The Cambrian explosion refers to the period of evolution of most modern animal groups, between 570 and 530 million years ago. The Cambrian explosion is an unparalleled period of evolutionary innovation in the history of our planet. During the Cambrian explosion, early organisms evolved into many different, more complex forms. During this period, almost all of the persistent basic animal body plans came into being. The first batch of back-to-back animals, also known as vertebrates, developed to about 525 million years ago during the Cambrian period. The earliest known vertebrate is considered to be Myllokunmingia, an animal believed to have a skull and bones made of cartilage. Today, there are approximately 57,000 vertebrates, accounting for about 3% of all known species on our planet. The other 97% of the species that are alive today are invertebrates, belonging to animal groups such as sponges, cnidaria, flatworms, mollusks, arthropods, insects, segmented worms and echinoderms, and many other lesser known fauna. . The first terrestrial vertebrates evolved about 360 million years ago. About 360 million years ago, the only organisms that inhabited terrestrial habitats were plants and invertebrates. Then, a group of fish knew that the finfish evolved the necessary adaptability to achieve a transition from water to land. Between 300 million and 150 million years ago, the first terrestrial vertebrates produced reptiles, which in turn produced birds and mammals. The first terrestrial vertebrates were amphibious tetrapods that had been in close contact with their aquatic habitat for some time. In the course of their evolution, early terrestrial vertebrates evolved adaptability, allowing them to live more freely on land. One such adaptation is amnion eggs. Today, animal populations including reptiles, birds and mammals represent the offspring of early amniotic animals.