An alternative hypothesis on the origin of Hominins and African Apes.

The ancestors of Humans are commonly thought to have evolved on the savannas of Africa, a theory which was originally proposed by Charles Darwin. This original proposition was based upon the fact that our closest ancestors, the African Apes, lived in the forests of Africa, and that our upright walking behaviour seemed like an adaption to open grasslands, rather than any fossil evidence available at the time. This is a plausible hypothesis, and much subsequent palaeoarchaeological evidence uncovered in Africa has appeared to support it. However, a number of recent discoveries have been hard to reconcile with this scenario, leading to the emergence of alternative ideas on Human origins.

There are a number of things about Human anatomy which appear to be inconsistent with an origin on the African Savannas. We have a furless body, a layer of subcutaneous white fat, feet much flatter than other primates, run slowly, and sweat heavily when hot or exercising, leading to the loss of fluids and minerals (our water consumption needs are high compared to other Primates even when not doing this).

For a long time it was hypothesised that knuckle-walking African forest Apes migrated onto the open grasslands, where they evolved into upright Australophithecenes, and subsequently into Humans. Over time, this theory has been modified as we have come to understand more about the environments favoured by Australopithecenes, they are now seen as inhabitants of mosaic woodlands who moved from a partially bipedal lifestyle to an obligate bipedal one to facilitate crossing more open areas, although why this should be the case remains unclear.

Another puzzle is the apparent absence of fossils ascribed to either the genera Pan (Chimpanzees) or Gorilla, despite the long lineage of fossil Human-ancestors dating back into the Pliocene. The genera Australopithecus, Paranthropus, Sahelanthropus, Orrorin, and Ardipithecus, have all been classified as Hominins, more closely related to modern Humans than either Pan or Gorilla. This is in contradiction to what would be expected, as genetic studies suggest modern Chimpanzee and Gorilla populations have diverged from large ancestral populations, while Humans apparently descend from a lineage with consistently small population sizes and repeated genetic bottlenecks. It has been suggested that this has come about because the ancestors of Chimpanzees and Gorillas lived in acidic forests, where there is lower potential for remains to become fossilised. However, this environment is more favoured by Orangutans, which do have a fossil record, than it is by Chimpanzees, which do not.

'Mrs Ples' a 2.1 million-year-old skull assigned to the species Australopithecus africanus in the collection of the Ditsong National Museum of Natural History. José Braga/Didier Descouens/Wikimedia Commons.

In a review article published in the journal Academia Biology on 10 June 2026, Marc Verhaegen of the Anthropology Study Center in Putte, Belgium, Stephen Munro of the National Museum of Australia, Kathelijne Bonne of GondwanaTalks in Madrid, Spain, Frances Mansfield, an independent researcher from Volos in Greece, and Mario Vaneechoutte of the Faculty of Medicine and Health Sciences at Ghent University, present a new hypothesis on the origins of both Humans and African Apes, in which they argue that the Australopithecenes were not, in fact the ancestors of Humans but rather of modern Gorillas and Chimpanzees. 

Verhaegen et al. not that genomic studies have found that the ancestors of modern Gorillas and Chimpanzees were infected with the Endoretroviruses PtERV1 (CERV1) and PtERV2 (CERV2) between 3 and 4 million years ago (Endoretroviruses are fragments of ancient viral DNA which have been inserted into the hosts genome), but Humans and Asian Apes show no signs of any such infections in their past. Since Humans are not immune to the effects of these Viruses, the most likely explanation is that our ancestors were geographically isolated from the epidemic. It has been suggested that Human ancestors may have moved out of Africa during much of the Miocene Epoch, when the Viruses were circulating, but Verhaegen et al. suggest that a more likely scenario is that they did not arrive in Africa at all until the Early Pleistocene. Few, if any, Animals are thought to have migrated from Africa to Europe during the Pliocene, while a range of European Animals migrated into Africa, driven by the hyper-arid climate that emerged in southern Europe at this time. 

Furthermore, Verhaegen et al. consider that the Australopithecines found in Africa during the Pliocene were not closely related to modern Humans, but rather that their smaller brains, short legs, and long arms indicate that they were related, or even ancestral, to modern African Apes. The evolution of Australopithecenes has often been seen as confusing, as earlier species often have more Human-like traits, seen as 'advanced', while later forms are often more Ape-like, or 'primitive', leading to speculation about undiscovered ghost lineages connecting earlier species to Humans. 

This distinction vanishes if Australopithecines are considered to be the ancestors of African Apes. For example, the Miocene Ardipithecus ramidus, which lived about 4.4 million years ago, had small canine teeth, similar to those seen in modern Humans, whereas the Pliocene Australopithecus africanus had much larger canines, comparable to a modern Ape. Other examples are the Miocene Orrorin tugenensis, which lived about 6 million years ago, had femurs more closely resembling those of Humans that any Pliocene Australopithecene, and the Early Pliocene Australopithecus anamensis, which lived about 4.2 million years ago, had a modified talar trochlea which would have enabled it to swing its leg forward during upright locomotion, something which is absent in subsequent members of the genus. Furthermore, later Austalopithecenes such as the Early Pleistocene Australopithecus sediba, which lived about 1.8 million years ago, appear better adapted to an arboreal lifestyle than earlier members of the group.

A cast of a skull of Ardipithecus ramidus from the Miocene of Ethiopia, in the collection of the Royal Tyrrell Museum. Chris Woodrich/Wikimedia Commons.

Other than an early adaptation to bipedalism, Verhaegen et al. see Australopithecenes as consistently more Ape-like than they are Human-like, in particular noting that Australopithecus afarensis appears more Gorilla-like and Australopithecus africanus appears more Chimpanzee-like.

Verhaegen et al. do not see Homo habilis, the smallest and earliest member of the genus Homo, which appeared in the Early Pleistocene, about 2.58 million years ago, to be a true member of the genus Homo, noting that the species shares a small brain, short leg, and long arm morphology with Australopithecenes and Apes, and in particular that the morphology of the enamel–dentine junction in this species is also more Australopithecene-like. The later, but equally small, Homo naledi is also considered to be an Australopithecene. 

In Verhaegen et al.'s view, the last common ancestor of all Australopithecenes, living Apes, and Humans, would have been an upright Miocene Ape, capable of bipedal locomotion, climbing trees, and wading in shallow water. They describe a scenario in which these Apes led a lifestyle they describe as 'aquaborealism', living in forests which were at least seasonally flooded, with a lifestyle which involved wading in waters on the forest floor, climbing trees in a vertical position, and swinging beneath branches (branchiating). From this ancestral state the Hyobatids (Gibbons and Siamangs) evolved into small, fast branchiating Apes living in the tree canopy, Orangutans into larger, slower, branchiating forms, Gorillas and Chimpanzees (separately) into knuckle-walking forms, and Humans into bipedal walkers.

