Ape

• 1. Introduction

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  Apes share a scientific name with humans: Hominoidea. That single Latin label encompasses gibbons swinging through the forests of Southeast Asia, gorillas roaming the highlands of central Africa, orangutans navigating the canopy of Borneo, and every human being who has ever lived. Yet for most of written history, people drew a hard line between themselves and these creatures. The story of how that line kept moving is one of the stranger dramas in the history of science.

  Hominoidea is a superfamily of Old World simians, native today to sub-Saharan Africa and Southeast Asia, though the fossil record shows they once ranged across Africa, Asia, and Europe as well. There are two living branches: the gibbons, also called lesser apes, belonging to the family Hylobatidae; and the great apes, belonging to the family Hominidae. That second family includes orangutans, gorillas, chimpanzees, and humans.

  What makes an ape an ape rather than a monkey? The most obvious answer is the absence of a tail. But the deeper answer lies in a mutation of the TBXT gene, anatomy shaped by a life spent hanging and swinging from branches, and a set of shared evolutionary origins stretching back roughly 25 million years. How that evolutionary story was pieced together, and how fiercely it was resisted, turns out to be just as fascinating as the animals themselves.

• 2. The Naming Problem

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  "Ape" descends from Old English apa, a word whose ultimate origin nobody has pinned down. For most of English history, the word meant any non-human primate that looked roughly human, and it was used loosely enough to be comedic or punning as much as scientific.

  The complications multiplied once the word "monkey" entered the language. At that point, "ape" narrowed to mean specifically a tailless, therefore exceptionally human-like, primate. The 1911 Encyclopaedia Britannica captures the tension neatly: it listed two distinct senses for the word, one as a synonym for monkey in general, and one for the tailless, human-like primate in particular.

  John Edward Gray introduced the taxonomic terms hominoid and hominin in 1824 and 1825, laying the groundwork for the formal classification that scientists still use. But in popular usage, the word "ape" kept drifting. Biologists once used it to mean all members of Hominoidea except humans. More recently, many have shifted to including humans in the category, treating "ape" as simply another word for hominoid. The primatologist Sir Wilfrid Le Gros Clark was among those who shaped the older view, arguing that the living members of the primate order could be arranged in an ascending series from monkeys to apes to humans. That picture has since been replaced by something messier and more accurate: a branching tree rather than a ladder, with humans as one branch among many.

• 3. Linnaeus and the Theologians

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  In 1758, Carl Linnaeus placed a second species alongside Homo sapiens in his genus Homo: Homo troglodytes, meaning cave-dwelling man. He had no physical specimen to work from, only second- or third-hand accounts, and it remains unclear which actual animal, if any, he was describing. Linnaeus may have been drawing on reports of mythical creatures, unidentified primates, or Asian people dressed in animal skins.

  Linnaeus also named the orangutan Simia satyrus, the satyr monkey. He grouped Homo, Simia, and Lemur together under the order Primates. This arrangement disturbed many readers. His own Lutheran archbishop accused him of impiety.

  In a letter to Johann Georg Gmelin dated the 25th of February 1747, Linnaeus was candid about the pressure he faced. He wrote: "It is not pleasing to me that I must place humans among the primates, but man is intimately familiar with himself. Let's not quibble over words. It will be the same to me whatever name is applied. But I desperately seek from you and from the whole world a general difference between men and simians from the principles of Natural History. I certainly know of none."

  Johann Friedrich Blumenbach responded in 1779 by proposing a compromise: split primates into the Quadrumana, four-handed creatures meaning apes and monkeys, and the Bimana, two-handed creatures meaning humans. Georges Cuvier adopted the distinction, and some naturalists elevated it to the rank of separate orders. But as Charles Darwin argued in his 1871 book The Descent of Man, and Selection in Relation to Sex, the affinities between humans and other primates were too numerous and too fundamental for such a clean division to hold.

• 4. Evolutionary Roots

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  Genetic analysis combined with fossil evidence places the split between hominoids and Old World monkeys at roughly 25 million years ago, near the boundary between the Oligocene and Miocene epochs. From there, the family tree continued to branch. Gibbons diverged from the rest of the hominoids about 18 million years ago. The split leading to the orangutan lineage, genus Pongo, happened around 14 million years ago. Gorillas, genus Gorilla, separated from the human-chimpanzee line around 7 million years ago. The divergence of humans and chimpanzees came later still, somewhere between 3 and 5 million years ago.

