Part 2: Perfecting life in -and abandoning – forests; evo-revolutionary ‘gambling’ in savannas; and taking up promiscuity, murder and cannibalism
Evolutionary ‘monkeying’ about
At ca 27 million years ago (Mya), the catarrhine Apes sensu lato split into:
- the Old World monkeys (Cercopithecoidea): tailed arboreal monkeys (e.g. the widespread Vervet Monkey Chlorocebus pygerythrus with many well-marked subspecies); much larger, ground-living Mandrill/Drill ‘Baboons’ (Mandrillus with long/furrowed-snouts and stub-tails); and ‘true’ baboons’ [Papio spp. sensu lato (ca 89% genetically similar to humans) including the Gelada Theropithecus with whom they have an “intertwined common ancestry”] and
- Apes sensu lato (Hominoidea).
Mandrill/Drill ‘baboons’ are forest-living, ground-dwelling monkeys (the world’s largest), evolutionarily closest to the semi-terrestrial mangabey monkeys (Cercocebus spp.). They independently acquired ‘baboonity’. This is an excellent example of convergent evolution in African mammals. Socially, Mandrills live in very large, stable groups – ‘hordes’ – numbering up to 800 individuals.
When the ‘island’ African Plate ‘collided’ with Eurasia ca 22 Mya, some hominoids dispersed out of Africa into Asian forests and, by 18-12 Mya, established the Gibbon lineage. These forest-specialist, tailless Lesser Apes (family Hylobatidae) have a highly specialized forearm anatomy that is an adaptation for arboreal locomotion. Indeed, gibbons are among nature’s best brachiators. At the level of the genome, gibbons are ca 96% similar to humans and, where gibbon species-ranges overlap, there is evidence of interspecific hybridization. Also, like the forest-specialist New World monkeys and modern Homo sapiens sapiens, Gibbons are socially monogamous and often retain the same mate for life – although they do not always remain ‘faithful’.
Great Apes radiated further evolutionarily – within and outside of Africa. Orangutans and gorillas specialized further, adapting to become inextricably linked to primary tropical forests. Chimpanzees and the Bonobo also exploited secondary forest, and chimps, ‘chimp-men’ and archaic humans ventured into savanna/woodland. This radiation of apes (ca 93% genetically similar to Old World monkeys) began within Africa at 18-16 Mya. At 16-12 Mya, the orangutans (Pongo spp. – ca 97% genetically similar to African Great Apes) split off and dispersed to – and speciated within – the tropical forests of Southeast Asia and China. They evolved into the most arboreal of the Great Apes and persist only in refugia in the rainforests of Borneo and Sumatra.
Within Africa, gorillas followed suit at 11-9 Mya, also finding refuge in primary forest. Then, at 8-6 Mya, when forest fragmentation continued apace in a ‘drying’ Africa, the hominins (Hominins): chimpanzees + the Bonobo (Pan spp.), extinct ‘chimp-men’ and ’humanoids’ (Homo spp. sensu lato) parted evolutionary company with them. The initial divergence between hybridizing basal populations may have begun much earlier. Many of the chimp-men and their ancestors who possessed a mosaic of chimp and human attributes failed to survive the Darwinian “Struggle for Existence” and went extinct. However, one, or an evolutionary nexus of several, australopithecine chimp-men evolved into the earliest putative Homo species, habilis, emerged at 2.5 Mya.
The two species of Gorilla (gorilla and beringei) are ground-dwelling herbivores patchily distributed within African primary equatorial forests in Central Africa. They exploit a wide range of elevations, and the distributions of the two species are bounded by the Congo River and its tributaries. The Mountain Gorilla (Gorilla. b. beringei) inhabits montane cloud forests ranging in altitude from 1650 to 5400 m and lowland gorillas live in dense swamp-forests as low as sea level.
Genomically, gorillas are 98% similar to humans, and the 99%-similar gorilla subspecies may have diverged from one another as recently as 250 Kya. Despite chimpanzees being the closest extant relatives of humans, 15% of the human genome is more like that of the gorilla. In addition, 30% of the gorilla genome is closer to human or chimpanzee than the latter are to each other.
Like chimpanzees and humans, gorillas are susceptible to Ebola virus and common infectious respiratory diseases, including mild colds, severe pneumonia and, most recently, COVID-19.
In short, the Great Apes are genetically and evolutionarily very closely intertwined.
Gorillas are strikingly sexually size dimorphic, and males have large canine teeth. The weight of wild male gorillas is 136-227 kg, more than twice that of adult females.
