Evolution and the Primates: Lamarckism to the Synthetic and Neutral Theories, Cladogenesis, Anagenesis and Punctuated Equilibrium, Selection, Trends in Primate Radiation, Primate Characteristics and Social Behaviour, and the Extant Primates from Prosimians to the Great Apes and Humans

Jean-Baptiste Lamarck's Philosophie Zoologique (1809) gave the first coherent theory of transformation: organisms have an inner drive to complexity, organs strengthen with use and wither with disuse, and characters so acquired in a lifetime are passed to offspring — the giraffe's neck, the blacksmith's arm. Neo-Lamarckism (Cope, Packard, Spencer, later Lysenko) kept the inheritance of acquired characters after Darwin. Darwin's On the Origin of Species (1859), with Wallace's parallel paper read to the Linnean Society in 1858, argued from variation, over-reproduction (Malthus), the struggle for existence and differential survival to natural selection, and from selection over time to descent with modification; it lacked a theory of heredity and Darwin himself allowed some use-inheritance ("pangenesis"). August Weismann's germ-plasm theory (1885–92) closed that door: the germ line is separate from the body, so nothing the body acquires can be inherited — he cut the tails off mice for many generations to show it — and this selection-only Darwinism is neo-Darwinism. The rediscovery of Mendel (1900) at first seemed to oppose gradual selection with discontinuous mutation (de Vries), until the population geneticists R.A. Fisher, J.B.S. Haldane and Sewall Wright showed in the 1920s and 1930s that Mendelian inheritance plus selection in populations produces Darwinian change. Dobzhansky's Genetics and the Origin of Species (1937), Huxley's Evolution: The Modern Synthesis (1942, which named it), Mayr's Systematics and the Origin of Species (1942) and Simpson's Tempo and Mode in Evolution (1944) completed the synthetic theory: evolution is change in allele frequencies in populations, driven by mutation, selection, gene flow and drift, with speciation usually by geographic isolation.

Anagenesis is change within a single lineage through time without branching — one species gradually becoming another, so that a chronospecies is a segment of a continuous line. Cladogenesis is branching: one species splits into two or more, which is the only way the number of species increases; Rensch coined both terms (1947) and Simpson used them. The synthetic theory's phyletic gradualism expected the fossil record to show slow anagenetic change; Niles Eldredge and Stephen Jay Gould (1972) argued instead that species typically persist unchanged for millions of years (stasis) and that change is concentrated in rapid cladogenetic events in small, peripherally isolated populations (Mayr's allopatric model), which leave no intermediate fossils because they are brief and local. The debate concerns tempo, not whether selection operates. Selection itself comes in kinds. Directional selection shifts the mean (larger brains in Homo); stabilising selection trims both extremes (human birth weight, where very light and very heavy babies survive less); disruptive selection favours both extremes against the middle and can split a population. Sexual selection (Darwin, 1871) works through mate choice and male competition (the peacock's tail, the silverback's size); kin selection (Hamilton, 1964) explains altruism toward relatives through inclusive fitness; balancing selection maintains two alleles, as heterozygote advantage does for sickle-cell haemoglobin in malarial regions. Artificial selection — Darwin's pigeons and the domestication of the dog, cattle and rice — is the human version.

The order Primates (Linnaeus, 1758) is defined less by any single feature than by a set of trends that Le Gros Clark (1959) listed as the products of an arboreal, visually guided way of life: retention of the primitive five-digit limb with grasping hands and feet and opposable thumbs and (except in humans) big toes; flat nails replacing claws, with sensitive tactile pads; reduction of the snout and of the sense of smell; forward-facing eyes with overlapping fields giving stereoscopic vision, colour vision and a bony post-orbital bar or plate; a large and complex brain, especially the neocortex; a generalised dentition with reduced tooth number (two incisors per quadrant; 2.1.3.3 in New World monkeys and most prosimians, 2.1.2.3 in Old World monkeys, apes and humans); a clavicle and a mobile shoulder; an upright trunk in sitting and climbing; single births, long gestation and prolonged infant dependence; and life in social groups. The radiation began in the Palaeocene and Eocene (about 55 million years ago the adapids and omomyids, lemur-like and tarsier-like); Old World anthropoids appear in the Oligocene of the Fayum, apes spread across Africa, Europe and Asia in the Miocene, and the hominin line separated from the chimpanzee line roughly six to eight million years ago. Today the prosimians survive mainly in Madagascar (lemurs), Africa and South Asia (lorises, galagos; the slender loris in South India and Sri Lanka) and island South-East Asia (tarsiers); the New World monkeys in Central and South America; the Old World monkeys across Africa and Asia (the rhesus, bonnet and langurs of India); the gibbons and orangutans in South-East Asia (the hoolock gibbon in North-East India is India's only ape); the chimpanzees, bonobos and gorillas in equatorial Africa.

