Reaching back almost to the beginning, our brain uses a communication system found in jellyfish, first appearing 600 million years ago. Their nerves and the manner they send signals are similar to ours. Your brain relies heavily on structures found in the animals that preceded us. The first known animal, sponges, appear just before jellyfish. They basically had a more primitive form of the communication system found in jellyfish BUT our brain still uses part of their communication system. How that process evolved between the first multicellular organism and sponges is unknown since fossils are not available.
Worms are the simplest organisms to have a central nervous system, allowing them to exhibit more complex forms of behavior. Insects have a small but remarkable brain which can, for example, permit the cockroach to dart away as soon as it senses the moving air preceding a quickly descending human foot. The insect brain controls crawling, hopping, swimming, flying, burrowing, mating, and you-name-it.
An animal’s information system runs up the spine to the brain. Vertebrates, with that stiff spine, improve the protection of the information system. In early vertebrates, one part of the brain controlled behaviors that had happened again and again. Those automatic responses are sort of like cruise control on a car. A bird flies straight at your head. You do not stop and ponder, “Hmm. Should I duck?” No. Your conscious brain is too slow; a reflex established in antiquity makes you duck.
Science quarrels when the first brain appears, but the more important question is “What does the brain do?” Basically, it signals; it communicates; it directs traffic.
Without using the term “brain,” signaling began with life. From the beginning, cells had a communication system. Without one, how else could the DNA of the first bacteria direct the organism to make a copy of itself and then reproduce by splitting that copy off? Each of those steps, from no-life to life, from bacteria to eukaryotes, eukaryotes to multicellular, multicellular to jellyfish, required cells signaling one another. For sure, the brain was more organized when vertebrates had it at the end of a central nervous system. But communication among cells had to begin with life itself.
The amphibian transition from water to land made a big impact on the brain’s development. By this time, the brain had a midbrain and forebrain where brain functions for hearing and seeing in a higher and drier world sharpened. One part of the brain included responses like dominance and submission. Sudden movements, intimidating objects and brighter light changed visions centers. A possibly threatening or sight or sound causes us to instinctively turn our face and eyes in that direction.
As the finished reptile appears, the brain controls vital functions like heart rate, temperature, breathing, and balance. The brain, at that point, has a brain stem connected to two spheres (called the cerebellum.) This section helps with learning new motor behaviors, such as swinging a golf club.
In addition, most of science agrees that emotional centers also existed in the reptilian brain. Likely members include the instinct to fight or run which might also be called terror or anger. Instincts regarding sexual drives as well as parenting can be found here.
Dinosaurs, birds and mammals all followed reptiles. Each of those began with the same brain structure – the same brain structure as the reptiles. The human brain contains that section (often called reptilian) which still controls involuntary and instinctive behavior. Contrary to the arrogant beliefs of too many humans, the brain did not start over when homo sapiens arrived.