The Holy Grail of evolutionary researchers is the missing link,the earliest common ancestor shared by humans and their ape cousins. Many have believed that identifying the transition to　bipedalism (walking on two legs) and upright posture could be the key to pinpointing the first clear divergence between man and monkey. Charles Darwin was the first to theorize that bipedalism freed up human arms for tasks such as tool making and tool use, activities that in turn led to greater intelligence and larger brain size―a theory that has become widely accepted.
  Now, it appears, researchers have found an important key as to when this transition occurred. Using a CT scan to analyze the fossilized thighbone of an early humanlike creature the size of a chimpanzee, scientists have determined that the creature walked on two legs. “Now, for the first time, we have solid evidence dated to 6 million years ago of an intermediate creature between humans and the apes that demonstrated upright posture and bipedalism,” says Robert Eckhardt, an evolutionary researcher.
　The lower part of the bone is about three times as thick as the upper part, whereas in modern chimps and gorillas, the thickness of the upper and lower parts of the bone is roughly equal. The shape of the fossil is closer to that of present-day humans, where the lower part of the thighbone is four times thicker than the upper part, indicating a transition to bipedalism.
  Though pleased with the discovery of the thighbone, known as Orrorin tugenensis, some genetic researchers are not surprised by this evidence of early bipedalism. Based on genetic data, they had earlier estimated that the divergence between early humans and chimpanzees had occurred sometime between 5 million and 7 million years ago, even though the most commonly acknowledged date for the dawn of bipedalism among most evolutionists has been set at about 4 million years ago. “Dating the beginnings of bipedalism is very important in the human story because, for many experts, it would mark a clear divergence from the ancestral-ape pattern and show that human linage had really begun,” said Chris Stringer of London's Natural History Museum.
  Researchers are also divided when it comes to explaining how and why bipedalism first evolved.One theory has it that an extended drought at the end of the Miocene Era over 6 million years ago reduced the forests in Africa and forced tree-dwelling creatures to the ground. Those creatures able to walk on two legs had clear advantages over those that could not. Many ape species likely became extinct at this time, while those that evolved to bipedalism survived.
  Owen Lovejoy, a biological anthropologist, theorizes that the differentiation of gender roles that bipedalism introduced enhanced species survival. While one parent foraged for food on the ground,the other could remain at home in the forest canopy and take care of children. The vast majority of four-legged species forage together, exposing the entire family, including the more vulnerable younger offspring, to attack from predators. “Both males and females would benefit from intensified cooperative care of offspring under such conditions, as this would reduce the burden on females and enhance infant survival rates,” Lovejoy said. Over time, females began to choose males who regularly offered food.
  Not all scientists buy into the idea of a smooth evolution to bipedalism, though. Anthropologists Brian Richmond and David Strait argue that more complete fossilized remains of 4-million-year-old human ancestors show that early-human wrist joints were very similar to those of modern apes. They believe that early humans were able to lock their wrists, indicating that they continued to support their weight on their knuckles as they walked, much as chimpanzees do today. What this implies could be significant for, as Richmond argues, “our earliest ancestors did not simply come down from the trees. Rather, they evolved from an ape that was already adapted to life on the ground.”
  Some researchers argue that the fossil Orrorin and the CT scans taken of it do not completely substantiate evolution to complete bipedalism. “I'm not sure the scans, which have some problems with them, are very convincing,” Lovejoy said. “I am convinced, based on morphology and anatomy, that [the owner of] Orrorin was bipedal at least frequently, if not habitually. The question is to what extent. Clearly, Richmond and Strait would like to examine the wrist of Orrorin's owner, if such a fossil were to exist.
  All scientists at least agree that human evolution into an upright creature meant major changes in skeletal and muscular structures, and that there must have been good reasons why these changes occurred. For now, though, the exact reasons for the changes are shrouded in mystery, and scientists continue to search for further fossil and geographical clues to obtain a clearer understanding of the driving force behind human evolution.