Apes are the primates most closely related to humans, and they include a surprisingly small club of species divided into two families: the great apes (chimpanzees, bonobos, gorillas, and orangutans) and the lesser apes (gibbons and siamangs). Humans belong to the great ape family too, though in everyday language “apes” usually refers to the non-human members. What makes this group so fascinating is not just their genetic proximity to us but the sheer range of what they do with bodies and brains that share so much of our evolutionary heritage.
The Ape Family Tree
All living apes descend from a common ancestor that split from Old World monkeys roughly 23 to 25 million years ago. From that shared trunk, the branches separated at different times. Molecular studies estimate that the lesser apes (gibbons) diverged from the great ape lineage somewhere around 18 to 20 million years ago, followed by the orangutan line branching off about 13 million years ago and the gorilla line about 7 million years ago. The split between the human lineage and chimpanzees happened most recently, around 5 to 7 million years ago.1Molecular Biology and Evolution. Estimation of Divergence Times for Major Lineages of Primate Species Different methods and calibration points produce slightly different dates. One analysis using complete mitochondrial DNA placed the gibbon-great ape split at about 36 million years ago and the human-chimpanzee split much more recently, though most genomic studies have converged on narrower and more recent windows.2PubMed. Pattern and timing of evolutionary divergences among hominoids based on analyses of complete mtDNAs
These dates come from comparing DNA sequences across species and calibrating the molecular clock against known fossil dates. A Miocene fossil from Spain, a small-bodied primate called Pliobates cataloniae that lived about 11.6 million years ago, has complicated the picture. It weighed only about 4 to 5 kilograms but showed a mix of primitive features and skeletal traits shared with modern apes, suggesting that the ancestor of all apes may have been smaller than previously assumed.3PubMed. Miocene small-bodied ape from Eurasia sheds light on hominoid evolution Fossils like this remind researchers that the story of ape evolution is far from settled.
The Chromosome 2 Puzzle
One of the most striking genetic differences between humans and all other apes is chromosome count. Humans have 23 pairs of chromosomes; every other living ape has 24. The reason is that two ancestral chromosomes fused end-to-end to form human chromosome 2. Researchers have pinpointed the fusion site at single-base-pair resolution and estimated it formed roughly 900,000 years ago, well after the human lineage split from chimpanzees.4PubMed Central. Revised time estimation of the ancestral human chromosome 2 fusion The fusion event was accompanied by multiple inversions and duplications of DNA segments, some of which originated more than 5 million years ago and were shared among African great apes through a process called incomplete lineage sorting, where ancestral genetic variants get distributed unevenly across descendant species.5Cell Genomics. Incomplete lineage sorting of segmental duplications defines the human chromosome 2 fusion site early during African great ape speciation The fusion itself did not cause humanity, but it is a useful genetic landmark that distinguishes our karyotype from every other ape’s.
