The earliest known squirrel species fossils come from Eocene strata. It is a long time from 65 million years ago to 35 million years ago. At that time, dinosaurs had just died out.

Mammals have not yet gained control of land, the Qinghai-Tibet Plateau has not yet become land, Europe and North America are still connected by land, and South America is a huge island. The most primitive squirrel lives in North America, and its shape is close to that of today's flying squirrel. A series of fossils of squirrel species have also been found in the late Eocene strata in North America, which are different from any kind of squirrel.

Extended data:

Mind mapping uses the skills of paying equal attention to graphics and texts, showing the relationship between topics at all levels with hierarchical diagrams of mutual subordination and correlation, and establishing memory links between topic keywords, images and colors. Mind mapping makes full use of the functions of the left and right brain and the laws of memory, reading and thinking to help people develop in a balanced way between science and art, logic and imagination, thus opening up the infinite potential of the human brain. Therefore, mind mapping has a powerful function of human thinking.

Radioactive thinking is a natural way of thinking in the human brain. Every kind of information that enters the brain, whether it is feelings, memories or ideas-including words, numbers, symbols, aroma, food, lines, colors, images, rhythms, notes and so on. -it can be a thinking center, from which thousands of joints radiate outward.

Each joint represents a connection with the central theme, and each connection can become another central theme, and then thousands of joint points radiate outward, showing a radioactive three-dimensional structure, and these joint connections can be regarded as your memory, just like neurons in the brain, and are your personal database.