They give two potential scenarios from which modern African Apes could descend from Australopithecenes; either Chimpanzees descended directly from earlier, more gracile Australopithecenes, such as Australopithecus afarensis and Australopithecus africanus, while Gorillas arose from later, more robust forms such as Paranthropus boisei and Paranthropus robustus, or they went through separate but parallel evolutionary paths, running something like Australopithecus africanus-Paranthropus robustus-Chimpanzees and Australopithecus afarensis-Paranthropus boisei-Gorillas, in response to similar environmental changes.

A skull reconstruction assigned to Paranthropus robustus in the collection of the National Natural History Museum of China. Erlend Bjørtvedt/Wikimedia Commons.

Verhaegen et al. believe that the view of the common ancestor of Apes and Humans as being Ape-like, and Apes therefore as the 'primitive' state, has distorted our view of Hominid evolution for over a century. They argue that while Australopithecenes do have some Human-like traits, these are indicators that Australopithecenes are ancestral to Humans, but rather that they share some traits derived from a mutual common ancestor that frequently adopted an upright bipedal posture as an adaptation to life in flooded forests. 

The analysis presented by Verhaegen et al. suggests two distinct phases of Human evolution associated with wet environments; an initial aquaboreal phase in flooded Miocene forests, followed by a littoral (beach dwelling) phase, which may have involved frequent shallow diving to access food, which probably continued into the Early Pleistocene.

Apes differ from Old World Monkeys in a number of ways, the most obvious of which is their much larger size. This appears likely to have been a trait found in the last common ancestor of all Apes; even the relatively small Hylobatids are thought to have evolved from a larger ancestor, due to their long gestation period, which is unusual in such a small Primate, and is thought to be a hangover from a larger ancestor. Another notable trait is the absence of a tail in Apes. This has become much reduced, and forms a part of the 'pelvic cup'; a modification of the pelvis which helps to support the intestines when in an upright position. There is no comparable tail loss in any other Primate to which this can be compared, but the idea that this was an adaptation to an upright posture while engaged in vertical climbing and branchiating seems reasonable. Furthermore, if these ancestral Apes were spending a lot of time in wading in water, then a tail might have been disadvantageous, prone to heat loss, adding to the friction of the Apes when moving through the water, and prone to infections or attacks by predators. 

Verhaegen et al. note that quadruped Animals returning to the water tend to evolve in one of two ways. Those that use spinal flexation as the main means of propulsion and the tail as the driving organ, such as Whales and Sirenians, tend to lose their hind limbs, whereas those that use their limbs for propulsion, such as Seals, Bears, Penguins, and Hippos, tend to lose their tails. An exception can be seen in Animals such as Otters and Beavers, which use their tails for propulsion but make extensive use of their limbs for foraging and movement on land. Apes, which use their limbs to swim when they enter the water, have lost their tails.

The lumbar spine of Apes is stiffer and further from their dorsal surface than is the case in most Mammals, and their forelimbs (arms) are notably long, adaptations which seems favourable to climbing in an upright position and hanging below branches. They also have wide hips compared to other Primates, and a flatter pelvis, facilitating lateral leg movements, as well as a broader sternum and thorax, which pushes their scapulas into a more dorsal position, facilitating lateral and upward arm movements. 

Fossils of Miocene Apes are often found in what have been interpreted as warm, wet, forest environments, which has led to the suggestion that they may have been aquaborreal in nature, spending their time wading through flooded forests or climbing in the branches above with their arms. Such behaviour is known in extant Apes, for example Gorillas have been observed entering forest swamps to forage for Sedges, Bonobos will wade through water to obtain Waterlilies, and Orangutans have been observed wading in shallow water in Borneo.

Bipedal Gorilla, wading. Female Gorilla with a walking stick, while crossing bipedally an Elephant pool at Mbeli Bai, Republic of Congo. Thomas Breuer & Emma Stokes in Verhaegen et al. (2011).

Verhaegen et al. propose that all modern Apes derived from this initial aquaboreal Miocene Ape, with Gibbons adopting a fast branchiating motion, Orangutans a slower branchiating movement combined with knuckle walking, Gorillas and Chimpanzees separately developing a knuckle walking gait, and the ancestors of Humans going through a distinct litoral (coast dwelling) phase.

A number of lines of evidence have pointed towards Early Pleistocene members of the genus Homo may have engaged in regular wading, swimming, and even diving. Homo erectus has been observed to have a pachyosteoscletotic skeleton (i.e. unusually dense bones), something associated with slow-moving, shallow-diving Tetrapods such as Sireneans, as well as the earliest Whales and Seals. The dense skeleton can help such Animals maintain their position in the water, particularly in saltwater environments (where the body is more buoyant). The occipital bones of Homo erectus are roughly twice as thick as those of comparably sized Apes, making the interpretation of the species as a swift bipedal predator chasing down prey hard to sustain. 

Furthermore, the low positioning of the braincase, receding forehead, absence of a chin, and forward projecting face of Homo eructus appear to be an adaptation to frequent shallow water diving, and possibly floating on their backs. The forward pointing face and paranasal sinuses of Homo eructus may indicate a habit of surfacing nose first, with the nostrils above water and the heavy occipital area at the back of the head beneath the water, something which would have worked well in a back-floating position. The basicranial flexation of modern Humans holds our face in a ventral, forward facing position. In contrast, Homo erectus would have tended to look upwards, in what would be a forward position when swimming or diving (Neanderthals are somewhere between these positions). This has been suggested to be an adaptation to foraging in coastal waters, where the easy availability of coastal food sources would compensate for the lack of stability associated with a bipedal gait on land.

Several skulls of Homo erectus have been shown to have bony growths in the inner ear called aural exotoses (or surfer's ear) which is caused by chronic exposure to cold water. This has also been observed in about half of all Neanderthals. Verhaegen et al. observe that this directly contradicts the frequent claim that there is no direct palaeontological support for the coastal Ape hypothesis.