How Apes Move
Apes do not all move the same way, and their locomotion is one of the clearest dividers between the two families. Gibbons and siamangs are built for brachiation, the arm-swinging mode of travel through forest canopies. Their shoulder flexors, elbow flexors, and wrist flexors are optimized for generating power and storing energy in long tendons during each swing, and their elbow flexors in particular are more powerful than those of any non-brachiating primate.6PubMed Central. Functional anatomy of the gibbon forelimb: adaptations to a brachiating lifestyle In siamangs, the shoulder adductors and elbow flexors are built for force, while the shoulder abductors and flexors are better suited for speed and rapid direction changes, a division of labor that lets these animals accelerate, brake, and pivot through three-dimensional canopy routes.7PubMed Central. Muscle moment arms and function of the siamang forelimb during brachiation
Great apes, being much heavier, spend more time on the ground or moving through branches using a combination of climbing and cautious quadrupedal travel. Chimpanzees and gorillas are knuckle-walkers, placing their weight on the backs of their curled fingers rather than their palms. This posture keeps the wrist relatively straight, which turns out to be important: chimpanzees have short digital flexor tendons that stiffen the wrist when the finger joints extend, creating a rigid strut that transmits propulsive force efficiently to the ground.8PubMed. A cadaveric study of wrist-joint moments in chimpanzees and orangutans with implications for the evolution of knuckle-walking Three-dimensional motion capture of chimpanzees walking on their knuckles shows that their wrists extend only about 5 to 20 degrees, far less than you would see in a monkey walking palm-down, and the finger joints extend more than previously thought, between roughly 26 and 59 degrees.9PubMed. The biomechanics of knuckle-walking: 3-D kinematics of the chimpanzee and macaque wrist, hand and fingers
Skeletal adaptations reinforce knuckle-walking too. In African apes, two small wrist bones (the scaphoid and the centrale) are fused together, and biomechanical simulations show that this fusion lowers stress in the wrist during knuckle-walking compared to an unfused arrangement.10Scientific Reports. The biomechanical importance of the scaphoid-centrale fusion during simulated knuckle-walking and its implications for human locomotor evolution Orangutans, by contrast, are overwhelmingly arboreal and do not knuckle-walk. They move through trees using a deliberate four-limbed style sometimes called quadrumanous climbing, grasping branches with hands and feet alike.
What Apes Understand
Cognition is where apes consistently surprise researchers. Great apes use tools in the wild, and some populations have developed elaborate toolkits. Chimpanzees in the Goualougo Triangle of the Republic of Congo use at least three different tool sets and are highly selective about materials, choosing specific plant species for specific tasks. Their tool repertoire is among the largest and most complex documented in any wild chimpanzee population.11PubMed. Chimpanzee tool technology in the Goualougo Triangle, Republic of Congo These are not behaviors every chimpanzee everywhere exhibits; they vary by community, which is part of why researchers describe them as cultural.
Great apes also plan for the future. In controlled experiments, chimpanzees and orangutans set aside a tool they would need later, even when it meant ignoring a tempting immediate reward. They did not simply learn an association between tool and food; they mentally pre-experienced the future task and acted accordingly.12PubMed. Chimpanzee (Pan troglodytes) and orangutan (Pongo abelii) forethought: self-control and pre-experience in the face of future tool use Related work on self-control shows that chimpanzees can delay gratification across extended sequences. When given the option to trade a less-preferred food for a better one, they successfully traded up through multiple exchanges rather than eating the food in hand, showing robust impulse control even when the payoff was several steps away.13PubMed Central. Trading up: chimpanzees (Pan troglodytes) show self-control through their exchange behavior In simpler delay-of-gratification tests, both chimpanzees and an orangutan waited through a series of twenty accumulating treats before accepting the reward.14PubMed. Maintenance of self-imposed delay of gratification by four chimpanzees (Pan troglodytes) and an orangutan (Pongo pygmaeus)
Mirrors, Minds, and False Beliefs
One of the classic tests of self-awareness is the mirror mark test: paint a spot on an animal’s forehead while it is unaware, then let it see a mirror. If the animal touches the mark on its own body, it recognizes its reflection as itself. Great apes pass this test. Lesser apes do not. Gibbons and siamangs fail to use mirrors to find marks placed on their heads, even though they are clearly motivated to investigate visible marks on their own limbs or on the mirror itself.15PubMed Central. The evolution of primate visual self-recognition: evidence of absence in lesser apes A comprehensive review of attempts to train or coax self-recognition in various primates reached the same conclusion: no compelling evidence for mirror self-recognition exists in monkeys, gibbons, or siamangs.16PubMed. Mirror self-recognition: a review and critique of attempts to promote and engineer self-recognition in primates This suggests visual self-recognition evolved after the great ape lineage split from gibbons, roughly 18 million years ago.
