The emergence of bipedalism is considered a defining event in the evolutionary history of humans. Understanding the evolutionary development of bipedalism in homo sapiens is contingent upon our understanding of the skeletal morphology of the last common ancestor of hominins, chimpanzees, and bonobos. Of primary importance to this discussion is the development of the vertebral column; specifically the number of lumbar vertebrae present. The vertebral column is a defining feature of the order Vertebrata, and the lumbar vertebrae of the trunk and lower back play a key role in mobility ad stability. The Mammalian Vertebral Number Evolution Project, led by Dr. Scott Williams, examines this question through the comparative analysis of lumbar vertebra across the Mammalia class. Working with grad students and colleagues, they have argued that hominins evolved from a short-backed ancestor and that bipedalism likely emerged with or just before the appearance of Australopithecines.