Final answer:
The earliest vertebrate characteristic that evolved before jaws, a bony skeleton, and four limbs, was the vertebral column. This essential structure offered support and protection for the spinal cord and evolved progressively into the complex backbones seen in later vertebrates.
Step-by-step explanation:
Earliest Vertebrate Characteristics
The earliest vertebrate characteristic that evolved was the vertebral column. The presence of a backbone is the primary feature shared by all vertebrates and is crucial for both structural support and protection of the spinal cord. Early vertebrates, like jawless fish similar to living hagfish, had a cranium but did not initially have a complete vertebral column. The vertebral column allows vertebrates to maintain shape and offers protection for the spinal nerve cord. Over time, this feature became more complex with the addition of intervertebral discs and a more articulated structure.
Following the evolution of the vertebral column, other characteristics such as jaws, bony skeletons, and eventually four limbs, evolved. Jaws appeared with the advent of the gnathostomes, and bony skeletons emerged in Osteichthyes, different from the cartilaginous skeletons of Chondrichthyes. Four limbs are a characteristic of tetrapods, which was a significant development as vertebrates started inhabiting terrestrial environments.