The skin of gorillas is darkly pigmented throughout life. Mature male gorillas have a bony sagittal crest running down the centre of the skull and have a pronounced supraorbital brow ridge. They move around by knuckle-walking and live in groups comprising one adult male or silverback and usually 5-10 adult females and their offspring.
Chimpanzees and the Bonobo
Although mid-Pleistocene chimpanzee fossils have been reported from Kenya in the East African Rift Valley, the Common Chimpanzee (Pan troglodytes troglodytes + verus, vellerosus and schweinfurthii) is currently disjunctly and patchily distributed in West and Central African tropical forests and savannas of equatorial Africa from Senegal in the west to Lake Albert and north-western Tanzania in the east. Its sister species, the Bonobo (Pan paniscus – Gracile or Pygmy Chimpanzee) is confined to a 500000 km2 block of primary and secondary forests, including seasonally inundated swamp forests in the Congo Basin. The current ranges of these two apes are separated by the Congo and the Lualaba Rivers. Untrained apes cannot swim and fear water barriers.
Geographical distribution of chimpanzees (Pan troglodytes subspp.) and the Bonobo (P. paniscus)
These apes began to evolutionarily diverge from one another at 2.0-1.8 Mya, but their DNA remains 99.6% similar. Indeed, the DNA of chimpanzees and humans is ca 98.4% similar to gorillas, and both Bonobo and chimpanzee DNA is ca 98.7% similar to that of humans. Genetic dissimilarity between chimpanzee subspecies is 0.3% – two to three times greater than that between the most different human populations.
In short, gorillas, chimpanzees, the Bonobo and humans are intimately evolutionarily intertwined. Bonobos and chimps are capable of hybridizing in captivity and morphometric analyses detect no clear morphological ‘boundary’ between the two species taxa. Analyses of complete genomes suggest that there were bouts of ancestral interbreeding in nature between them 55-100 Kya during episodic connection and fission of forest refugia that occurred during relatively dry and wet intervals that characterized the Plio-Pleistocene. Moreover, chimpanzees currently thrive in insular patches of forest/savanna outside equatorial forest.
Biological similarities and differences
Morphology – The chimpanzee’s brain volume is 282–500 cc, averaging larger than that of the Bonobo – 358 cc. Humans and bonobos share one thing that chimpanzees do not possess – a pattern in the brain cells called spindle cells or Von Economo Neurons. This is a pattern associated with empathy and other complex social skills, that is also found in whales and elephants. Bonobos also have more grey matter volume in the right anterior insula, right dorsal amygdala, hypothalamus, and right dorsomedial prefrontal cortex, all of which also are regions assumed to be vital for feeling empathy, sensing distress in others, and feeling anxiety.
Both chimpanzees and bonobos are sexually size dimorphic, but much less so than gorillas. An adult chimpanzee male weighs up to 70 kg and can be as tall as 1.6 m. Females are smaller – 27-50 kg. Bonobos are smaller still (ca 1.15 m) and males range from 34 to 60 kg, against an average of 30 kg in females. They are more gracile and not as muscular as chimpanzees. Both species have long and powerful arms – longer than their legs – which are very important in climbing and locomoting within trees as well as in walking on the ground.
The Bonobo’s skull has a less prominent brow ridge. It has a black face with bright pink lips, small ears, wide nostrils, and long hair on its head that forms a parting, and a tail-tuft through adulthood. Females have more prominent breasts, in contrast to the flat breasts of other female apes – although not so prominent as those of humans.
Chimpanzee skin, particularly that of juveniles, is whitish and not black as in bonobos and gorillas. The colour of the face of chimpanzees varies with age, becoming darker in older adults.
Although both species can stand up and walk upright, bonobos employ this form of locomotion much more frequently. Chimpanzees usually knuckle-walk on all fours, clenching their fists to support themselves. Moreover, although both chimpanzees and bonobos are highly vocal, bonobos have higher-pitched voices whereas chimpanzees tend to hoot, scream, and grunt and sometimes drum on hollow trees.
Sociobiology – Both of these apes live in social groups, sometimes exceeding 100 individuals and, at the end of each day, build leafy nests in trees where they sleep for the night. Chimpanzees are much more aggressive than bonobos, and resolutely defend of their territories. ‘Wars’ between different chimpanzee tribes are common, sometimes resulting in the death of combatants. Chimpanzee groups are male-dominated with an overtly violent alpha male ranking above all others – both males and females. Alpha male chimpanzees typically attain dominance by cultivating allies who will support that individual during future ambitions for power. Young female chimpanzees gain favour by submitting to dominant males. They also have a hierarchy and young females may inherit high status from a high-ranking mother.