Morphologically, primates keep a mammalian coat of hair, reduced in the great apes and most of all in humans, whose "nakedness" with abundant sweat glands is read as a cooling adaptation to long-distance walking in open country; tactile whiskers are lost in the anthropoids. The skeleton keeps the clavicle, a mobile shoulder and forearm capable of full rotation (pronation and supination), and pentadactyl hands and feet; cranially, the orbits move forward and are enclosed behind (a bar in prosimians, a full plate in anthropoids), the face shortens and the braincase expands over it; post-cranially the trunk becomes more upright in the apes, whose broad, shallow chest and long arms suit hanging. Dentally the incisors are spatulate, the canines projecting (with a diastema to receive the upper canine in apes, absent in humans), the premolars two or three, the molars low-crowned and bunodont — bilophodont (four cusps in two ridges) in Old World monkeys, and with the Y-5 pattern (five cusps in a Y-shaped groove) in the apes and, in a reduced form, in humans. The brain grows relative to body size along the order, with the visual cortex and the neocortex expanding; the human brain (about 1,350 cc) is three times a chimpanzee's (about 400 cc). The opposable thumb and big toe give the power grip and, with a lengthened thumb and shortened fingers in humans, the precision grip. Locomotion falls into quadrupedalism (arboreal and terrestrial; the knuckle-walking of chimpanzees and gorillas; the fist-walking of orangutans), vertical clinging and leaping (tarsiers, indri), brachiation (arm-swinging under branches: gibbons, with long arms, hook-like hands and short thumbs), quadrumanous climbing (orangutans) and bipedalism (humans alone habitually). Bipedalism rebuilds the body: the foramen magnum moves under the skull, the spine gains lumbar and cervical curves, the pelvis becomes short and bowl-shaped with flaring ilia, the femur angles inward at the knee (the valgus angle), the foot loses its opposable big toe and gains longitudinal and transverse arches.

Primate social behaviour is as much a defining trait as any bone. Most primates live in stable groups whose form varies with ecology: solitary foraging with overlapping ranges (orangutans, many prosimians), monogamous pairs with offspring (gibbons, indris), one-male groups with several females (gorillas, hanuman langurs, where the takeover of a group by a new male is followed by infanticide — Hrdy's Abu langurs), multi-male multi-female troops with dominance hierarchies (macaques, baboons, chimpanzees) and the fission–fusion communities of chimpanzees and spider monkeys. Dominance ranks are established by threat and grooming alliances; grooming is the primate social currency. Communication uses calls, facial expression and posture; vervet monkeys have distinct alarm calls for eagle, leopard and snake. Chimpanzees at Gombe (Jane Goodall, from 1960) make and use tools — termite-fishing probes, leaf sponges, nut-cracking hammers — hunt cooperatively, share meat and wage lethal inter-group conflict, and different communities show different tool traditions, which primatologists call chimpanzee cultures. Bonobos resolve tension through sexual behaviour and are female-bonded. These observations frame every model of early hominin society.

Humans share about 98.8% of their DNA sequence with chimpanzees and bonobos, the closest living relatives, about 98.4% with gorillas and about 97% with orangutans; the older grouping of the three great apes in Pongidae apart from Hominidae is therefore a grade, not a clade, and modern classifications put all of them in Hominidae, with humans and their fossil relatives after the chimpanzee split as the tribe Hominini — the reason the recent literature says "hominin" where the syllabus says "hominid". Uniquely human traits, apart from bipedalism, are the enlarged brain, the small face and reduced canines, the chin, the loss of the diastema, the parabolic dental arcade, the long thumb, the arched foot, the barrel chest, prolonged childhood and delayed maturity, hidden ovulation, menopause, a descended larynx and language, and cumulative culture; the great apes, in turn, share with us self-recognition in mirrors, tool use, learned traditions and a long infancy.