At the same time, Human-ancestors underwent both an increase in both overall body size, and relative brain size. This is a common adaptation to moving from a terrestrial to an aquatic lifestyle, seen for example in Whales and Dolphins, but is not typical when non-aquatic Mammals increase in size; for example, the largest Apes, Gorillas, do not have relatively large brains. Homo erectus underwent a significant increase in brain size, with later specimens had a brain twice as large as that of an equivalent-sized Ape. This may have benefited from the higher proportion of nutrients such as docosahexaenoic acid, folic acid, selenium, taurine, and iodine, in aquatic-derived foods, all of which are needed for brain-growth. Such disproportionate brain growth is not seen in terrestrial carnivores, and therefore seems unlikely in a Hominin chasing prey in open grassland. Notably, Australopithicenes never showed any notable increase in brain size over their two-million-year history. Verhaegen et al. take this as evidence that early Homo did not evolve from such ancestors on the African grasslands, since there seems no good reason that our brains should have started to grow rapidly while remaining in the same environment.

There is also considerable evidence that Homo erectus did consume aquatic foods. Fossils of Homo erectus from Mojokerto on Java were found in association with numerous marine Bivalve shells, while those at Trinil on the same island, were found with the shells of  freshwater Bivalves, such as Pseudodon and Elongaria, including specimens engraved with geometric patterns. Furthermore, palaeoarchaeological remains from the Koobi Fora Formation in the Turkana Basin of northern Kenya have yielded stone tools from a Oldowan technology alongside the remains of aquatic Animals including Fish, Turtles, and even Crocodiles. Early Homo specimens from a variety of locations have dental wear associated with grit and the oral processing of marine Molluscs. Archaic members of the genus Homo are known to have reached Sulawesi by one million years ago, despite this island never being connected to the Eurasian mainland, suggesting an early ability to cross open water.

A Pseudodon shell with geometric engravings from the Trinil archaeological site in East Java, dated to about 500 000 years ago. The shell was excavated by Dutch palaeoarchaeologist Eugène Dubois in the 1880s and taken to the Netherlands. It was returned to Indonesia by the Naturalis Biodiversity Center in 2025. Naturalis Biodiversity Center/Wikimedia Commons.

The genus Homo first appeared around the beginning of the Pleistocene, with the first evidence of the consumption of aquatic foodstuffs appearing about two million years ago, across the tropical and temperate regions of the Old World. This has led to the suggestion that Homo was able to spread across this area rapidly by following coasts and rivers, wading and diving for food. There is also increasing evidence for the consumption of aquatic foods by Neanderthals across their range, as well as Modern Humans across the world, and from their earliest appearance. 

Humans differentiate from the standard morphology and physiology of Primates, and indeed terrestrial Mammals in general, in several ways. We have fleshy outward lips, a small oral opening, a closed tooth row, a more globular tongue, a descended larynx, and an external nose that still today some individuals can partially close, using nasal muscles. We also have a distinct 'Cupid's bow' to our upper lips (technically the philtrum) to our upper lips, another feature which some modern individuals can use to close their nasal airway. All of these are adaptations which enable us to regulate breathing and seal our airways better, and which may have been more pronounced in Homo erectus.

Feeding on shellfish would also have required adaptations which required improved abilities to control the sucking and swallowing of food, particularly if this was done in the water. Adaptations which improved fine control over the lips, jaw, tongue, glottis, and larynx, would also have acted as pre-adaptations for the later evolution of speech. Hunting for foodstuffs underwater, particularly if we were doing this blind (by touch alone) is also likely to have improved both our manual dexterity and the sense of touch in our fingers, something also seen in Raccoons and Otters.

In Humans, the connective tissues, tendons, ligaments, and muscles of the human foot are aligned with the hallux to form a longitudinal arch. This is unlike the foot of any other Primate, and is a good adaption to walking, and acts as a shock-absorber when running. However, it is also a particularly good adaptation to swimming, particularly as our feet are relatively larger and more paddle-shaped, and makes us particularly sure-footed on wet or infirm terrain, where other Mammals often struggle. Humans are able to swim using axial undulation, a coordinated wave of motion from trunk to hips to legs, something other Primates are unable to do. Our feet can be seen as a trade-off between something useful for swimming and something useful on land. Notably, we are less efficient runners than most other Mammals, which directly contradicts the idea that we evolved to chase large prey across grasslands.

Notably, Humans have a layer of white fat beneath our skins (the adipose layer) considerably thicker than found in any other Primate, as well as an overall fat content which is also distinctively high. In a healthy male Human, between 12% and 23% of the body mass is fat, while in a female it is between 25% and 35%. In Chimpanzees and Bonobos body fat typically makes up less than 5% of the body mass of males, and less than 8% of the bodymass of females. Thus even the leanest of Humans have a significantly higher proportion of fat than healthy Chimpanzees. This suggests that Humans have undergone a significant ecological shift after our ancestors split from those of Chimpanzees, and one which decoupled the fat content of our bodies from the its use as an energy store. Such a change is again typical of aquatic and semi-aquatic Mammals, where fat has an important role both as an insulator and as a buoyancy aid. Such a layer provides no particular role during activities such as sustained running, and may be disadvantageous, as it can hamper the dissipation of heat.

Verhaegen et al. believe that plate tectonics played an important role in the evolution of Hominoids, Hominids, and Hominins, and in particular the formation of the Red Sea and the African Rift. Until about 30 million years ago (Early Oligocene), Africa and Arabia were a single island continent, separated from Eurasia by the Tethys Ocean. Over time, this continent drifted northward, the Tethys Ocean narrowed, and a series of islands and archipelagos formed between the two continents.

Between about 30 million years ago and about 20 million years ago (during the Oligocene and Early Miocene), an episode of plume volcanism beneath the Afar Triangle led to uplift, basalt volcanism, and the formation of a triple junction rift system. The three branches of this rift would go on to form the Gulf of Aden to the east, the Red Sea to the northwest, and the Ethiopian Rift to the south. At this time Eurasia and Africa-Arabia were still separated, although the Mesopotamian Seaway, which separated Arabia from what would become Mesopotamia and Persia, was becoming increasingly narrow.

Between about 20 million years ago and about 14 million years ago (Early-Middle Miocene) the Mesopotamian Seaway slowly closed, leading to the formation of a connection called the Gomphotherium Landbridge (Gomphotherium being a type of early Elephant that migrated out of Africa across this landbridge), leading to faunal exchange between Africa-Arabia to the south and Eurasia to the north. forming the first wave of the 'Great Old World Biotic Interchange', while the Mediterranean Sea and Indian Ocean became isolated from one-another. This also led to uplift and mountain formation along the Bitlis-Zagros Suture Zone, and the development of the Dead Sea Transform and Aqaba faults in response to the added tectonic stress. At this time the Red Sea had opened, and was connected to the Mediterranean by by the Gulf of Suez, but a land bridge at Bab-al-Mandeb, connecting Arabia to Africa, separated it from the Gulf of Aden.