On the other hand, bonobos are peaceful and, although territories are clearly marked, there is usually no fighting over territory between groups and group members may intrude into neighbouring territories, allowing them to be overlapping. Indeed, bonobos may even be described as ‘considerate’ and ‘empathetic’, even sharing food with strangers. They live in gynecocratic, female-led societies. A young female bonobo finds acceptance by pandering to the dominant females within her group. Bonding among females enables them to dominate males. Community female acceptance is necessary for reaching alpha male status to ensure that he helps the group to find high quality food. The influence of females is so great that the mothers even help their male offspring to procreate. They intervene if their offspring have competitors and may even chase rival males if necessary. As a result, these males are up to three times more likely to mate. Adolescent females often leave their native community to join another community. Males remain in their natal troop with their mother until they are adults. They generally defer to females, wait for them to eat first, give up favourite sunning spots or just simply move out of the way.
Bonobos are also far more playful than chimps, and remain so into adulthood, with adults regularly playing with other adults.
Reproduction – There are also marked differences in reproduction and sexual relations. Within chimpanzee groups, alpha-males guard fertile females and often fight over reproductive access. Sex is strictly about reproduction, and reproductive tactics can include infanticide – the killing of offspring unrelated to a male chimp. Infanticidal individuals remove potential competitors to their own offspring; and the mother, without an infant to care for, will become available for mating again much sooner.
In bonobos, in order to avoid conflict, there is casual copulation – colloquially dubbed as the ‘bonobo handshake’ – among members of the different sexes and as well as members of the same sex. In that sense, it is not uncommon to see two males or two bonobo females copulating. Bonobos even sometimes mate in the ‘Missionary’, face-to-face position, which is rarely seen among chimpanzees. They exhibit a wide repertoire of sexual activities such as kissing, oral sex, and rubbing of the genitals – referred to as genito-genital (GG) rubbing, the non-human analogue of tribadism engaged in by some human females. The female’s clitoris is, proportionately, three times larger than the human equivalent.
While most female chimpanzees are sexually active only seasonally, female bonobos may engage in copulation with males at any time. There is virtually no competition between males, and no pair bonding. Either sex may initiate sexual activity. Nearly 30% of intercourse occurs with females who were not ovulating, with mutual gazing and even ‘French kissing’. Young female bonobos will leave their birth troop at about the time of their first monthly menstrual cycle, usually around 9 or 10 years old. They may join a new troop and gain acceptance from the dominant females through grooming, play, and sexual contact.
A bonobo female’s first pregnancy is usually at about 13 or 14 years old and bearing her first infant will raise the young mother’s social standing within the troop. Pregnancy lasts 8 months, and a single 2–3-pound infant is born well-furred with eyes open. Bonobo infants develop more slowly than baby chimpanzees. They do not leave their mother for the first three months of life, clinging tightly to her with hands and feet while being held against her chest.
Adult male bonobos do not contribute to the care of youngsters but will protect them from predators. Unlike male chimps who can be quite violent towards infants and juveniles, male bonobos rarely exhibit any aggression towards youngsters, and infanticide among bonobos has never been observed.
Tools – Tool use and tool making among chimpanzees is widespread, very common, and is cultural in nature. Adults will demonstrate the use of a tool to youngsters, and different troops may have different techniques in using the same tool or may use different tools for similar tasks. Bonobos in the wild have demonstrated very limited tool use and little, if any, tool making.
Diet – Chimpanzees and bonobos are much more omnivorous than gorillas and cooperatively hunt in groups of up to 100 individuals. Because of the complex fission-fusion nature of chimp society, in which there are no stable groups, only temporary sub-groupings known as ‘parties’ that congregate and separate throughout the day. The size and membership of hunting parties varies greatly, from a single individual to as many as thirty-five. Foragers feed principally upon fruits and supplement their diet with a variety of foods including leaves, worms, insects, eggs and animals (including fish, reptiles, wild pigs, small antelopes and monkeys). Chimps even kill and feed on one another, even infants!).
Bonobos are also omnivorous, but hunt less often in groups. Unlike chimpanzees, bonobos do not hunt monkeys but instead play with and groom them. Furthermore, the phenomena of infanticide, cannibalism, and lethal invasion seen among chimps have never been observed among the bonobo.
In terms of having an evolutionary ‘toolkit’, ‘chimp-obos’ were ‘all dressed up’ and ready to evolve into humans. But, this transformation was hit-and-miss and lasted at least 5 million years. For details, see Part 3.