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Chororapithecus

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Chororapithecus
Temporal range: Tortonian–Messinian
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Gorillini
Genus: †Chororapithecus
Suwa et al., 2007
Species:
†C. abyssinicus
Binomial name
†Chororapithecus abyssinicus
Suwa et al., 2007

Chororapithecus is an extinct hominine genus that lived during the Miocene and is represented by one species, Chororapithecus abyssinicus. It is believed to be the earliest known species of the tribe Gorillini. It was originally interpreted as living about 10 to 10.5 million years ago, and its existence was thought to indicate that the last common ancestor between the human/chimpanzee lineage and gorillas may have lived greater than 10 to 11 million years ago, which is at least 2 million years earlier than the previously thought date of divergence of about 8 million years ago.[1] However, a subsequent study by Katoh et al. (2016) dated its fossils to ~8 million years ago.[2]

The only evidence found of this extinct ape is currently nine fossilized teeth of at least three individuals, recovered from the Chorora Formation which runs along the southern Afar Depression of Ethiopia (the same place where the remains of Lucy were discovered in 1974). Analysis of eight molars (two of them fragmented) and a canine tooth show that their structure is partly similar to modern gorillas.[3]

The researchers compared the make-up of the teeth to those of other current and fossil apes, and concluded that the new ape fossils were possibly those of a species of gorilla which ate mostly high-fiber plants, and that the fossil species is likely a 'direct ancestor' of the gorillas currently living in Africa. Alternatively, the idea that the finds are the remains of early hominins has not been ruled out entirely.[1]

Taxonomy

Chororapithecus teeth were discovered in Ethiopia in a 2005–2007 survey in the Beticha locality of the Chorora Formation, hence the name, and the formation itself is named after the Chorora village about 8 km (5.0 mi) south of the locality. The species name, abyssinicus, is in reference to Abyssinia, the former name of Ethiopia.[3]

The holotype specimen, CHO-BT 4, is a right upper second molar, and the paratypes are a left lower canine, 3 right upper third molars, a left lower third molar, a left lower first molar, and a left and a right lower molar fragment. The discoverers noted the teeth have several features in common with those of gorillas, and classified it as the first fossil member and the only other genus of the tribe Gorillini. Because the Chororapithecus teeth have several unique specializations (they are derived), they did not consider it as ancestral to the gorilla.[3] However, the discovers also conceded it is possible that Chororapithecus and gorillas instead convergently evolved the same teeth due to a similar diet.[3]

The teeth were originally dated to 10.5–10 million years ago (mya), and the discoverers then concluded that the gorilla–human last common ancestor (LCA) existed about 12 mya, but they were re-dated to about 8 mya, which is more consistent with the timing of 8 mya for the LCA according to molecular data.[4] Based on the revised date and similarly large premolar size, the 10 million year old Kenyan Nakalipithecus has been proposed to have been the ancestor to Chororapithecus.[5]

Previous efforts to find fossils of great ape ancestors in Africa from between 12 and 8 million years ago had been largely unsuccessful. This absence led some to hypothesize that apes were absent from Africa during this time, and had recolonized Africa from areas in Asia where fossils from this time period are more plentiful. Molecular estimates that humans and gorillas diverged after 8 million years ago were consistent with this idea, in which an adaptive radiation might have occurred after a single species entered Africa and began to adapt to different environments.

Current fossils and research prior to this finding indicate that the evolutionary split between gorillas and humans occurred around 8 million years ago. The new fossils indicate that the split may have happened as long as 10.5 million years ago. It is thought that humans shared a common ancestor with chimpanzees as recently as 4 to 7 million years ago.

Despite the finds, other researchers are not convinced that the conclusions are correct. Although the teeth are very similar to those of modern gorillas, they could have been shaped by parallel evolution of a genetically different species which consumed similar foods. "It is stretching the evidence to base a time scale for the evolution of the great apes on this new fossil. These structures appear on at least three independent lineages of apes, including gorillas, and they could relate to a dietary shift rather than indicating a new genetic trait," said Professor Peter Andrews at the Natural History Museum in London, UK, who also added, "but the fossil evidence for the evolution of our closest living relatives, the great apes, is almost non-existent."[6]

Palaeoanthropologist Jay Kelley of the University of Illinois at Chicago was also skeptical. He remained unconvinced that it was a gorilla, saying that more work would need to be done to determine where this specimen may fit in hominid evolution. He said he would be "very cautious" about realigning divergence dates between species based on this specimen.[1]

These teeth are collectively indistinguishable from those of modern gorilla subspecies in dental size and represented proportions. This modest sample nevertheless exhibits substantial size variation, with molars at both the largest and smallest end of the modern gorilla ranges of variation.

