Age of Pelvic Bone Fusion

There are differences in the rate of growth of the os coxa bones prior to fusion. The anterior length of the ilium is absolutely larger and the growth rate is significantly faster in Gorilla, Pongo and Pan compared to humans (fastest growth in descending order respectively). The breadth of the ilium has a reversed pattern in rapid growth in the relationship between humans and great apes (except for gorillas). Humans have significantly larger dimensions and rapid growth in iliac breadth compared to Pongo and Pan, but growth is still slower compared to gorillas. At infancy, gorillas already exhibit a relatively large iliac breadth, a curved iliac crest and a more pronounced greater sciatic notch compared to infant chimpanzees, this is likely due to gorilla’s larger body mass. Overall, the relative growth of the ilium in humans is significantly slower in the rate of shape change compared to great apes. The ilium of Australopithcine juveniles are similar in size to adults suggesting that the ilium increases in breadth more rapidly during early postnatal ontogeny compared to modern humans.

In humans, the first elements to fuse are the ischium and pubis, which unite anteriorly to form the ischiopubic ramus between 4 and 8 years of age. Next, the ilium fuses to the combined ischiopubic portion at the acetabulum between 11 and 15 years in females and 14 to 17 years in males to form the os coxa. The secondary ossification centers in the pelvis are complex due to the union of the three bones at the acetabulum. Due to available comparative great ape data only two secondary ossification centers will be highlighted in human development, the iliac crest and the ischial epiphysis. The iliac crest epiphysis begins to ossify in the middle of the crest and extends outward in either direction that form from two separate (an anterior and posterior) ossification centers. The anterior epiphysis commences ossification first at around 12-13 years of age in females and 14-15 years of age in males. The iliac crest epiphysis is fully formed prior to union with the ilium, in females union commences at 15 years and in males at 17 years of age. Partial fusion of the iliac crest can range from 15-22 years of age, with complete union in all individuals by the age of 23. Unlike the iliac crest, the ischial epiphysis commences union prior to complete formation. The ischial epiphysis appears between 13-16 years of age and begins to fuse at the superior rim of the epiphyseal surface and continues to develop into the ramal epiphysis which will continue to fuse towards the pubic body with complete union by 20-21 years of age.

In chimpanzees and bonobos, the pattern of fusion of the os coxa is the same as humans, but the timing in growth and maturity occurs earlier. Although only small sample sizes are available to identify the age of fusion in the os coxa, it has been shown that the ischium and pubis unite anteriorly to form the ischiopubic ramus between 1 and 4 years of age in captive bonobos, captive chimpanzees and wild chimpanzees. Similar to humans, the ilium is next to fuse to the ischiopubic portion around 7 years of age in captive bonobos and chimpanzees, while data from wild chimpanzees suggest fusion between 5 to 11 years of age. Complete fusion of the acetabulum in wild chimpanzees occurs by the age of 12.5 years for females and 14.5 years for males, while captive chimpanzees exhibit an earlier age of fusion with complete fusion for females by 9 years of age and males by 13 years of age.

There are few fossil juvenile hominins to base the age of fusion in the os coxa. One example comes from the Nariokotome youth (KNM-WT 15000). Recent work on the histological data of the dental remains suggests an earlier estimated age of the individual. This suggests that the later age based on skeletal elements is really indicating that early hominins achieved a greater proportion of adult body mass, stature and maturity before modern humans.