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Description
Biologists divide the entire animal world into two groups: vertebrates and invertebrates. Vertebrates are animals that have a backbone, while invertebrates do not. Currently, vertebrates include all the largest and most powerful animals, while invertebrates are much more diverse and are much more widespread than vertebrates, since 97% of all animal species on Earth are they. It is certain that the first were invertebrates. Fossilized remains of Ediacaran invertebrates, for example, date back to a period 50 million years older than the beginning of the Cambrian period. During the Cambrian period, a number of invertebrate animals with hard skeletons appeared. This very extensive series included sponges and arthropods, mollusks and similar animals called brachiopods. But the phylum Chordata, which includes vertebrates, is much older than previously thought. The first chordates were most-likely soft-bodied animals, since very few of their remains survive, but when chordates began to develop hard skeletons consisting of cartilage and bones, the fossil remains of these animals became more distinct and numerous. In the Burgess Shale in Canada, the fossilized remains of a Pikaia gracilens(1), a soft-bodied animal with a chord that gives strength to the body, were found. This feature means that this animal was probably one of the very first representatives of chordates - a group to which vertebrates also belong. In 1999, Chinese scientists announced that they had found two vertebrate fossils dating back more than 500 million years, close to the beginning of the Cambrian period. These animals - Haikouichthys ercaicunensis(2) and Myllokunmingia fengjiaoa(3) - are the most ancient fish-like animals known so far. Their length is not even 3 cm, but they represent a certain step along the path of the evolution of the animal world, which led to the emergence of amphibians and reptiles, including giant dinosaurs.

Description
Biologists divide the entire animal world into two groups: vertebrates and invertebrates. Vertebrates are animals that have a backbone, while invertebrates do not. Currently, vertebrates include all the largest and most powerful animals, while invertebrates are much more diverse and are much more widespread than vertebrates, since 97% of all animal species on Earth are they. It is certain that the first were invertebrates. Fossilized remains of Ediacaran invertebrates, for example, date back to a period 50 million years older than the beginning of the Cambrian period. During the Cambrian period, a number of invertebrate animals with hard skeletons appeared. This very extensive series included sponges and arthropods, mollusks and similar animals called brachiopods. But the phylum Chordata, which includes vertebrates, is much older than previously thought. The first chordates were most-likely soft-bodied animals, since very few of their remains survive, but when chordates began to develop hard skeletons consisting of cartilage and bones, the fossil remains of these animals became more distinct and numerous. In the Burgess Shale in Canada, the fossilized remains of a Pikaia gracilens(1), a soft-bodied animal with a chord that gives strength to the body, were found. This feature means that this animal was probably one of the very first representatives of chordates - a group to which vertebrates also belong. In 1999, Chinese scientists announced that they had found two vertebrate fossils dating back more than 500 million years, close to the beginning of the Cambrian period. These animals - Haikouichthys ercaicunensis(2) and Myllokunmingia fengjiaoa(3) - are the most ancient fish-like animals known so far. Their length is not even 3 cm, but they represent a certain step along the path of the evolution of the animal world, which led to the emergence of amphibians and reptiles, including giant dinosaurs.