Owen Lovejoy's Theory about the evolution of upright walking:

This excerpt is taken from the book, ORIGINS, by Richard Leakey

Owen Lovejoy has provided a different model for the origin of hominid bipedality. Lovejoy believes that opportunistic hunting on the emerging African savannah was an important component in the origin of human bipedality. His theory is that with the shrinking of the Miocene forest, the savannah would have gradually become available for exploitation by proto-hominids. These early forms would have explored the savannah during the day but returned to the safety of the forest for the night.

\However, in traversing the savannah, they would have encountered an unexpected food source in the form of carrion left by the predators as well as young or ailing animals. Getting this food source back to the forest would have required that it be carried. Lovejoy believes that this food source would have been useful only to those early hominids who, due to natural genetic variation resulting in some forms being more disposed to carrying than others, were able to walk bipedally enabling them to carry objects, in this case, food. Lovejoy also believes that this early hominid behavior might have resulted in the early formation of hominid pair-bonding which would have led the males to begin to recognize their mates, to provision their mates and to become more attentive to their offspring than are modern day non-human primates.

This pair-bonding, again according to Lovejoy, might have lead to the beginnings of the nuclear and ultimately the extended family. Such a circumstance would have had a profound effect on the reproductive behavior of these early hominids. For example, the closest living relatives of human beings are the bonobo and common chimpanzee whose reproductive behaviors and cycles are vastly different from modern humans. Lovejoy hypothesizes that if the nuclear and extended family had its beginnings in these early hominid groups, females would not have been totally responsible for the care of the offspring. With the care of the offspring now shared by a nuclear group, hominid females were then free to produce more offspring and thus the human birth cycle was established. It cannot be an accident that living chimps, particularly the common chimp, generally have only one offspring which matures in six years and for which she is responsible. During that period, if the offspring is healthy, the chimp mother does not go into estrus. She is solely responsible for the care of the infant. Having more than one infant at a time therefore would prove evolutionarily disadvantageous since two offspring could not be cared for properly by only one mother. The human condition however is remarkably different which explains why the number of chimpanzees who live on this planet number only several thousand while human beings number into the billions. Returning to Lovejoy's model, the elements of bipedality, carrying and sharing and the notion of reduced birth spacing in humans are all interlinking ideas intertwined in an elaborate feedback system.

Other models for the origin of bipedality do of course exist. But there is currently no consensus among anthropologists regarding the origins of this unique form of human locomotion. What is commonly accepted however is that, whatever its cause, the first element in the human evolutionary journey was the origin of upright walking.