Human beings, homo sapiens to be exact, are defined as having an interior supporting skeleton.
This is what makes people and primates different from, say, jellyfish, which have no bones, or lobsters, which have a type of skeleton on the outside.
The skeleton is what makes homo sapiens unique in the animal world. The skeleton helps humans support their weight and walk upright using two legs. It provides a special mechanism for thumbs to move, allowing the thumb bone to rotate within the socket, helping fingers to pick up and hold objects.
"A skeleton is the scaffolding, or support, for the body," said Dr. Rebecca Carl, assistant professor of pediatrics at Northwestern University and pediatric sports medicine physician at the Ann & Robert H. Lurie Children's Hospital Institute for Sports Medicine.
Technically, babies are born with a large proportion of cartilage, soft bone, as well as bone. The cartilage begins the process of converting to bone in the womb and continues to harden for a long time.
"In fetal development, there's a cartilage skeleton and it begins to harden into bone before birth. It is possible to have some missing individual bones; you're born with more of a skeleton that's a mixture of cartilage and hard bone, which gradually hardens into adult bone," Carl said of the process called endocrinal ossification.
"It's like a seed that expands from a small center, like the top of the femur or thigh bone, and within six months it quickly spreads to cover the whole bone."
Only the top of the thigh bone is cartilage at birth -- the remainder is hard bone, she added.
Bone serves a very important protective function.
"The brain is vulnerable to injury and is soft. The skull, a bony cap covering the brain, cradles the brain and acts as the first line of defense when there's an injury," Carl said. …