|Scientific name Chororapithecus abyssinicus|
Similar Nakalipithecus, Primate, Samburupithecus, Ouranopithecus, Ankarapithecus
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. However, a subsequent study by Katoh et al. (2016) dated its fossils to ~8 million years ago.
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.
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.
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.
"Based on this fossil, that means the split is much earlier than has been anticipated by the molecular evidence. That means everything has to be put back," said researcher at the Rift Valley Research Service in Ethiopia and a co-author of the study, Berhane Asfaw. This refers to the calibration of the molecular clock, the average rate of fixation of neutral mutations in the human and ape ancestors living in this environment over the past 10 million years. The human-gorilla split could have occurred 10 million years ago only if the mutations arose five times more slowly than previously thought, suggesting that other common ancestors of great apes likewise arose longer ago than believed.
Other expert opinions
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."
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.
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.