The most important organ present inside an organism is the BRAIN. It is said that brain weighs about 1.4 kilograms inside the body. It is a mass of nerve bundles that cause transmission of messages to other body parts. Also sensing of surroundings is an aspect of the brain.
Brain is enclosed in a thick bony structure called as the skull. The skull provides protection to the brain from external injuries and also the brain is quite sensitive internally. The brain does not allow the entry of any kind of drugs inside the brain. Brain is an important organ of the central nervous system inside the body along with spinal cord. The brain sends the stimulus according to the responses received.
We all know that a series of evolutionary steps have taken place in the history that helped in evolution of humans to what we are today. From chimpanzees to man, the evolution has taken place over a vast span of years. Along, with that from organisms that do not have a brain to today’s modern man also under went various series of evolution.
First of, during the time when there were organisms which do not have need of a complex structure as brain, the growth was bilateral or radial symmetry. Also, as the complexity of the organisms started to increase, the sense organs such as eyes that help in movement started to evolve. Then, the eye structures and other body senses were connected through lines like in telegram. Then, chordates showed the presence of a basic nervous system. As the structures evolved, the central nervous system consisting of brain, a spinal cord and a mass of nerve bundles came into existence.
The brain is present only in the head because of presence of sense organs and it even more convenient to distinguish the head and the tail part of the organisms. And the hox genes present in the chromosomes help in distinguish of head region and tail region. The brain also as the level of complex nature increased, formed as a big mass of nerve bundles inside the head that helps in entire controlling of the body.