Approximately 1.9 million years ago, a new form of homininHomininMember of the subtribe Hominina, comprising the human lineage (Homo, Australopithecus, Paranthropus…) but excluding orangutans and gibbons. The term progressively replaces "hominid" in its narrow sense.→ emerged in East AfricaAfricaThe cradle of humankind: the continent where the first hominins appeared, then Homo sapiens around 300,000 years ago, before the expansion to the rest of the world.→. Taller, more robust, and endowed with a significantly larger brain than its predecessors, it opened a decisive chapter in the history of human evolution. This species is known as Homo ergaster, the "Working Man," and represents for many paleoanthropologists the first true representative of the genus Homo in the modern sense.

The Discovery of KNM-ER 3733

In 1975, on the eastern shores of Lake Turkana in Kenya's Koobi Fora region, fossil hunter Bernard Ngeneo unearthed a nearly complete skull. Catalogued as KNM-ER 3733, this specimen belonged to an adult female who lived approximately 1.63 million years ago. The skull's morphology immediately surprised specialists: far more gracile than earlier known forms, with a rising forehead, reduced brow ridges, and a cranial capacity estimated at 850 cc1.

KNM-ER 3733 skull of Homo ergaster
The KNM-ER 3733 skull, type specimen of Homo ergaster, discovered at Koobi Fora (Kenya) in 1975.

Turkana Boy: The Most Complete Ancient Hominin Skeleton

On August 23, 1984, Kamoya Kimeu discovered fragments near Nariokotome, West Turkana, leading to the excavation of KNM-WT 15000, the "Turkana Boy." This nearly complete skeleton belonged to a male adolescent aged between 7 and 11 years, already standing 1.60 m tall and projected to reach 1.80 m in adulthood2. The skeleton revealed for the first time the body proportions of an ancient hominin: long legs adapted for distance walking and running, arms proportionally shorter than in australopiths, and a body structure remarkably close to modern humans.

Physical Characteristics

Homo ergaster differs from australopiths and earlier Homo through several progressive anatomical features: cranial capacity between 700 and 900 cc, reduced sexual dimorphism, modern body proportions with long lower limbs, and reduced facial prognathism. These characteristics indicate an animal fundamentally different from its predecessors, the first hominin to have definitively left forest-edge niches for the open African savanna.

The AcheuleanAcheuleanLower Palaeolithic technical culture characterised by hand axes, present across three continents.→ Industry

The appearance of Homo ergaster coincides with the emergence of the Acheulean stone tool tradition around 1.76 million years ago. Carefully bifacially knapped handaxes imply multi-step mental planning absent in earlier OldowanOldowanThe oldest known stone-tool industry (c. 3.3–1.7 Ma), characterised by flaked pebbles (choppers) and basic flakes. Named after Olduvai Gorge (Tanzania).→ toolmakers4. This cognitive revolution persisted for over a million years.

Taxonomic Debate and Evolutionary Position

Whether H. ergaster represents a distinct African species or simply an African population of H. erectus remains debated. Regardless, these African fossils occupy a pivotal position: likely descended from Homo habilis, they gave rise to Asian H. erectus, H. heidelbergensis, and ultimately H. sapiens. The first migrants to leave Africa were actually more primitive than H. ergaster, as shown by the Dmanisi fossils from Georgia, dated to 1.77-1.85 million years ago.