  In 2015, a newly described genus and species, Pliobates cataloniae, added a wrinkle. This creature lived 11.6 million years ago and appears to predate the split between Hominidae and Hylobatidae, suggesting the early hominoid family tree was more complex than previously understood.

  Earlier estimates had placed the human-chimp divergence at 15 to 20 million years ago. Ramapithecus, a fossil primate from that time period, was once seriously proposed as a human ancestor. Later fossil work showed Ramapithecus was actually more closely related to orangutans, and biochemical evidence revised the divergence estimate sharply downward. The true last common ancestor of humans and chimpanzees lived much more recently than those early estimates suggested.

• 5. Anatomy of the Hominoids

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  The siamang, the largest of the gibbons, weighs up to 14 kilograms. The bonobo, the smallest of the great apes, ranges from 34 to 60 kilograms. That gap in body size between the two ape families corresponds to a whole cluster of physical and behavioral differences.

  Gibbons are defined above all by their arms. Long limbs evolved for brachiation, swinging hand over hand through the canopy, and the wrists became ball-and-socket joints as a direct adaptation to that lifestyle. All sixteen gibbon species are native to Asia and spend most of their lives in trees.

  Across all hominoids, shared skeletal features distinguish them from their Old World monkey relatives. Apes have more mobile shoulder joints because the scapula sits on the back rather than the side of the ribcage. Their ribcages are broader and flatter from front to back. Their spines are shorter and less mobile, with the caudal vertebrae reduced to the point of disappearing entirely, which is why no living hominoid species has a tail. The molar teeth also differ: hominoids carry five cusps in a Y-5 pattern, whereas Old World monkeys have only four in a bilophodont pattern.

  One biochemical peculiarity links all hominoids: the enzyme urate oxidase is inactive across the entire group. Its function was lost twice during the middle Miocene, first in the ancestors of Hominidae and later in the ancestors of Hylobatidae. One hypothesis holds that both losses occurred in apes living in Europe during a period of cooling climate, when the ability to accumulate fat more easily became a survival advantage during periods of starvation. When ape populations later migrated to Asia and Africa, this trait traveled with them.

• 6. Intelligence and Culture

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  Wolfgang Kohler's early studies of chimpanzees demonstrated problem-solving abilities that Kohler attributed to insight rather than simple trial and error. That work opened a line of research that has continued for decades.

  The use of tools by non-human apes has now been repeatedly documented, and the manufacture of tools has been recorded both in laboratory settings and in the wild. Chimpanzees in different regions of Africa have developed distinct tool traditions used in food acquisition, a pattern that researchers interpret as a form of animal culture. Imitation is far easier to demonstrate in great apes than in other primate species.

  Nearly all the major studies in animal language acquisition have involved great apes. Whether those studies demonstrate genuine language ability remains disputed, but the feats of learning involved are not in question. Gibbon cognition has received comparatively little study, so the picture is thinner for the lesser apes than for great apes.

  In November 2023, scientists reported the first documented evidence that groups of primates including bonobos are capable of cooperating with members of other primate groups. Jane Goodall, Dian Fossey, and Biruté Galdikas conducted the foundational field studies on the three best-known great apes, and those studies revealed that social structure varies sharply across species: gibbons form monogamous territorial pairs, orangutans are largely solitary, gorillas live in small troops led by a single adult male, and chimpanzees live in larger and more complex social groups.

• 7. Conservation Crisis

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  Ebola virus has killed at least one third of all gorillas and chimpanzees since 1990, and it is currently considered the single greatest threat to the survival of African apes. That figure sits alongside the longer-running devastation of habitat loss.

  Every non-human hominoid species is rare and threatened with extinction. The eastern hoolock gibbon sits at the least severe end of the spectrum, classified as vulnerable. Five gibbon species are critically endangered, as are all species of orangutan and gorilla. The remaining gibbon species, the bonobo, and all four subspecies of chimpanzee are classified as endangered.

  The primary driver of decline is the loss of tropical rainforest. Hunting for bushmeat adds pressure on some populations. For Africa's great apes, logging, agricultural expansion, and mining compound the habitat threat that deforestation alone creates. Humans and other apes occasionally eat other primates, and some of those prey species are now themselves approaching extinction, with habitat loss as the underlying cause there as well.

  The precariousness of wild ape populations gives the long taxonomic debate over human membership in Hominoidea a particular weight. Gorillas and chimpanzees, the animals whose kinship with humans Darwin argued was too close to deny, now depend on legal protections and conservation programs that flow partly from the recognition of that kinship.