An animal study was carried out to gain better understanding on the effect of erect posture in the development of the lumbosacral junction. Rhesus monkeys which are able to sit in an upright position were compared with quadrupeds, including rats, rabbits and dogs, whose lumbosacral spines are normally more or less horizontal. The important finding was the presence of the iliolumbar ligament in the rhesus monkey which attached to the last, occasionally the second last, lumbar transverse process and the adjacent iliac wing. A similar ligament was absent in the quadrupeds. We postulate that the stresses created across the lumbosacral junction as a result of the erect posture in the rhesus monkey have played an important role in stimulating the formation of this ligament.