A sagittal crest is a ridge made of bone that is atop the cranium. In hominids, they are generally there to provide more surface area for muscle for the purpose of powering the jaws. Only hominids like robust Australopithecines or Neaderthals need them because of their gigantic jaws. Usually, these gigantic jaws, and hence the sagittal crest, are the result of the adaption to chomping down roots, fibers, and so forth. Most humans lack this feature, but some Eskimoes apparently have one.