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How did Turkana Boy's body proportions resemble those of modern humans?

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Illustration showing side-by-side reconstructions of five hominins in the human lineage: Australopithecus afarensis, Homo habilis, Homo erectus, Homo neanderthalensis, and Homo sapiens, with each figure labeled and depicted in a walking pose to highlight evolutionary changes in posture and body structure. A scale bar at the top right indicates relative height. — human evolution An artist's depiction of five species of the human lineage.

The skeleton of the Nariokotome youth, commonly known as Turkana Boy (Homo erectus or H. ergaster), provides significant evidence that the basic human body size and shape had evolved by approximately 1.5 million years ago.1

In contrast to earlier hominins such as Australopithecus, Turkana Boy’s body proportions closely resembled those of modern humans in the following ways:

Pelvis and Hips: The hips were narrow, a characteristic shared with modern people.1
Lower Limbs: The thighs were long, and the species possessed long femurs of a modern human configuration.12
Thoracic Shape: H. ergaster is the earliest hominin documented with a human-like thoracic (chest) shape, which is distinct from the rib cage configurations of earlier, more arboreally active species.2

These anatomical adaptations, along with internal knee structures similar to those of H. sapiens, indicate that Turkana Boy was well-adapted for efficient, long-distance walking and endurance running.12