Finds in East Africa over the past two decades of this very important species in the sequence of human evolution have led physical anthropologists to rename the East African form. What was once considered Homo erectus in the time period from 1.75 MYA to around 0.7 MYA, no matter where it was found in the Old World, is now separated into the species Homo ergaster in Africa and Homo erectus in Asia.
We know that Homo ergaster was the first human form to migrate out of Africa into Asia, somewhere between about 1.7 and 1.1 MYA. In Asia these humans developed then into the classic Homo erectus forms as found, for example, in China's "Peking Man" and Indonesia's "Java Man". This Asian species then apparently became extinct sometime in the past hundred thousand years when anatomically modern Homo sapiens populations, having first appeared in Africa, migrated into the rest of the Old World.
This particular fossil, found in Kenya in 1975 by Bernard Ngeneo, is dated at 1.75 MY and thus is one of the oldest known examples of a crucial evolutionary shift. In a relatively brief period of time these humans evolved from an early Homo species (probably Homo habilis) which was small-brained, small-bodied (maximum adult male height about 5 feet), and highly sexually dimorphic to the larger-body sizes of contemporary humans and with the relatively modest degree of sexual dimorphism seen in people today.
In addition it is only with Homo ergaster that we see the appearance of contemporary human limb proportions. In all earlier Homo and Australopithecus species the arms are much longer relative to the legs than is the case with Homo ergaster and all later human populations.
The consensus is that it was only with Homo ergaster that humans evolved into a species that was totally committed to terrestrial life, living completely on the ground in normal circumstances, rather than being dual-adapted to both arboreal and terrestrial living (for instance, commonly sleeping in trees at night).
This skull has a cranial capacity of 850 ml., and is considered to have been probably female, given the less rugged construction of the brow ridges and other skull features.
Around 1.5 MYA Homo ergaster populations invented a key improvement in the lithic tool technology, in developing the Acheulean tool tradition, commonly known for its signature tool, the hand ax. This was the first stone tool, to our knowledge, which early humans made to a preconceived design, and hand axes are found by the thousands in the Old World.
The catalog number for this famous and important fossil is KNM-ER 3733.