The Turkana Boy skeleton is human in every meaningful measurement. The detailed specifics indicate that this boy of 9-12 years old was completely human.
Earlier humans had roughly the same body size as modern chimpanzee. Yet this immature male had already surpassed a height of five feet at the time of his death, and probably would have attained a height of 6 feet and a weight of roughly 150 lbs., assuming Homo ergaster underwent an adolescent growthspurt as modern teenagers do. The hips were more slender and adapted to walking and running over long distances. The proportions of his arm and leg bones were like those of modern humans, as opposed to the shorter legs and longer arms (more ape-like proportions) of Homo habilis and A. afarensis. The cranial capacity of WT 15000 is measured at 880cc. Using the same extrapolations that were used for height, it is estimated that he would have attained an adult cranial capacity of 909cc. His body was long and slender, probably adapted to a tropical environment, given that most tropical populations of modern humans are also tall and slender.
KNM WT 15000 "The Turkana Boy". Human Family Tree
The only thing that makes Turkana Boy different then modern humans is the size of the skull which is well below the average cranial capacity but well within the range of modern humans. The immediate ancestor was thought to be Homo habilis who was for all intents and purposed nothing more then an ape. The brain of the Homo ergaster populations would have had to double the cranial capacity roughly 2 million years ago and fully develop the skeleton structure with virtually no precursors.
The co-discoverers describe the detailed specifics in the following book. The skeleton for all intents and purposes is simply human. Something I might add, no one would have expected to find from a nearly complete skeleton between 1.5 and 1.9 million years ago.
An finally, it is striking to us to see how humans this early member of our own genus was over 1.5 mya. In limb proportions and body proportions, in tooth features, in the spinal column with it's curvatures, in the thorax with the characteristic hominid rib torsion, this skelleton looks human. It is possible to find several primitive features, such as the relatively small brain, but one has to search for them. (The Nariokotome Homo erectus Skeleton. Alan Walker and Richard Leaky)
