The ‘lesser apes’ or ‘gibbons’ belong to the family Hylobatidae of the order Primates [1]. They are typical of subtropical and tropical rainforests, ranging from those of Eastern Bangladesh and Northeast India to southern China and Indonesia (including islands of Sumatra, Borneo, and Java). This family is divided into four genera, namely Hylobates, Hoolock, Nomascus, and Symphalangus, with three currently recognized extinct genera, Bunopithecus, Junzi, and Kapi. [2,12] In India, the Northeastern states are known to house Hoolock Gibbons. However, two out of the three species of Hoolock Gibbons, namely the Western Hoolock Gibbon (Hoolock hoolock) and the Eastern Hoolock Gibbon (Hoolock leuconedys), are not equally distributed in these states. While the Western Hoolock Gibbons are predominant in Northeast India [1], the Eastern Hoolock Gibbons are not widespread and are found in specific pockets of Arunachal Pradesh and Assam. [3]
Hoolock Gibbon with its baby. Image Source: B. G. Thomson [4]
The evolutionary history indicates that the gibbons diverged from the Old World Monkeys around 29 million years ago [5]; there is a mystery around the Hylobatid origin due to the lack of fossil record of the lesser apes. While the fossil record of the great apes is diverse and well-represented in Asia by approximately 12.7 million years, the fossil record of the lesser apes is rather incomplete, with only a handful of teeth specimens of stem hylobatids (gibbons from which succeeding species stemmed) contributing to the studies till now. This has caused a large gap in the timeline and knowledge between the early fossil apes in Africa and the earliest fossil hylobatids in China. Only Yuanmoupithecus of the Late Miocene (approx. 7–9 million years ago) [6], the first widely recognized fossil taxon of the primitive sister taxon of crown hylobatids, is considered the closest known fossil to the present-day gibbons.
This probably gives rise to a certain thought in your minds: Is it possible to find fossil hylobatid records of the times way before the Yuanmoupithecus?
The answer is yes. Since the gibbons diverged from other primates at least 20 million years ago, we might as well find such records in Africa and/or Asia.
The discovery of a new fossil ape near Ramnagar in Jammu and Kashmir back in 2015 proved to be a boon in filling the gaps in the ape fossil record. The findings were published in the journal Proceedings of the Royal Society B in 2020 after years of study and phylogenetic analysis to confirm its novelty and figure out the primate phylogeny. The research team comprised well-equipped palaeontologists from the USA and India. Christopher Gilbert of Hunter College, the lead author of the study said, “We initially started working at Ramnagar because we were interested in ape and primate evolution. This site is a relatively well-known area that has produced fossil ape specimens in the past.” [7]
The field team in 2015 that discovered Kapi ramnagarensis, from left: Dr. Ningthoujam Premjit Singh (Panjab University), Dr. Rajeev Patnaik (Panjab University), Dr. Chris Gilbert (Hunter College, City University of New York), Dr. Biren Patel (University of Southern California), and Dr. Chris Campisano (Arizona State Univ.). Image source: Chris Gilbert [8]
The small-bodied ape specimen, named Kapi ramnagarensis, was reported to be from the late Middle Miocene period. It was found at the site of Ramnagar situated in the Indian Lower Siwaliks with a stratigraphic correlation to the middle or lower half of the Chinji Formation on the Potwar Plateau of Pakistan. The Hindi genus name Kapi means ape or monkey, and the species name ramnagarensis refers to Ramnagar, the site of discovery. This animal’s single molar specimen has morphological similarities to that of extant hylobatids which are stronger than Yuanmoupithecus and has managed to extend our knowledge of fossilized gibbons by about 5 million years. The geological age of this ape specimen has been estimated to be about 12.5–13.8 million years, which is contemporaneous with the great ape fossils, thereby providing a minimum age for gibbon evolution and dispersal into Asia.
“What we found was quite compelling and undeniably pointed to the close affinities of the 13-million-year-old tooth with gibbons,” said the Leakey Foundation grantee Alejandra Ortiz. She added, “Even if, for now, we only have one tooth, and thus, we need to be cautious, this is a unique discovery. It pushes back the oldest known fossil record of gibbons by at least five million years, providing a much-needed glimpse into the early stages of their evolutionary history.” [8]
Map illustrating the location of Kapi (black star) relative to the modern (dark green) and historical (light green) populations of lesser apes, and the approximate distribution of early fossil apes in East Africa (blue triangles). Green triangles mark the location of previously discovered fossil gibbons. Image source: Luci Betti-Nash
The molar specimen VPL/RSP2 (Vertebrate Paleontology Laboratory, Panjab University Department of Geology/Ramnagar Sunetar Primate 2) corresponds to that of a bunodont ape, an ape whose molars have low and rounded cusps, which is slightly smaller than the Hoolock in molar size. The tooth is longer than broad but relatively narrow as compared to that of many modern gibbons and slightly narrower than that of the Yuanmoupithecus and Bunopithecus.
CT scan images of VPL/RSP2 in various views. Clockwise from top left: Oc, Occlusal; Li, Lingual; Di, Distal; Me, Mesial; Bu, Buccal; Ob, Oblique. Image source: Proceedings of the Royal Society B [9]
There are five well-developed cusps in VPL/RSP2 that are low and conical in shape, arranged around the periphery of the crown, which is ovoid in occlusal outline. The analysis of the molar made it clear that the specimen has a damaged metaconid, which is the highest cusp with the most volume, followed by subequal hypoconid and protoconid. Even though the entoconid is similar in elevation to the hypoconid and protoconid, it is relatively smaller in the basal area. The hypoconulid is the smallest out of the five cusps; this is very typical of the apes. The talonid basin is expansive, supporting a Y-shaped groove.
