DIFFERENCES BETWEEN ADULT AND SUBADULT SKELETONS
The adult skeleton consists of 206 bones. Each of these bones develops from a number of centers of ossification. It is estimated, then, that a baby is born with approximately 450 bones that grow from their centers of ossification and eventually become the 206 bones of the adult skeleton. For example, a typical long bone (e.g., tibia) has three centers of ossification: one primary center, the diaphysis; and two secondary centers, the epiphyses. In between epiphysis and diaphysis is an epiphyseal growth plate of cartilage that will remain unfused until postnatal growth is complete after puberty (and for some bones, well into adulthood). There is a relatively well documented order in which bones of the subadult postcranial skeleton reach full fusion, which is used as the basis for age estimation in forensic contexts (Bass 2005; White and Folkens 2000).
Similarly, the sutures between cranial bones in children are unfused, which allows skull growth to coincide with brain growth and provides a basis for age estimation based on suture fusion. The skulls of babies are marked by several fontanelles (soft spots), which are areas of the skull filled with membrane that has not been replaced with bone through intramembranous ossification.
Finally, the age of the subadult skeleton can be estimated based on teeth. All mammals develop two sets of teeth: deciduous (baby) teeth and permanent (adult) teeth; humans are no exception. Permanent human teeth were described immediately above. It is worth spending a few words to describe deciduous human teeth. At birth, humans usually display no teeth, but by about six months of age, the deciduous lower central incisors usually appear (see Bass 2005). When the complete sequence of deciduous teeth has erupted, there are five teeth in each quadrant: two incisors, one canine, and two molars. Deciduous incisors and canines are eventually replaced by their adult counterparts; deciduous molars are replaced by adult premolars, and there is no deciduous precursor to adult molars. The eruption patterns of deciduous and adult teeth is well documented and is used as in forensic contexts to estimate age (Bass 2005).
CONCLUSION
Over the last six to seven million years, humans have been evolving to become more efficient at walking around on two limbs (bipedal locomotion), resulting in skeletal anatomy that is divergent from our closest living relative, the chimpanzee (Pan troglodytes). These differences can be seen in both the axial and appendicular skeletons. In the axial skeleton, for example, the foramen magnum in humans is more anteriorly positioned than that of chimpanzees, which places the vertebral column directly underneath the skull as opposed to behind it as it is in chimpanzees and other quadrupedal mammals. Chimpanzees also do not have an S-shaped curvature to their vertebral column; they simply retain the gentle primary kyphosis developed during the fetal period. Furthermore, they actually have one fewer lumbar vertebrae compared to humans, which results in a stiffer lower back.
As far as differences in the appendicular skeleton are concerned, the shape of the pelvic girdle is dramatically different in humans, where the ilium flares out laterally compared to the posterior flare of the ilium in chimpanzees. This reorganization of the pelvis has changed the function of two muscles, gluteus medius and gluteus minimus, from hip extensors in chimpanzees to hip abductors in humans. The angle of the femoral diaphysis is more oblique in humans because one of the demands of efficient bipedal locomotion is that humans require their knees to remain under their center of mass when they are standing on one limb during walking; in chimpanzees, the knees are not moved under their center of mass, so the femoral diaphysis is nearly vertical in orientation from the hip to the knee joint. In addition, humans have oversized hip and knee joints for their body size compared to chimpanzees, likely because they require more surface area to keep from damaging joint surfaces when they support their entire body mass on a single limb during walking. Chimpanzees spend more time engaging in climbing behavior than humans do, and they are known to have glenoid fossae and scapulae that are oriented more superiorly than humans, which allows them to support their body weight better on their upper limbs than humans can. Finally, the phalanges of the hand and foot are less anteroposteriorly curved in humans than they are in chimpanzees, and instead of having an opposable big toe like chimpanzees, humans have a big toe that is in line with the other digits and more efficient for bipedal locomotion.
