What do all chordates have in common during at least one stage of their life cycle?

All chordates, which include animals from various groups such as mammals, birds, reptiles, amphibians, and fish, possess a notochord at some stage in their life cycle. The notochord is a flexible rod-like structure that provides support and can be found in the embryonic development of all chordates. In many vertebrates, the notochord is replaced by the vertebral column (backbone) during development, but its presence during the embryonic phase is a key characteristic that unites all chordates. This feature plays a critical role in the development of the nervous system and serves as a primary support structure in the early stages of growth. The other traits listed, such as having a backbone, feathers, or a four-chambered heart, are specific to certain groups within the chordates and do not apply universally to all members of the phylum at any stage.