Photograph of the fossil tooth (Kapi ramnagarensis). Image source: Chris Gilbert
The analysis of this lower molar specimen has made it clear that Kapi ramnagarensis is more similar to the extant hylobatids as compared to the widely accepted stem hylobatid Yuanmoupithecus. Even though only a single molar was documented, we absolutely can’t stray away from the fact that this new hominoid species is also a stem hylobatid, that too one of the earliest ones in the fossil record.
The discovery of the 12.5–13.8-million-year-old Kapi in the Lower Siwalik deposits has given proof of the dispersal of lesser apes from Africa to Asia by the end of the Middle Miocene, possibly at the same time as the great apes, such as the Sivapithecus. This has proved valuable in filling the temporal, morphological, and biogeographic gaps in hominoid evolution.
“We found the biogeographic component to be really interesting,” said co-author Dr. Chris Campisano, a researcher at the Arizona State University. He also added, “Today, gibbons and orangutans can both be found in Sumatra and Borneo in Southeast Asia, and the oldest fossil apes are from Africa. Knowing that gibbon and orangutan ancestors existed in the same spot together in northern India 13 million years ago, and may have a similar migration history across Asia, is pretty cool.” [8]
The analysis of the affinities of both Kapi and Yuanmoupithecus has also shed some light on the fact that hylobatids most likely evolved from an African taxon dentally similar to dendropithecids (members of the extinct genus of apes that are native to East Africa between 20 and 15 million years ago) or proconsulids (members of the extinct genus of primates that existed from 21 to 14 million years ago, native to Eastern Africa). Since these two advanced catarrhine groups (primates of a group that comprises the Old World monkeys, gibbons, great apes, and humans) are native to East Africa [10], there is a possibility that the early stem hylobatid fossil record is actually present in the East African Miocene, but it cannot be distinguished.
Kapi presents a dental morphology between that of Early Miocene dendropithecids / proconsulids and extant hylobatids by showcasing features of the living hylobatids and that of the primitive ones. Such dental features mirroring that of the extant hylobatids, particularly the bunodonty and expansion of the occlusal basin, indicate an adaptive shift to a fruit-dominated frugivore diet by the end of the Middle Miocene period.
Even though the Siwaliks have housed fossilized apes, none of these has actually provided a clear understanding of the gibbon fossil record. A worn upper molar found in the Middle Siwalik locality of Haritalyangar, India, was initially thought to be of a hylobatid ancestor, but it actually turned out to be from a late-occurring pliopithecid (member of the extinct superfamily Pliopithecoidea comprising catarrhine primates that inhabited Asia and Europe during the Miocene), Krishnapithecus. [11] The South Asian Dionysopithecus sp. found in the Lower Siwalik Kamlial Formation and Manchar Formation in Pakistan (approx. 16–17 million years ago) has also been regarded as a possible stem hylobatid. However, re-examining the specimens proved that it is very unlikely for them to be fossil hylobatids. Such exemplary findings have not contributed to hylobatid studies as much as this specimen from Ramnagar has.
Studies on hylobatid evolution are finally gaining pace, but clear insight into the first approximately 6–8 million years of hylobatid evolution is still necessary.
Quoting Dr Rajeev Patnaik, Professor at Panjab University and an author of the study, serves the apt conclusion, “The next step would be to study the habitat of such primates and associated mammals to understand how climate change has influenced their evolution.” [7]
References
[1] Brockelman, W., Molur, S. & Geissmann, T. (2008). “Hoolock hoolock”. IUCN Red List Of Threatened Species. (Accessed: 17 July, 2022).
[2] Geissmann, Thomas (2003). "Taxonomy and evolution of gibbons". Evolutionary Anthropology: Issues, News, and Reviews. (Accessed: 8 February, 2022)
[3] Brockelman, W., Geissmann, T. (2019). "Hoolock leuconedys". IUCN Red List of Threatened Species. (Accessed: 17 July, 2022)
[4] Thomson, B.G. (2015). Hoolock Gibbon, India Photograph by BG Thomson | Fine Art America (Accessed: 8 February, 2022)
[5] Carbone, Lucia; et al. (2014). "Gibbon genome and the fast karyotype evolution of small apes". Nature. 513 (Accessed: 9 February, 2022)
[6] Harrison (2016). “The Fossil Record and Evolutionary History of Hylobatids.” (Accessed: 9 February, 2022)
[7] Anand, A. (2020) Ape fossil from Ramnagar could fill the gaps in understanding the evolution of gibbons | Research Matters. (Accessed: 9 February, 2022)
[8] New fossil ape discovered in India. (2020) The Leakey Foundations. The Leakey Foundation | New fossil ape discovered in India (Accessed: 9 February, 2022)
[9] Gilbert, Christopher C.; et al. (9 September 2020). "New Middle Miocene ape (Primates: Hylobatidae) from Ramnagar, India fills major gaps in the hominoid fossil record". Proceedings of the Royal Society B. (Accessed: 8 February, 2022)
[10] Andrews, Peter; Simons, Elwyn (1977). "A New African Miocene Gibbon-Like Genus, Dendropithecus (Hominoidea, Primates) with Distinctive Postcranial Adaptations: Its Significance to Origin of Hylobatidae". (Accessed: 10 February, 2022)
[11] Sankhyan, Kelly, Harrison (2017). “A highly derived pliopithecoid from the Late Miocene of Haritalyangar, India.” (Accessed: 10 February, 2022)
[12] Sonstige, Wilson, Don E. 1944- Hrsg. Cavallini, Paolo (2013). Handbook of the mammals of the world (Accessed: 8 February, 2022)
Great reference. 👌 Well written.