Normal vertebrates, such as fish, have a long axis in a horizontal position. The front end is the head, in which all the sense organs are concentrated. The spine is either made of cartilage or hard bone. Along both sides of the spine, in addition to the ribs, there are pairs of fins, which are connected to the body through shoulder straps and belts and are used for navigation and balance, or to propel the body forward. A major characteristic of a vertebrate is the location of its spinal cord, circulatory and digestive systems. The spinal nerves are located above the notochord, or spine, while circulation and digestion are located below the spine. Fish use gills to breathe. Except for the most primitive types, all fish have jaws, which are transformed from the third pair of gill arches in the front.

Comparing the structure of amphioxus with the structure of fish with backbones in normal conditions, it is easy to see how similar they are. As long as nature slightly processes and transforms the structure of amphioxus, it will become a perfect real fish. Therefore, the structure of amphioxus is very close to the common ancestor of all vertebrates. It probably appeared very early, but over a long period of time, because it was content with the status quo and did not seek progress, it has evolved very little. Not much has changed. However, at least we can see through amphioxus that the ancestors of all living vertebrates, including humans, who lived 600 million years ago, looked roughly the same.

Early fish

The earliest known fish fossils are some scattered scales about 500 million years ago. The earliest fish to appear were jawless fish. They had no upper and lower jaws, only a funnel-shaped mouth at the front of the body, and could not actively feed. The earliest jawed fish were placoderms, which had upper and lower jaws and could actively feed. The earliest cartilaginous fish appeared in the early Devonian period 300 million years ago, and the rip shark is often regarded as one of the most primitive representatives. Bony fishes are the most advanced fish and are generally believed to have evolved from sticklebacks 400 million years ago.