernard Wood and Terry Harrison also criticized the assignment of Sahelanthropus as well as Ardipithecus and Orrorin to the clade of hominini in a review article as premature. In particular, the relatively small canines, the absence of the diastema and the position of the foramen magnum are indications of a possible belonging to the early hominini; However, these characteristics are by no means exclusive characteristics of the hominini, but also, for example, for Oreopithecus , which - like Ramapithecus punjabicus - was initially placed at the base of the hominini, but was later classified unequivocally apart from the hominini due to other characteristics. It cannot therefore be ruled out that the similarity of these characteristics of Sahelanthropus with those of the oldest species, Australopithecus anamensis, which is undoubtedly considered hominin , is to be assessed as synapomorphism and indicates convergent developments .

See also

literature

  • Franck Guy, Daniel E. Lieberman, David Pilbeam et al .: Morphological affinities of the Sahelanthropus tchadensis (Late Miocene hominid from Chad) cranium. In: PNAS . Volume 102, No. 52, 2005, pp. 18836-18841, doi: 10.1073 / pnas.0509564102

Web links

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

Individual evidence

  1. ↑ Michel Brunet , Franck Guy, David Pilbeam et al .: A new hominid from the Upper Miocene of Chad, Central Africa. In: Nature . Volume 418, 2002, pp. 145-151, doi: 10.1038 / nature00879
  2. ^ Bernard Wood , Hominid Revelations from Chad. In: Nature. Volume 418, 2002, pp. 133-135, doi: 10.1038 / 418133a , full text
  3. ↑ Bernard Wood , Nicholas Lonergan: The hominin fossil record: taxa, grades and clades. In: Journal of Anatomy . Volume 212, No. 4, 2008, p. 356, doi: 10.1111 / j.1469-7580.2008.00871.x , full text (PDF; 285 kB) ( Memento from October 20, 2012 in the Internet Archive )
  4. ^ "Sahelanthropus is the oldest and most primitive known member of the hominid clade, close to the divergence of hominids and chimpanzees." Michel Brunet et al., Nature , Volume 418, 2002, p. 151.
  5. ↑ In the original: "the term hominid is used here for convenience to denote all taxa that are closer to humans than to chimpanzees, and does not connote any taxonomic scheme." M. Brunet et al., Nature , Volume 434, 2005, p 753.
  6. ↑ Michel Brunet et al., Nature , Volume 418, 2002, p. 151.
  7. ^ Michel Brunet et al .: New material of the earliest hominid from the Upper Miocene of Chad. In: Nature. Volume 434, 2005, pp. 752-755, doi: 10.1038 / nature03392
  8. ↑ Christoph PE Zollikofer et al .: Virtual cranial reconstruction of Sahelanthropus tchadensis. In: Nature. Volume 434, 2005, pp. 755–759, doi: 10.1038 / nature03397 - in the original: "The reconstruction confirms that S. tchadensis is a hominid and is not more closely related to the African great apes."
    Facelifting for the oldest hominid. On: uzh.ch from April 7, 2005
  9. ^ David R. Begun : The earliest hominins - is less more? In: Science . Volume 303, No. 5663, 2004, pp. 1478-1480, doi: 10.1126 / science.1095516
  10. ^ Yohannes Haile-Selassie , Gen Suwa, Tim D. White: Late miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. In: Science. Volume 303, 2004, pp. 1503-1505.
  11. ↑ Ewen Callaway: Femur findings remain a secret. In: Nature . Volume 553, 2018, pp. 391–392, full text (PDF) with an illustration of the found situation of the type specimen
  12. ↑ Patrick Vignaud et al .: Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad. In: Nature. Volume 418, 2002, pp. 152-155, doi: 10.1038 / nature00880
  13. ↑ Fossil finds of animals that could definitely be dated elsewhere are used in these cases as a yardstick for determining the age.
  14. ↑ so Michel Brunet for example in: New Gorilla species rewrites ape evolution. In: New Scientist . Volume 195, 2007, p. 12
  15. ↑ Alain Beauvilain: The contexts of discovery of Australopithecus bahrelghazali (Abel) and of Sahelanthropus tchadensis (Toumaï): unearthed, embedded in sandstone, or surface collected? In: South African Journal of Science , Volume 104, No. 5–6, 2008, pp. 165–168, full text (PDF; 479 kB)
  16. ↑ Greek πίθηκος, old Gr. pronounced píthēkos means "monkey".
  17. ↑ Milford H. Wolpoff et al .: Palaeoanthropology (communication arising): Sahelanthropus or 'Sahelpithecus'? In: Nature , Volume 419, 2002, pp. 581-582, doi: 10.1038 / 419581a
  18. ↑ Michel Brunet et al .: Reply. In: Nature. Volume 419, 2002, p. 582, doi: 10.1038 / 419582a
  19. ↑ Y. Haile-Selassie et al., Science , Volume 303, 2004, pp. 1503 ff.
  20. ↑ C. Zollikofer et al., Nature , Volume 434, 2005, pp. 755 ff.
  21. ↑ Milford H. Wolpoff, John Hawks, Brigitte Senut, Martin Pickford and James Ahern: An Ape or the Ape: Is the Toumaï Cranium TM 266 a Hominid? In: PaleoAnthropology. Volume 6, 2006, pp. 36–50, full text (PDF; 2.19 MB)
  22. ↑ The anthropologist Bernard Wood had already raised concerns about something in 2002: Sahelanthropus shows a much too human-like face far too early to be placed at the beginning of human development. - B. Wood, Nature , Volume 418, 2002, p. 134.
  23. ↑ Emma Young: New gorilla species rewrites ape evolution. In: New Scientist . Volume 195, 2007, p. 12. - Literally it says: “The earlier date for the start of the gorilla lineage would also fit with the age of the earliest known remains from the lineage leading to humans, found in Chad and dated about seven million years old, Brunet says. "
  24. ↑ Bernard Wood , Terry Harrison : The evolutionary context of the first hominins. In: Nature. Volume 470, 2011, pp. 347-352, doi: 10.1038 / nature09709