Between about 14 million years ago and about six million years ago (Late Miocene), movement on the Dead Sea Transform Fault, combined with the mass of sediments around the Nile Delta, closed off the connection between the Red Sea and the Mediterranean at the Gulf of Suez. Following this, the Red Sea underwent a desiccation crisis, drying up and leaving vast salt deposits, albeit with occasional marine incursions.

Between about 5.9 and 5.33 million years ago (latest Miocene), the Gulf of Gibraltar also closed, cutting off the inflow of water from the Atlantic to the Mediterranean, and triggering the Messinian Salinity Crisis, in which the Mediterranean largely dried up, leaving a vast and inhospitable salt plain. 

After about 5.33 million years ago (Pliocene), the straits of Bab-al-Mandeb opened up allowing the Red Sea to flood from the Gulf of Aden, and the Strait of Gibraltar reopened, allowing the Mediterranean to refill from the Atlantic (the Zanclean Megaflood). Around this time the Gulf of Suez reconnected to the Mediterranean, cutting off Africa from the Red Sea until the beginning of the Pleistocene, when a land bridge formed across the Sinai again.

Simplified tectonic and palaeogeographic evolution of the Mediterranean Sea and the Arabian Peninsula, with key marine connections between Africa, Arabia, and Eurasia (details of European and Paratethys geographies not given). (1) Before 30 million years ago (until the Rupelian, Oligocene); (2) 30–20 million years ago (Rupelian–Burdigalian); (3) 20–14 million years ago (Burdigalian–Langhian); (4) 14–6 million years ago; (5) From about 5.9 to 5.33 million years ago: the Strait of Gibraltar closed due to plate tectonics; (6) From 5.33 million years ago onwards. Red circle in (2): Afar plume basalt eruptions; brown line in (3): uplifting mountain front at the Bitlis-Zagros Suture Zone; red line in (3) and (4): active Dead Sea Transform Fault; pale orange in (4) and (5): desiccated marine domains; blue arrows in (6): marine gateways and direction of filling of basins. Verhaegen et al. (2026).

Verhaegen et al. suggest two alternative scenarios for the emergence of the first Apes. One scenario sees a group of Early Miocene Primates living in coastal forests and islands along the Mesopotamian Seaway, which gave rise to both the Apes and the Old World Monkeys. The other sees these ancestors living in coastal forests in East Africa, which migrated northward along the Red Sea Rift as it opened, migrating into the Arabian Peninsula and then eventually into Eurasia. This latter scenario is supported by the presence of the possible Ape Morotopithecus in Uganda about 20.6 million years ago, although the exact status of this fossil is unclear.

In either scenario, Early Apes increased in size rapidly after splitting from the Old World Monkeys, at the same time developing a very broad and strong sternum in a broad thorax, a somewhat shorter lumbar spine with only five lower and more centrally placed lumbar vertebrae (indicative of a vertical body posture), an enlarged sacrum equipped with a coccyx in a broad pelvis (cup form, supporting the intestines), external tail loss, relatively longer legs (indicative of wading), and longer arms (indicative of below-branch hanging). These early Apes are thought to have lived in (probably coastal) swamp forests, hanging beneath branches, practising aquarbourism, and developing to an upright bipedal stance. 

These early Apes probably spread along the northern coast of the Tethys Ocean, with different groups splitting off and going their own way, beginning with the Gibbons. Between 22.4 and 16 million years ago the ancestors of the Orangutans had split from those of the African Apes, moving eastward towards the coastal forests of Southeast Asia, while the proto-African Apes colonised forests along the Western Tethys (what would become the Mediterranean). 

There is a surprising absence of Ape or Hominid fossils from the Middle Miocene of Africa (between about 13 and 10 million years ago). At the same time, Eurasia has an abundance of both, particularly in southern Europe and Anatolia, with many forms looking like plausible ancestors for African Apes. Verhaegen et al. cite this as support for the idea that the ancestors of modern Humans and African Apes were not in Africa during this time, but instead in southwestern Eurasia. At this time much of southern Europe was covered by  inland seas, mega-lakes, swamps, and coastal forests, and home to Dryopithecine Apes such as Dryopithecus, Pierolapithecus, Danuvius, and Rudapithecus, which are potentially ancestral to modern Gorillas, Chimpanzees, and Humans.

A partial skull of Pierolapithecus catalaunicus, a Dryopithecine Ape from the Middle Miocene of Spain. Institut Català de Paleontologia Miquel Crusafont.

During the Vallesian Crisis (between about 11.6 and 8.7 million years ago) the climate of southern Europe became much drier and the extensive forests shrank, being replaced by open grasslands. Many of the Apes there died out, while surviving forms, such as Ouranopithecus and Graecopithecus adapted to the new environment by becoming more bipedal, and foraging in mixed woodlands and river valleys. These Apes could potentially be ancestral to later Australopithecenes and African Ape (one Turkish Ape from this period, Anadoluvius turkae, has been suggested as an offshoot from the line which led to Gorillas).

The scenario envisaged by Verhaegen et al. has a group of Dryopithecine Apes taking advantage of the closure of the Mesopotamian Sea, and migrating to the swamp forests surrounding the early Red Sea. The ancestors of Orangutans must have split from these western Apes before 14 million years ago, when the Badenian Transgression would have blocked migration between Europe and East Asia. The lineage which led to Gorillas probably split off next, migrating from Europe, where taxa such as Ouranopithecus remained, while closer ancestors such as Anadoluvius turkae migrated through Anatolia, and on to Africa, where they gave rise to Gorilla-like African Apes such as Chororapithecus and Sahelanthropus, then eventually the East African Australopithecenes and modern Gorillas.

At the same time, Graecopithecus and similar species around the Mediterranean could provide plausible ancestors for Chimpanzees and Humans, explaining the Hominid-like footprints seen at Trachilos on Crete about six millions of years ago (long before the earliest such footprints in Africa). The lineage which led to Humans and Chimpanzees must also have migrated southward, at latest during the onset of the Messinian Salinity Crisis about 5.9 million years ago, which would have made the Mediterranean Basin uninhabitable. The route from Europe into Africa across the Sinai Peninsula would have been cut off during the Zanclean Megaflood (5.33 million years ago), which filled the Mediterranean and over-spilled into the Red Sea, filling that too. The final connection between Arabia and Africa was lost about 5 million years ago, when the Bab-el-Mandeb Strait opened, connecting the Red Sea to the Gulf of Aden.

Verhaegen et al. suggest that some of these Apes crossed into Africa before 5 million years ago, giving rise to the Southern African Australopithecines and eventually modern Chimpanzees. Another group remained on the southern shore of Arabia, where they were forced to turn increasingly to the littoral environment for survival, as the land became increasingly arid and hostile.

