542-488 million years ago
The notochord is one of the basic features which all chordates share and it evolved in ancestral urochordates. Its evolution allowed chordates to swim from side to side in S-shaped lateral undulations which are still used by a diversity of chordates ranging from primitive chordates to fish to snakes and crocodiles.
The notochord is a flexible, elastic rod that runs down the longitudinal axis of the body made of a core of cells in a tough fibrous coat. It is important for movement: without it muscle contraction would cause the entire body to shorten instead of the side-to side body flexion which the chordates depend on. Although the notochord no longer serves this support function in most modern vertebrates (because it has been replaced by a vertebral column), the notochord is still present in all vertebrate embryos where it promotes the formation of other structures (like the central nervous system). In adult humans, the remnants of the notochord form the central region (nucleus pulposus) of the intervertebral disks between our vertebrae.
The notochord of the primitive chordate, the lancelet
(Amphioxus), is depicted in the following images.