Even more intriguing is the question of whether apes understand that others can hold false beliefs. In eye-tracking studies, three species of great apes looked toward the location where an agent would mistakenly search for an object, even though the apes themselves knew the object had been moved. In other words, they anticipated someone else’s mistaken action.17PubMed. Great apes anticipate that other individuals will act according to false beliefs A follow-up experiment strengthened the finding by showing that apes who had personally experienced a barrier as opaque anticipated that an agent behind it would not know the object had been moved, while apes who had experienced the barrier as transparent did not make that prediction. The apes appeared to draw on their own perceptual experience to reason about what someone else could or could not see.18PubMed Central. Great apes use self-experience to anticipate an agent’s action in a false-belief test A control experiment using an inanimate object instead of an agent found no significant tendency for apes to look at the “correct” location, ruling out a simpler explanation that they were just tracking object motion rather than reasoning about an agent’s mind.19PubMed Central. A test of the submentalizing hypothesis: Apes’ performance in a false belief task inanimate control The evidence points toward at least an implicit understanding of false belief in great apes, though researchers still debate whether this constitutes true “theory of mind” in the human sense.
Gesture, Voice, and the Roots of Language
Great apes communicate extensively through intentional gestures, and the repertoires of closely related species overlap more than you might expect. Bonobos and chimpanzees share about 88 percent of their gesture types, and the differences that exist appear to stem from their different social systems rather than from one species inventing gestures the other lacks.20PubMed Central. The gestural repertoire of the wild bonobo (Pan paniscus): a mutually understood communication system A detailed analysis of over 2,300 intentional gesture instances across both species found 33 gesture types used to convey 14 different outcomes, from “follow me” and “initiate grooming” to “move away” and “stop what you’re doing.” The overlap in gesture-to-meaning mappings between bonobos and chimpanzees was greater than expected by chance, suggesting a shared biological repertoire inherited from their common ancestor.21PubMed Central. Bonobo and chimpanzee gestures overlap extensively in meaning Bonobos also showed greater flexibility in combining gestures with facial and vocal signals, and these multimodal combinations had a stronger effect on the recipient than gestures alone.22PubMed Central. Ape gestures and language evolution
On the vocal side, the bottleneck to speech is not the mouth. Imaging studies of macaque vocal tracts showed that their anatomy could theoretically produce a range of vowel-like sounds, meaning the primate vocal tract has been “speech-ready” for a long time. What macaques and apes lack is the neural wiring to control those sounds with the precision and flexibility that human speech requires.23PubMed Central. Monkey vocal tracts are speech-ready Ape vocal behavior nevertheless follows social rules that parallel human conversation. Bonobos and gorillas, the more socially tolerant species, tend to take turns vocalizing. Chimpanzees, whose social lives are more competitive, overlap each other’s calls more frequently. Orangutans, the most solitary of the great apes, mostly call in isolation.24PubMed. Social pressure drives “conversational rules” in great apes
Social Lives and Conflict
Great ape societies vary enormously. Gorillas typically live in groups dominated by one or two silverback males. Chimpanzees form large fission-fusion communities where subgroups shift constantly. Bonobos live in mixed groups where females hold considerable social power and tension is often resolved through sexual contact rather than aggression. Orangutans are semi-solitary, with adult males maintaining large ranges that overlap those of several females.
Cultural learning also runs deeper than researchers initially assumed. Great apes engage in prolonged “peering,” where one individual watches another’s actions at close range, and analysis of this behavior suggests apes possess many more culturally transmitted practices than have been formally catalogued.25PubMed. Social Learning: Peering Deeper into Ape Culture These cultural differences between populations are part of what makes each community unique and, from a conservation standpoint, irreplaceable.
The dark side of chimpanzee sociality can be striking. A long-term study documented a community fission in which one group split into two, and over seven years the members of one faction launched 24 attacks on the other, killing at least seven adult males and seventeen infants.26PubMed. Lethal conflict after group fission in wild chimpanzees The researchers noted that group identities shifted and escalated into lethal hostility without the cultural markers often thought necessary for human warfare. Observations like this do not mean chimpanzees are inherently violent; most of their social time is spent grooming, playing, and traveling peacefully. But the capacity for organized lethal aggression is there, and it complicates any simple narrative about whether violence is uniquely human.
