The skeleton has three main functions of support, movement, and protection. The upper limb is for manipulation and is specialized for manual dexterity. The lower limb is for posture and mobility and is specialized for weight bearing and walking. The sophisticated human hand is responsible for the advanced human civilization by making tools. The upper and forearms are levers to position the hand. The human upright posture allows use of upper limb free for manipulation. Control of posture and movement in humans is very advanced to enable upright walking. The musculo-skeletal system act as levers and pulleys with a mechanical efficiency higher than that of man-made machines.

Continuous movement and exercise of the musculo-skeletal system is a physiological necessity for growth in infants and to prevent disuse atrophy. Physical exercise is best undertaken while doing some work that is useful for the community.

The human facial muscles are used in both verbal and non-verbal communication. Body language and body posture can convey very sophisticated messages and emotions.

The vocal apparatus, the mouth, the tongue, pharynx, and the larynx, help in producing noises that may be happiness, anger, or fear. They also produce words that have enabled humans have the most sophisticated language.

THE SKIN

The functions of the skin are: protection against mechanical injury, excretion through sweat, sensory input, temperature regulation, immune protection, protection against UV rays. Skin color is controlled by 4 pairs of genes. Chemically and optically, it is the melanin in the epidermis and the blood vessels in the dermis that define skin color. The density of melanocytes in the skin does not differ between darker and lighter races. Darker races have more active melanin production than lighter races. The primary purpose of melanin is to protect the skin against cosmic radiation especially ultra-violet radiation. Skin whorls and patterns are unique for each individual. The probability of two individuals having the same pattern is very low.

Human nails unlike those of animals do not play a significant role in self-defense against attack. They are however used to scratch the skin and to improve grip for some small objects.

Tears, sweat, and sebum play non roles in non specific immune protection.

The number, size, and depth of melanin particles determine the color of hair from black to blonde. White hair has no melanin. Red hair has iron-rich pigment. Straightness and curliness of hair of hair depend on the shape of the pit of the follicle (straight sharply curved) or shape of hair shaft (round or oval).

The pattern of baldness is under genetic control.