A notochord is the defining characteristic of members of the phylum Chordata, a large and diverse biological group that includes all animals with spines, or backbones, along with more primitive chordates. The structure has very large cells that are densely arranged within a protective sheath. Chordates are divided into three subphyla, depending on what form their notochords take and when they appear. At any developmental stage, it acts as a form of support for the animal that possesses it, giving animals the ability to do things such as walking upright.
The most primitive group of chordates, urochordates, also called tunicates, only have a notochord in the larval stages of development. The animals in this group are pelagic, meaning that they are found in the world's oceans, and there are a number of representative species, most of whom are only known to biologists. These animals do provide an insight into the development of chordates, however, showing the notochord at an early stage of development.
The next group of chordates, cephalochordates, also called lancelets, possess a notochord into adulthood, and also live in the ocean. The structure runs all the way along the body, even up into the head, and the animals lack a protective layer of bone such as a spine. It acts as an axial support, providing a strong core for the animal. The notochord is highly flexible, but not compressible, allowing the animal to move freely without damaging the structure.
In the highest class of chordates, the vertebrates, the notochord only exists when the animal is in an embryonic form. As the vertebrate develops, it is first ensheathed in and then replaced by spinal vertebrae, protective cases of bone that cover the delicate spinal cord. The spinal column is able to support a much larger and more complex organism, and is much stronger than the notochord. This allowed early vertebrates to make the leap to the land and develop into well known species, such as humans.
In all chordates, the notochord exists in some form when the animal is in a larval or embryonic stage. Its development from there helps shape what sort of animal it will turn into, whether it be a sac-like tunicate or a pure bred Norwegian Fjord horse. The development of this structure represents a major advance in evolution, as it allowed animals to get much larger and more complex, a drastic departure from simpler orders of animals which existed previously.