Fossil Primates and the Earliest Hominins: The Oligocene and Miocene Apes from Parapithecus and Aegyptopithecus to Dryopithecus, Sivapithecus, Ramapithecus and Gigantopithecus, the Pre-Hominins Sahelanthropus, Orrorin and Ardipithecus, the Australopithecines and Paranthropus, and Homo habilis

Molecular clocks place the separation of the human and chimpanzee lineages at roughly six to eight million years ago, and three fossils fall in or near that window. Sahelanthropus tchadensis, nicknamed Toumaï ("hope of life" in the Goran language), is a nearly complete but crushed cranium found in 2001 at Toros-Menalla in the Djurab desert of Chad by Michel Brunet's team and published in 2002; dated to about seven (six to seven) million years, it combines a small, ape-sized brain (about 360 cc) with a flat face, small canines, thick enamel and a foramen magnum placed forward enough to suggest an upright posture — though a 2020 study of its femur questioned bipedalism. It is the earliest candidate hominin, and the first from west of the Rift Valley, which overturned the "East Side Story" that tied hominin origins to the Rift. Orrorin tugenensis ("original man" in Tugen), found in the Tugen Hills of Kenya in 2000 by Brigitte Senut and Martin Pickford and called "Millennium Man", is about six million years old and known from teeth, jaw fragments and three femora whose neck and head suggest bipedal walking, together with curved finger bones for climbing. Ardipithecus ramidus, from Aramis in the Middle Awash of Ethiopia, was announced by Tim White, Gen Suwa and Berhane Asfaw in 1994 as Australopithecus ramidus, moved to the new genus Ardipithecus in 1995, and described in full in 2009 from the 4.4-million-year-old partial skeleton "Ardi" (ARA-VP-6/500): a woodland-living form about 1.2 m tall, with a small brain (300–350 cc), an opposable big toe for climbing, a pelvis adapted to upright walking on the ground, and small canines without honing in males — read as evidence that bipedalism arose in woodland rather than savanna and that the last common ancestor was not chimpanzee-like. The older Ardipithecus kadabba (5.8–5.2 million years) precedes it.

Australopithecus ("southern ape") was named by Raymond Dart in 1925 for the Taung child, a juvenile skull with a natural brain endocast found in a limestone quarry in South Africa in 1924: a small brain, but a forward foramen magnum and human-like teeth. Dart's claim of an African ape-man was ridiculed for twenty years until Robert Broom's finds at Sterkfontein (the adult "Mrs Ples", Sts 5, 1947) and elsewhere confirmed Australopithecus africanus, now dated about 3.3–2.1 million years, with a brain of 430–520 cc, a projecting face and a bipedal pelvis. In East Africa, Donald Johanson's team found "Lucy" (AL 288-1), a 40%-complete female skeleton about 1.1 m tall, at Hadar in the Afar region of Ethiopia in 1974, and the "First Family" (AL 333) in 1975; with Mary Leakey's finds at Laetoli in Tanzania — where the 3.66-million-year-old footprint trails discovered in 1978 record fully bipedal walking — these were named Australopithecus afarensis by Johanson, White and Coppens in 1978. Dated about 3.9–2.9 million years, afarensis has a brain of 380–430 cc, a prognathic face, canines larger than ours but reduced, a dental arcade between U and parabolic, long arms and curved fingers alongside a bipedal pelvis and knee, and marked sexual dimorphism; "Selam", the Dikika child (2000), and the 2016 study of Lucy's fall from a tree belong to it. Australopithecus bahrelghazali ("Abel"), a mandible found by Brunet at Koro Toro in Chad in 1995 and dated about 3.5 million years, is the only australopithecine from outside eastern and southern Africa; many regard it as a western population of afarensis. Australopithecus anamensis (Kenya, 4.2–3.9 million years; Meave Leakey, 1995) precedes afarensis; A. garhi (2.5 million years, Ethiopia) and A. sediba (about 2 million years, Malapa, South Africa, 2008) follow.

Between about 2.7 and 1.2 million years ago a second australopithecine lineage specialised in heavy chewing. Its skulls carry a sagittal crest for the temporalis muscles, flaring cheekbones, a dished face, small incisors and canines, and enormous, thick-enamelled premolars and molars; the brain stayed at 500–550 cc. Robert Broom found the first, at Kromdraai in South Africa in 1938, and named it Paranthropus robustus ("beside man"); more came from Swartkrans. Mary Leakey found the East African form at Olduvai Gorge in 1959: the skull OH 5, called Zinjanthropus boisei by Louis Leakey (after Charles Boise, who funded the work) and "Nutcracker Man" and "Dear Boy" by the press, dated by the new potassium-argon method to 1.75 million years — the date that first showed how old the hominin record was. The still more primitive "Black Skull" (KNM-WT 17000, Paranthropus aethiopicus, 2.5 million years, west of Lake Turkana, 1985) links the robust line to afarensis. Whether the robust forms are a genus of their own (Paranthropus, the usage the syllabus follows in parentheses) or robust species of Australopithecus is a matter of taxonomic taste; what is agreed is that they are a side branch that went extinct about 1.2 million years ago, overlapping with early Homo and probably eating hard or fibrous plant foods, though isotopes show boisei ate mostly C4 grasses and sedges. The syllabus's spelling "Paranthropous" is a misprint of Paranthropus.

In 1960 Jonathan Leakey found a juvenile mandible, parietals and hand bones (OH 7) in Bed I at Olduvai Gorge, close to the Oldowan pebble tools that Louis Leakey had long attributed to a toolmaker more advanced than Zinjanthropus. In 1964 Louis Leakey, Phillip Tobias and John Napier named Homo habilis, "handy man" (Dart suggested the name), on a brain estimated at about 640–680 cc — below the 750 cc "cerebral Rubicon" that Arthur Keith had set for Homo, so they lowered the threshold to about 600 cc — together with smaller cheek teeth, a more parabolic arcade, a hand capable of the precision grip and a foot (OH 8) with arches. Later finds include the 1.9-million-year-old skull KNM-ER 1470 from Koobi Fora (Richard Leakey's team, 1972; about 750 cc), which many separate as Homo rudolfensis, and OH 62 (1986), whose ape-like limb proportions surprised those who expected a more human body. Dated about 2.4–1.6 million years and known only from Africa (Olduvai, Koobi Fora, Omo, Hadar, Sterkfontein), habilis is the syllabus's "early transitional human": it retains australopithecine body size and limb proportions while showing the enlarged brain, reduced dentition and (by association) stone tool-making that define Homo. The 2015 description of a 2.8-million-year-old Homo mandible from Ledi-Geraru in Ethiopia pushes the genus back further, and the debate whether habilis is a true Homo or should be returned to Australopithecus (Wood and Collard, 1999) is still open. Its successor, Homo erectus, is the subject of the next chapter.