Babies start with about 270 bones; adults keep roughly 206
The human skeleton begins near 270 bones at birth and settles around 206 after fusions, not counting tiny accessory bits. Those bones support, move, protect, make blood cells, store minerals, and join endocrine regulation, an interior scaffolding doing six jobs at once.
Bone accounts for roughly 14 percent of body weight, around 10 to 11 kilograms in an average adult, and it reaches peak mass somewhere between 25 and 30. Anatomists split the frame in two. The axial part, about 80 bones, is the central column: a spine of 32 to 34 vertebrae depending on how the sacrum and coccyx are counted, twelve pairs of ribs with the sternum, and a skull of 22 bones. The appendicular part, about 126 bones, hangs from it: shoulder girdles, arms, pelvis and legs.
Each half has its own job. The axial skeleton keeps us upright, carrying the weight of head, trunk and arms down to the hips, helped by spinal ligaments and the erector spinae muscles that aid balance. The appendicular skeleton makes walking possible and shields the organs of digestion, excretion and reproduction. Joints vary in freedom: a ball and socket swings far more widely than the pivot joint in the neck. Muscles anchored to bone supply the force, and the nervous system coordinates it all. Without the rib cage and the muscles between the ribs, the lungs would simply collapse.
Bone is also a factory and a warehouse. Blood cells form in the marrow, mostly in the femur and tibia during childhood but chiefly in the pelvis, skull, vertebrae and sternum in adults. The mineral matrix banks calcium, and marrow holds iron in the protein ferritin. Despite the popular picture, bone is not pure calcium: about 70 percent is hydroxyapatite, itself under 40 percent calcium by mass, with the rest largely chondroitin sulfate, a sugar built mainly of carbon and oxygen.
Human skeletons differ less between the sexes than those of many primates, though female bones tend to be smaller and lighter and the female pelvis is shaped for childbirth. Unlike most primates, men have no penile bone. Researchers also think our bones grew less dense in prehistory, and that the move from hunting to farming thinned them significantly, possibly costing some agility.
Source: Human skeleton