Around 2.8 million years ago the Earth's climate cooled sharply, leading sealevels to drop abruptly. At this time early Homo migrated from Arabia into Africa and Eurasia, leading to the sudden appearance of Homo erectus across the Old World. 

A reconstruction of the skull of Homo erectus in the collection of the University of Michigan Museum of Natural History. Thomas Roche/Wikimedia Commons.

Under this scenario, the genus Homo is absent from Africa until the Early Pleistocene appearance of Homo erectus/Homo eregastor, with the Pliocene Australopithecines not being ancestral to Modern Humans, but instead relatives of Chimpanzees and Gorillas. This view is broadly in-line with several recent cladistic analyses of Human origins, which suggest African Apes and Humans are derived from Eurasian Apes, and that there were multiple crossings from Eurasia into Africa. 

Verhaegen et al. take the view that East and Southern African Australopitecines largely evolved in parallel, in response to similar ecological pressures. Thus the gracile forms, Australopithecus afarensis and Australopithecus africanus, appeared in the Pliocene, when they were able to live in forest, and in particular swamp forest, environments, whereas the more robust Early Pleistocene forms, Paranthropus boisei and Paranthropus robustus, evolved in response to cooler, drier conditions, although generally sticking close to large bodies of water, such as Papyrus swamps. Eventually the modern African Apes adopted to a life on dry forest floors.

It is possible that the Southern African Australopithecines were more omnivorous, whereas the East African forms adapted to the processing of tougher plant materials, something which is reflected in the diets of modern Chimpanzees and Gorillas. 

This later arrival of Human-ancestors into Africa provides an explanation for the absence of any trace of the Endoretroviruses PtERV1 (CERV1) and PtERV2 (CERV2) in our genomes, while the ancestors of Gorillas and Chimpanzees were affected. The potential littoral phase also explains the many morphological adaptations which set Humans apart from other Apes, as well as behavioural traits such as a fondness for water and sea coasts. 

Verhaegen et al. believe that the earliest members of the genus Homo were shelfish divers, something which would have provided them with the resources for accelerated brain growth, as well as adaptions such as an external nose and pachyosteosclerotic skeleton. As supporting evidence for this, they cite the frequent occurrence of ear exostoses (surfer's ear) in early Homo, as well as tooth wear associated with a shellfish diet, the early arrival of Homo on remote islands such as Flores and Sulawesi, and the co-occurrence of their fossils and tools with shells, and even engraved shells.

See also...

Lethal conflict during the fission of a Chimpanzee group in Kibale National Park, Uganda.

Warfare, and other forms of collective violence, are one of the more distinctive, if less pleasant, forms of Human behaviour. In these acts, Humans are able to sort themselves into groups defined by traits other than kinship, such as ethnicity, religion, language, or other cultural traits, in order to engage in conflict against other groups of Humans. It has been proposed that this is a result of our ability to use cultural traits to define cohesive social groups, giving us a distinct sense of who is 'us' and who is 'them', enabling closer cooperation with members of our group, while at the same time maintaining a hostility to outsiders. 

However, this cannot explain how conflict can erupt within formerly cohesive groups, leading to rebellions and civil wars. This has been explained by suggesting that shifting interpersonal ties and rivalries can sometimes reach a point where internal hostilities overcome the cohesion of the group, regardless of the cultural connections which have been built up. This hypothesis has some evidential support, with observations suggesting that Humans can rapidly come to regard former members of an in-group as members of an out-group for the most arbitrary of reasons. 

Non-Human Animals also engage in territorial aggression, and sometimes lethal conflict with cospecifics, despite not having the religious, cultural, or political ideologies seen in Humans. This has been observed in a variety of Animals, including Banded Mongooses, Lions, Wolves, and Primates. Non-Human Animals living in social groups also have episodes of fission, in which one group splits permanently into two or more, something which is often explained in terms of feeding competition and social pressures, although most Animal groups do not engage in lethal combat during such fission episodes.

Chimpanzees, Pan troglodytes, are among our closest relatives, and in some ways can be seen as a bridge over the gap between Human and non-Human behaviour. Male Chimpanzees stay within the group they were born in, and engage in cooperative defence of the group's territory, as well as raids on the territories of neighbouring groups. Lethal violence between males of different groups is quite common during both of these activities. This can be explained in terms of hostility towards outsiders, while remaining loyal to members of the kin group. Chimpanzee groups are known to occasionally split, a process which is thought to involve lethal violence between males familiar with one-another.

In the 1970s, a group of Chimpanzees living in the Gombe National Park in Tanzania was thought to split in two, with the males of one of the new groups subsequently killing one of the adult females and all six adult males in the other group over a period of about four years. However, this group was not under constant observation, so that much of this activity is inferred rather than having been directly witnessed, and no subsequent observations of similar splits in Chimpanzee groups were recorded in the following decades. Furthermore, a study published in 2014 which looked at the genetic structure of Chimpanzee groups suggested that such ruptures were extremely uncommon, with groups splitting on average once every 500 years.

In a paper published in the journal Science on 9 April 2026, a group of scientists led by Aaron Sandel of the Department of Anthropology at the University of Texas at Austin, present a detailed and extensively documented study of a fission event which split a group of Chimpanzees living in the Kibale National Park in Uganda, and the lethal violence associated with this split.

Sandel et al. analysed 24 years of data on the social interactions of the group, ten years of GPS data, and 30 years of demographic data on the Ngogo Chimpanzee Group, which demonstrated a three step process, in which a formerly cohesive group polarised into two clusters with little social interaction between these clusters, these clusters then actively avoided one-another for two years, before engaging in a series of lethal aggressive actions. Sandel et al. take this as evidence that Chimpanzee groups can fracture and engage in collective violence against members of the same community without any of the cultural markers seen in Humans.

The Ngogo Chimpanzees have been the subject of a continuous research project since 1995. For the first two decades of the study, they remained a single group, although they did go through a regular fission-fusion dynamic in which the larger group split into parties which changed throughout the day, though individuals frequently moved between parties and all remained within the overall territory; this is something commonly seen in Chimpanzee groups. Females typically migrated from (or to) the group at adolescence, while males born into the group remained there for life. The males formed a strict dominance hierarchy, associated in mixed-sex parties, hunted together, and cooperated in territorial patrols.

Each adult male in the group was followed for 2-3 months each year between 1998 and 2024 to see which other individuals they associated with, stayed close to, and engaged in grooming with. Despite being a single group, the Ngogo Chimpanzees typically split into two-to-four clusters over the course of a year, with two persistent and long-lived clusters, the Western and Central clusters. Membership of these clusters was fluid, with 29% of Chimpanzees switching cluster each year, and extensive ties maintained between Chimpanzees in different clusters. 

