It has been 4.5-4.6 billion years since the earth was born, but today we only know its history of nearly 600 million years clearly. What happened in the history of the earth was mainly recorded by the rock formations formed at that time and the paleontological fossils contained in them; Although life on earth appeared as early as 3 billion years ago, it has been stagnant at a very low stage for a long time, mainly some low-level fungi and algae. The fossils they left behind can't explain much, and most of the rock formations that preserved these fossils have undergone different degrees of metamorphism, which makes this early history of the earth even more difficult to understand. Only about 600 million years ago, a large number of advanced organisms appeared, leaving a large number of undisturbed sedimentary rocks and animal fossils, thus providing many reliable materials. Therefore, the present historical description of the earth for 600 million years is detailed and credible. This is similar to the exposition of human history. Before written records, human history was vague and brief, but after written records, human history became clear and informative. In short, whether it is the history of the earth or the history of mankind, the farther away it is, the more vague and concise it is, and the closer it is, the clearer it is and the more information it contains. From the birth of the earth to 600 million years ago, this period of time is called the hidden generation in the history of the earth. Although it lasted about 4 billion years, due to the lack of data, it is impossible to divide it into Archean and Proterozoic in detail, and there is no clear boundary between them, so it is often called pre-Paleozoic. When life on earth evolved from lower plants to invertebrates with shells, the history of the earth entered Phanerozoic from Proterozoic. Organisms are constantly evolving from low to high, and invertebrates give way to vertebrates; There are new "strong" vertebrates, from fish, amphibians, reptiles, mammals to us, which decline and rise again, playing leading roles on the earth in turn. In the Early Paleozoic, vast areas in the north and south of China were submerged by seawater. In the ocean, algae are still multiplying, but more advanced creatures have appeared in large numbers. An animal called trilobite rules the oceans all over the world. At this time, there is still no life on the land. Trilobite is a kind of arthropod. The whole body is divided into three parts: head, chest and tail, with a prominent central axis running through the head and tail. It can be divided into three parts: horizontal and vertical, and has a carapace on its body, which plays a protective role. Trilobites are usually a few centimeters long, and they were a big animal at that time. Most of them live on the bottom of the sea, and a few live in mud or swim in the water. In the late CAMBRIAN, trilobites flourished. By the time of Ordovician, there were still many trilobites, but there were more powerful animals in the sea. This animal is a mollusk. It has a conical hard shell. At the open end of the cone, that is, its head, there is a circular tentacle, which is used to catch food, crawl and swim. They are very big, usually tens of centimeters long, move quickly and have hard mouths, so trilobites are no match for them. These mollusks are distant relatives of octopus and squid, but most of them are extinct, leaving only some fossils of their conical hard shells in rocks. These fossils are called "hornstones", and one of them is called "Nautilus", which actually exists in today's oceans.