The Slowest Life on Land
Orangutans have the slowest-paced life history of any land mammal. Females in wild Bornean populations give birth on average every 7.6 years, and pre-weaning survival is remarkably high. Female orangutans had a 94 percent chance of surviving to age of first reproduction, around 15 years old, higher than reported for any other mammal under natural conditions.27PubMed. The slow ape: High infant survival and long interbirth intervals in wild orangutans This strategy works because orangutans face very low natural predation risks: they are large, arboreal, and not very social, which limits disease transmission and competition. But it also means populations recover agonizingly slowly from losses. A population crash caused by habitat destruction or hunting can take generations to reverse, if it can be reversed at all.
Diet connects to broader ecology in unexpected ways. The gut microbiomes of great apes shift dramatically depending on lifestyle. Captive chimpanzees and gorillas harbor gut microbes more similar to those found in non-industrialized human populations than to their own wild counterparts. Captive apes also carried up to roughly 34 times the abundance and up to 5 times the diversity of antibiotic resistance genes compared with wild apes, a reflection of the antibiotic-saturated environments of captivity.28PubMed Central. The microbiome and resistome of chimpanzees, gorillas, and humans across host lifestyle and geography The finding underscores how much an ape’s microbial world depends on how and where it lives, not just its species identity.
Sharing Diseases Across the Species Boundary
Because apes and humans are so closely related, pathogens can jump between us in both directions. Research in Cameroon and Tanzania detected parasites and viruses in wild gorillas and chimpanzees that are closely related to human strains, with simian adenoviruses showing high sequence similarity to human versions. This suggests past cross-species transmission events and ongoing risk of future ones, particularly as human activities like tourism, research, and crop-raiding bring people and apes into closer contact.29Emerging Infectious Diseases. Potentially Zoonotic Enteric Infections in Gorillas and Chimpanzees, Cameroon and Tanzania Field site employees who spend extensive time near great apes are considered a priority population for preventing both zoonotic spillover (animal to human) and zooanthroponotic spillover (human to animal), and many lack adequate preventive or curative healthcare.30PubMed Central. Employee health at wild great ape sites: A qualitative inquiry
The risk runs both ways. A respiratory virus that causes mild cold symptoms in a human can devastate a habituated chimpanzee community. Ecotourism and research, the very activities that generate funding and data for ape conservation, also create pathways for disease. Minimum distance rules and mask mandates for tourists visiting gorilla or chimpanzee groups exist for exactly this reason, though enforcement varies.
Threats and What Is Working
Habitat loss is the most visible threat to wild apes, but it is not the whole story. A detailed assessment of Bornean orangutans found that between 1999 and 2015, about half of the population was affected by logging, deforestation, or industrial plantations. Land clearance caused the sharpest declines, but the majority of orangutans actually disappeared from selectively logged and primary forests, where decline rates were lower per area but where far more animals lived. Hunting appeared to be a major additional driver.31Current Biology. How Much Is Left on Borneo? The Recent Distribution and Status of the Bornean Orangutan Conserving habitat alone is not enough when hunting pressure persists inside those forests.
Community-based approaches show promise. In Cameroon’s Ebo Forest, a community-led conservation group maintained about 80 percent membership among local residents, and over 90 percent of respondents expressed positive views about the program’s role in both biodiversity protection and community well-being.32Biodiversity and Conservation. Assessing the impact of community-led conservation approach in the preservation of the cryptic gorilla (Gorilla gorilla) population in the Ebo forest, Littoral Region-Cameroon In Gabon, indigenous trackers demonstrated near-perfect accuracy in identifying great ape fecal samples by species, validating traditional ecological knowledge as a low-cost, reliable monitoring tool that could expand surveillance well beyond the boundaries of formal protected areas.33PubMed Central. Traditional ecological knowledge for great ape conservation in Gabon Conservation programs that treat local communities as partners rather than obstacles tend to produce more durable results, and the data from these programs increasingly backs that up.