As well as clusters, Sandel et al. identified a number of 'cliques' of males that consistently stayed together, even when switching clusters. One of these cliques comprised a group of three males that would go on to form the core of the post-fission Western Group. These three males remained together consistently, even when forming clusters with males that would go on to be in the post-fission Central Group. A cluster comprising exclusively males that would go on to form the post-fission Western Group first appeared in 2014.

In 2015, Ngogo Chimpanzees at the edge of their territory hear calls from neighbouring Chimpanzees; adult males embrace each other in an act of reassurance before moving toward the calls and engaging in an intergroup encounter. 'C' and 'W' designations refer to males who would later become members of the Central and Western groups, respectively. Sandal et al. (2026).

Despite the different social clusters, all of the Ngogo Chimpanzees, including all of the males, had overlapping space use patterns, and all males shared the same set of reproductive partners. All Chimpanzees born within the Ngogo Group between 2004 and 2014 that it was possible to genetically sample had both parents from within the group, though 44% had parents from two different clusters. 

The first sign of a split between the clusters was observed on 24 June 2015, when members of the Western and Central clusters were seen to approach one-another near the centre of the territory. Unexpectedly, rather than the two groups merging as usually happened on such occasions, the Chimpanzees of the Western Cluster ran away, with the Chimpanzees of the Central Cluster chasing them. The two clusters then avoided one-another for six weeks, something which had never been observed before.

The events of 2015 precipitated the greatest change of social change seen throughout the decades-long study of the group. What had been a single large group of Chimpanzees split into a number of smaller units, stabilising as two new groups by 2018.

The first patrol by one group against another happened in 2016, when males of the Western Group, accompanied by two of the Central Group males, staged a patrol against the Central Group. All subsequent patrols by the Western males contained only members of that group. In 2017, the Central Group males staged their first patrol against the Western Group, with aggressive interactions between the two groups escalating rapidly from that point. During one encounter in 2017 the males of the Western Cluster attacked the alpha male of the Central Cluster (who had been part of the Western Cluster before 2014), severely injuring him. Both groups subsequently increased the number of patrols against the other group.

By 2017 the two groups were using largely distinct territories, with the overlap between the two groups being similar in size to that seen between unrelated groups. The centre of the shared territory had become a border. This was accompanied by reproductive isolation between the two emerging groups, with the last infant with parents from different groups being conceived in March 2015. All subsequent births had both parents from the same new group.

By 2018, the original group appeared to have split into two new groups, entirely separated from one another. At this time the Western Group included 10 males and 22 females over the age of 12, and the Central Group comprised 30 males and 39 females over the age of 12. Until 2018, a few of the females and infants from the Central Group would occasionally join the Western Group when foraging from Fig trees. After 2018 all such activity ceased.

Following this complete split, the Western Group initiated a series of lethal attacks against members of the Central Group. On all occasions members of the Central Group were attacked by multiple members of the Western Group, during a patrol by members of the Western Group into the territory of the Central Group. Six lethal attacks on males belonging to the Central Group were observed between 2018 and 2024, with a seventh attributed with a high degree of confidence. From 2021 these attacks were also carried out on infants from the Central group, with fourteen infanticides observed, and another three inferred. 

Sandal et al. note that this is a conservative estimate. Between 2021 and 2024 another 14 adult and adolescent males from the Central Group disappeared, and are thought likely to have been killed. None of these individuals showed signs of any illness when they were last observed.

In 2019, the Western Chimpanzees, including W1, W2, and W3, attack and kill the adult male Central Chimpanzee C1, who they had associated with before the fission, marking a notable shift in relationships. Sandal et al. (2026).

By using decades of data gathered on the Ngogo Chimpanzees in Kibale National Park, Sandel et al. have been able to demonstrate the occurrence of a fission event within a wild group of Chimpanzees, something predicted to happen only once every 500 years. This was followed by a series of lethal aggressive interactions, with targeted violence continuing years after the split, something not observed in any other non-Human Primate. The rate at which killings occurred was far higher than that seen in small Human societies. This demonstrates that Chimpanzees can develop new group boundaries and defend them violently, despite not having any concept of ethnicity, religion, or political ideology.

Sandal et al. speculate that a number of factors might have contributed towards the division of the Ngogo Chimpanzee Group. Firstly, the group was unusually large, with almost 200 individuals, and almost 30 males, much larger than other known Chimpanzee groups, which may have strained the males ability to maintain good relations with all other members of the group. Secondly, feeding competition has been shown to play a role in group fission in other Primate species. The area occupied by the Ngogo Group had abundant food, but the large size of the group may have caused strain at some times of the year. Thirdly, the two groups became reproductively isolated before finally separating into two groups, something which may have increased male-male aggression as they had to compete for a smaller number of mates.

Other factors which may have played a role are also observed by Sandal et al.. Firstly, six adult members of the group, five males and a female, died in 2014. The cause of these deaths is unknown, but two showed signs of illness before their deaths, making it possible that the group was hit by a disease. The loss of more than 10% of the male Chimpanzees in the group may have weakened the groups network of social relationships, leading to the group beginning to break up in 2015. 

Next, the dominant male in the group changed in 2015, immediately before the fission of the group. Such changeovers are known to raise tensions among male Chimpanzees, leading to increases in behaviours such as aggression and avoidance. The former dominant male belonged to the Central Group, whereas the new dominant male came from the Western Group, but moved to the Central Group when he ascended to the top of the hierarchy, which Sandal et al. suspect may have increased tensions between the two groups.

Finally, in January 2017, the group was hit by a respiratory epidemic which killed 25 Chimpanzees, including four adult males and ten adult females. Two of the males that died were from the Western Group, including one of the last males in that group to be maintaining relations with the Central Group. Thus, even though this event happened after the groups had started to split, it may have contributed to the final breakdown in relations between the two. 

Chimpanzees are known to have a strong sense of who is in their group, and who is not. Female Chimpanzees leave the group they were born into as adolescents and look for a new group to join, but males remain in the group they were born into their entire lives. Thus any unfamiliar male Chimpanzee is treated as a stranger, with no recorded instances of wild male Chimpanzees forming relationships or cooperating with Chimpanzees in other groups. Under these circumstances, intense hostility towards outside males appears to be an adaptive trait in male Chimpanzees, leading to potential territorial extensions for the group, which in turn leads to more food and other resources, increased female fertility, and a greater survival rate among juveniles. 

