Mammals range from chimpanzees to gibbons. Are close relatives of humans with similar structures, such as elephants, whales, etc. Dolphins whose absolute brain capacity is greater than that of human beings and whose relative brain capacity is close to that of human beings. Therefore, it is biased to talk only about brain volume and basic structure. The particularity of the human brain mainly lies in its shape and function. First of all, discussing the common features of mammalian brains is helpful to understand their differences. Mammals all have cerebral cortex, but different sizes, thalamus and limbic system, but the midbrain is generally smaller. Compared with the reptile brain structure, it shows that the cerebral cortex and thalamus take over many functions of the midbrain, which leads to midbrain atrophy.

Reptiles' brains. The old cortex, also known as primitive (reptile) brain or "basic brain", including brain stem and cerebellum, is the first brain component, which McLean called "R- complex area". It consists of brain stem-medulla oblongata, pons, cerebellum, midbrain and the oldest basal nuclei-globus pallidus and olfactory bulb. For reptiles, the brain stem and cerebellum control the behavior of species. For this reason, people call the old cortex "reptile brain". Under the control of reptile brain, people have the same behavior pattern as snakes and lizards: dull, paranoid, impulsive, unchanging, suspicious and delusional, just like "branding the survival mark of ancestors in the wild times in memory". Copy the same behavior endlessly and never learn from previous mistakes (corresponding to the mechanical mind mentioned by Sri Arrondo). This brain controls the muscles, balance and automatic functions of the body, such as breathing and heartbeat. This part of the brain remains active all the time and does not rest even during deep sleep.

Limbic system (brain of ancient mammals). 1952, McLean first coined the term "limbic system" to refer to the middle part of the brain, which can also be called the old cerebral cortex or midbrain (ancient mammalian brain), corresponding to the brains of most mammals, especially those in the early stage of evolution. The ancient mammalian brain located in the limbic system is closely related to emotion, intuition, feeding, fighting, escape and sexual behavior. As McLean observed, the emotional system has always been a clear distinction between love and hate. A thing is either "pleasant" or "unpleasant", and there is no intermediate state. In the harsh environment, it is this simple principle of "seeking advantages and avoiding disadvantages" that ensures survival.