Final answer:
The truly hominine characteristic of the early Australopithecines is their ability to walk upright on two legs, known as bipedalism. This trait is a major evolutionary step distinguishing them from great apes and aligning them closer to the lineage leading to modern humans.
Step-by-step explanation:
The question is asking about the truly hominine characteristic of early Australopithecines. While Australopithecus species shared many traits with great apes, they also exhibited one crucial feature in common with modern humans—bipedalism. This means their ability to walk upright on two legs is the distinctly hominine characteristic. Unlike apes that move about on all fours, Australopithecines' adaptation to bipedal locomotion marked a significant divergence from other apes and is considered a fundamental step towards the evolution of later hominins, including Homo sapiens.
Fossil evidence, including the well-known Laetoli footprints, indicates that as far back as 3.6 million years ago, these early hominins were walking with a human-like gait. This form of mobility freed their hands for tool use and allowed for greater energy efficiency while traversing open landscapes. Thus, despite having other primitive features such as small brains relative to body size and pronounced sexual dimorphism, it was their bipedalism that truly differentiated them as part of the human lineage.