This cannot, however, explain the lethal aggression sometimes displayed to members, or former members, of the same group. During the fission of the Ngogo Group, males which had lived, fed, groomed, and patrolled together for years became targets of lethal attacks on the basis of their new group membership. This leads Sandal et al. to conclude that Chimpanzees have a sense of who belongs to their group which is based upon more than simple familiarity, and which can be updated to reflect changes in circumstances.

These results challenge previous assumptions about intergroup conflict in Chimpanzees in a number of ways. All observed attacks were initiated by the numerically smaller Western Group, which contradicts the assumption that larger groups have an advantage in such conflicts. The emergence of greater social cohesion in the smaller group appears to have given them an advantage in conflict which more than made up for the greater numbers of their opponents. These closer bonds appear to have developed before the onset of aggressive activity, suggesting that an external threat is not needed to forge these bonds. Conversely, the original Ngogo Group underwent a territorial expansion in 2009, which appears to have reduced the threat that it faced from rival groups, something which may have contributed to the long-term decline in social cohesion within the group. If this is the case, than an external threat may not be needed to forge close bonds, but it may be needed to maintain them.

Sandal et al.'s findings also have implications for Human behaviour. If Chimpanzee groups can split in this way without any of the cultural markers associated with Human conflict, then these may be less important to Humans than we generally assume, masking the actual roots of aggression, which may have more to do with interpersonal relationships. Humans tend to attribute the conflict to ethnic, religious, or political divisions, but this may be misleading, covering the actual causes of conflict. If this is the case, then  it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace.

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Tapanuli Orangutans brought closer to extinction by Cyclone Senyar.

The Tapanuli Orangutan, Pongo tapanuliensis, is thought to have moved significantly closer to extinction after Cyclone Senyar passed across Sumatra on 25 November 2025. The species, which was only discovered in 2017, is considered to be the world's rarest Ape, with a population of about 800 all living within an area of less than 1000 km² in North Sumatra. It is considered to be Critically Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species.

A male Tapanuli Orangutan, Pongo tapanuliensis. Maxime Aliaga/Sumatran Orangutan Conservation Program.

Cyclone Senyar caused more than 1000 mm of rain to fall in a day in parts of North Sumatra, triggering a series of catastrophic floods and landslides which killed over a thousand people on the island. The three districts where the species is found, North, Central, and South Tapanuli, have been particularly badly hit, with satellite images showing that between 48 and 72 km² of the forest inhabited by Tapanuli Orangutans has been destroyed, according to Erik Meijaard of Borneo Futures, who has been studying the storm's impact on the Apes. This could potentially equate to between 33 and 55 Orangutans, or between 6.2% and 10.5% of the total population.

So far, only a single dead Orangutan has been found, leading to the possibility that the Apes may have been able to escape the area before the worst of the event, although this is not typical Orangutan behaviour; they usually seek the nearest shelter and attempt to wait out large storms. Even should this have been the case, the storm appears to have destroyed large areas of their native habitat, including food sources, which makes it likely that more Orangutans will be lost in the near future. 

A dead Tapanuli Orangutan found amid storm debris in the village of Pulo Pakkat, North Sumatra, following the passage of Cyclone Senyar. Decky Chandra/The Guardian.

The population was already considered to be threatened by the expansion of mining, hydropower projects and palm oil plantations, within their habitat, as well as a rapidly changing environment, with global warning already having led to an increase in rainfall of between 28 and 160% across Sumatra. One possible benefit to the Orangutans is that the Indonesian government has ordered a halt to all new development projects in the Tapanuli area, pending a survey of the region, which may lead to their gaining some extra long-term protection. The government has also indicated its support for forest restoration projects in the region, which could lead to an increase in the available habitat for the Apes.

The storm is also reported to have completely destroyed the Ketambe Research Station, within the Gunung Leuser National Park in Aceh Province, which was the first specialist Orangutan research centre in the world, opened in 1971 by Dutch primatologist Herman Rijksen, and a leading centre for research into the Sumatran Orangutan, Pongo abelii, which is also considered to be Critically Endangered.

Damage caused to the Ketambe Research Station, within the Gunung Leuser National Park by Cyclone Senyar. American Association of Zookeepers. 

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The origin of kissing.

Kissing, if interpreted as mouth-to-mouth contact between members of the same species with no food being transferred, is widespread in the Animal Kingdom, as well as in most (but not all) Human societies. Variants on this have been seen in Mammals, Birds, Fish and even Insects. If this process is narrowed to oral-oral contact with some movement of the lips, then it is still widespread in Old World Monkeys and Apes. 

The advantages of kissing, viewed in a strictly evolutionary sense, are hard to define. Kissing does not appear to aid survival, or enhance reproductive success, and presents an opportunity for the transfer of infections. It has been suggested that kissing may enhance mate selection, enabling one kisser to evaluate the odour, health and/or social skills of the other, although there is no real way to prove this. Another suggestion is that kissing enhances arousal, thereby increasing the likelihood of mating success, though again, this is unprovable. Neither of these explanations can account for kissing between individuals who do not seem to be intent on mating, which is just as widespread as mating-related kissing. One theory which could account for this is that kissing is a social gesture which displays trust, coming intentionally close to the (potentially dangerous) mouth and teeth of a social peer or potential mate, while another is that the practice might facilitate the transfer of potentially beneficial microbes, though again, these hypotheses are impossible to prove one way or the other.

The Kiss, by Gustav Klimt. Österreichische Galerie Belvedere/Google Arts & Culture/Wikimedia Commons.

The meaning of kissing is subject to cultural interpretation in Human societies. Some societies do not kiss at all. In others kissing is an everyday, yet meaningful, event. It can also be a highly symbolic activity. Given this cultural variation in Humans, it is quite possible that kissing is also cultural in non-Human Primates, and may be present in some populations and not others. 

While a number of researchers have come up with different possible explanations for the purpose of kissing, the origin of the practice does not appear to have been examined to date, despite the insight that understanding this could provide on the purpose of the practice. The presence of kissing in different, but closely related, species of Primates suggests that it has not appeared separately in each of these species, but that it first appeared in a common ancestral species, and was then adapted by each evolving lineage to meet their needs.

In a paper published in the journal Evolution and Human Behaviour on 19 November 2025, Matilda Brindle of the Department of Biology at the University of Oxford and the Department of Genetics, Evolution and Environment at University College London, Catherine Talbot of the School of Psychology at the Florida Institute of Technology, and Stuart West, also of the Department of Biology at the University of Oxford, present the results of a study in which they developed a comparative framework for the practice of kissing across different species of Primate, which was then used to examine the evolutionary history of the behaviour within the group.