Anatomy

The teeth, both in absolute size and relative proportions, are the same as in gorillas. Like in gorillas, the upper molars have a long protocone crest, and the lower molars have a correspondingly long trigonid crest, which increasing shearing efficacy. Compared to gorillas, the cusp tips are relatively peripheral, are not well pronounced, and the enamel is thicker especially at the side cusps where the tooth borders other teeth. This causes a wide basin on the middle of the molar. The upper molars are elongated and narrow.[3]

Palaeobiology

The teeth exhibit adaptations for shearing, but the low cusp height indicates the teeth likely folded and pulverised tough plant fibres rather than cut through them as gorillas and other folivores with higher cusps do. The thick enamel is more similar to that of orangutans, and may indicate the consumption of hard, brittle foods. Chororapithecus and Oreopithecus are the only known folivorous Miocene apes.[3]

The Chorora Formation represents a braided river system, possibly a forested area alongside a lake in a forest-savanna mosaic environment.[3] The formation also records the earliest known occurrences of cercopithecine monkeys, hippos, and rabbits in Africa. Two different horses and a colobine monkey are also known.[7]

See also

Notes

  1. ^ a b c Dalton, Rex (2007-08-23). "Oldest gorilla ages our joint ancestor". Nature. 448 (7156): 844–5. doi:10.1038/448844a. PMID 17713490.
  2. ^ Shigehiro Katoh; Yonas Beyene; Tetsumaru Itaya; Hironobu Hyodo; Masayuki Hyodo; Koshi Yagi; Chitaro Gouzu; Giday WoldeGabriel; William K. Hart; Stanley H. Ambrose; Hideo Nakaya; Raymond L. Bernor; Jean-Renaud Boisserie; Faysal Bibi; Haruo Saegusa; Tomohiko Sasaki; Katsuhiro Sano; Berhane Asfaw; Gen Suwa (2016). "New geological and palaeontological age constraint for the gorilla–human lineage split". Nature. 530 (7589): 215–218. doi:10.1038/nature16510. PMID 26863981.
  3. ^ a b c d e f g Suwa, G.; Kono, R. T.; Katoh, S.; Asfaw, B.; Beyene, Y. (2007). "A new species of great ape from the late Miocene epoch in Ethiopia". Nature. 448 (7156): 921–924. doi:10.1038/nature06113. PMID 17713533.
  4. ^ Shigehiro Katoh; Yonas Beyene; Tetsumaru Itaya; Hironobu Hyodo; Masayuki Hyodo; Koshi Yagi; Chitaro Gouzu; Giday WoldeGabriel; William K. Hart; Stanley H. Ambrose; Hideo Nakaya; Raymond L. Bernor; Jean-Renaud Boisserie; Faysal Bibi; Haruo Saegusa; Tomohiko Sasaki; Katsuhiro Sano; Berhane Asfaw; Gen Suwa (2016). "New geological and palaeontological age constraint for the gorilla–human lineage split". Nature. 530 (7589): 215–218. doi:10.1038/nature16510. PMID 26863981.
  5. ^ Katoh, S.; Beyene, Y.; Itaya, T.; et al. (2016). "New geological and palaeontological age constraint for the gorilla–human lineage split". Nature. 530: 215–218. doi:10.1038/nature16510.
  6. ^ "Ancient ape fossil found". Natural History Museum. 2007-08-23. Archived from the original on 2012-02-06. Retrieved 2007-08-28.
  7. ^ Suwa, G.; Beyene, Y.; Nakaya, H.; et al. (2015). "Newly discovered cercopithecid, equid and other mammalian fossils from the Chorora Formation, Ethiopia". Anthropological Science. 123 (1): 19–39. doi:10.1537/ase.150206.

References

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