Top panel: kissing across the Animal Kingdom (clockwise): Rhesus Macaques, Macaca mulatta; Galapagos Albatross, Phoebastria irrorata; Polar Bears, Ursus maritimus; Wolves, Canis lupus; Prairie Dogs, Cynomys ludovicianus. Bottom panel: non-kissing mouth-to-mouth behaviours (left to right): premastication in Orangutans, Pongo sp.; trophallaxis in Indian Black Ants, Camponotus compressus; and Kiss-fighting in French Grunts, Haemulon flavolineatum. Brindle et al. (2025).

A full systematic review of kissing in Primates in academic literature proved to be impractical. The lack of clear scientific terminology around kissing, combined with a general reticence to discuss such matters in scientific studies, and a very large body of work on kissing in fields such as literature, rendered specialist search engines such as Web of Science unable to produce useful results. Consequently, Brindle et al. settled on a non-systematic review of the literature, combined with searching platforms such as YouTube for evidence of kissing behaviour, with the intent of establishing evidence of kissing in a species, not any data about this. They note that absence of such data does not mean the behaviour does not exist.

In doing this, Brindle et al. set out to answer seven questions, namely: (i) Which primates have been observed kissing? (ii) Does kissing show a phylogenetic signal? (iii) When did kissing first evolve in this group? (iv) How many times did kissing evolve? (v) Has kissing been lost across the course of evolution in any lineages? (vi) Are Neanderthals likely to have kissed? And (vii) how well do different life history variables correlate with the occurrence of kissing?

For the purpose of their study, Brindle et al. defined kissing as 'non-agonistic interactions involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer'. This definition still included behaviours seen in Animals such as Ants, Birds, and Polar Bears, but was much more common and widespread in Primates. Brindle et al. further restricted their study to Old World Monkeys and Apes, groups where there was sufficient data for useful comparisons to be made. 

Non-sexual kissing in Orangutans, a mother kissing her child. Sumatran Orangutan Society.

Brindle et al. make it clear that they were looking for the ultimate cause of kissing rather than the proximal one, which is to say, the reason why Primates first started kissing rather than the reasons for this occurring in any group of Primates who already had the behaviour.

They further note that kissing is likely to have begun as a modification of another behaviour. They note that a recent suggestion has been made that kissing began as a part of a oral grooming behaviour, but doubt the veracity of this, noting that this is not usually how Primates groom, and that no evidence was offered to support the hypothesis. They suggest instead that kissing might be a modification of oral-to-oral food transfer, a common phenomenon in Primates.

Because kissing has been speculatively linked to mating success, Brindle et al. compared the presence of kissing in a Primate species to the mating system used by that species. They also compared the presence of kissing to the diet, food sharing and premastication (chewing food before giving it to another individual, typically an infant) habits of each species, since these behaviours might give a species a preadaptation towards kissing. They note that species with frugivorous or omnivorous diets are more likely to share foods with infants, due to the patchy distribution of foods within the environment. Fruits and meats are also the foodstuffs most commonly premasticated before giving them to infants. Reliable data on premastication was only available for the Great Apes. Data on diets from Neanderthals and Modern Humans was included in the dataset.

Kissing the War Goodbye by Victor Jorgensen. US National Archives and Records Administration/Wikimedia Commons.

Brindle et al. used a phylogenetic tree constructed using the 10kTrees Project V3.0 resource, which was cut to include only Old World Monkeys and Apes, including Humans and Neanderthals. Neanderthals were included not just because they nested within the available data, but because it has been suggested that there is evidence from the oral microbiome that Modern Humans and Neanderthals may have exchanged microbial species via kissing some time after the two species split.

The phylogenetic tree was constructed using a Bayesian Markov chain Monte Carlo framework with the practice of kissing identified as either present or unknown (since it is impossible to assume that a species does not kiss simply because we have not observed it). Neanderthals were also classified as unknown. 

Kissing was found in all species of Great Ape except the Eastern Gorilla, Gorilla beringei. It was also observed in a variety of Old World Monkeys, although chiefly within the Papionini (Macaques and Baboons). Brindle et al. note that much of the data on the Papionini relates to same-sex kissing, at least in part because that was the subject of the studies which had looked at the behaviour in this group.

Brindle et al. conclude that kissing appeared in the common ancestor of all extant Great Apes some time after the split with the common ancestor of all extant Lesser Apes (Gibbons), i.e. between 21.5 and 16.9 million years ago. The trait has subsequently been retained, with the one possible loss in Eastern Gorillas. Kissing was found in eight species of Papionini, but it was impossible to determine that the trait was present in the common ancestor of the group; it has either evolved or been lost multiple times (possibly both). Brindle et al. also conclude that kissing was almost certainly present in Neanderthals.

Phylogeny illustrating the reconstructed evolutionary history of kissing within the Apes (Hominoidea). The occurrence of kissing and other life history variables is displayed at the tips of the tree. Left to right: kissing (observed/not observed); mating system (single/multi-male); diet (folivorous, frugivorous, omnivorous); food sharing (present/absent); and premastication (present/absent). At the tips and nodes of the tree, black circles indicate a trait has been reported or reconstructed as ‘present’; white circles indicate kissing has not been reported or was reconstructed as ‘absent’; grey circles indicate equivocal reconstructions (mean probability less than 0.65); circles are not present where data are missing. The Neanderthal tip represents the mean probability that kissing was ‘present’, based on Bayesian estimation. Maximum Clade Credibility tree created from a sample of 10,000 molecular phylogenies from the 10kTrees project. Brindle et al. (2025).

Brindle et al. also note that there seems to be a strong association between kissing and multi-male mating systems (i.e., systems in which multiple males may compete for the right to mate with a female), with the kissing existing alongside long bonding of females to a single male only in Western Gorillas and some Human societies. Premastication was found in every species in which kissing has been observed, but there was generally an absence of data for species where kissing had not been observed, leading Brindle et al. to refrain from making a judgement on the relevance of this trait.

Brindle et al. conclude that kissing is present in all Great Apes except Eastern Gorillas, and that there is strong evidence that the trait appeared once in the group, between 21.5 and 16.9 million years ago. The data is less clear for Old World Monkeys, where the trait may have appeared once and subsequently been lost in many lineages, or appeared several times in different lineages.

The retention of kissing as a behaviour in Great Apes strongly suggests that it provides an evolutionary advantage, which Brindle strongly suspect is associated with sexual selection, although they do not believe this is sufficient to assert that this was the reason it originally evolved. They note that the limitations of the data they used leave plenty of opportunity for further research on the subject.

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