Knuckle-walking as chimps do is actually the transition stage between walking on four legs to walking on just two, bipedal-walking. The change from knuckle to bipedal walking started while our ancestors lived mostly in trees then improved during increased time on the ground as trees became scarce. It gave us many evolutionary advantages when returning to the ground after climate changes decimated the forest, leaving wide belts of open terrain with no trees. Being up on two legs allowed us to see further and also freed up our hands to carry tools and perform other tasks. It also conserved energy. Metabolic and biomechanical comparisons indicate that humans walking on two legs consume only a quarter of the energy that chimpanzees use while knuckle-walking on all fours.

The changes required for walking upright were having a skull placed on top of the spine instead of in font, thighbones angled inward to place support directly under the ribcage and over the feet, a broader pelvis angled to support internal organs, and a s-shaped spine to move the balance point over the feet. These changes however did come at a cost.

Our spines are a heritage from distant ancestors who carried themselves horizontally, in water and on land. In quadrupeds the spine functioned more like a flexible suspension bridge, supporting the body's organs. The human spine has been transformed into a weight bearing column, putting it under unprecedented stresses causing the discs to dislodge and pinch spinal nerves resulting in pain and fractures.

Perhaps the greatest downside of walking upright is the difficult passage through the birth canal. Unlike monkeys and apes, whose infants are able to move around independently soon after they are born, humans give birth to very helpless young. This is because human babies develop very slowly in the womb and are born at a stage when they are still underdeveloped compared to the ape species. The brain of human newborns is only one quarter of its full size, whereas those of chimpanzee infants are already half grown at birth. Most of the baby's development therefore takes place outside the womb, with the brain only reaching full size at around a year of age. Being born so early is what makes human infants so uniquely vulnerable and helpless. Human babies are born so early because of the size and structure of the mother's pelvis. For a species to walk efficiently on two legs, the pelvis must provide stable support for the upper body and the internal organs. This has given rise to a bowl-shaped pelvis in modern humans that works very effectively as far as walking is concerned, but creates some major problems for giving birth. It restricts the size of the birth canal and even causes it to be twisted. The inlet to the birth canal is widest from side to side, so the baby enters it facing sideways. However the outlet of the birth canal is widest from top to bottom, so that halfway through birth, in order to squeeze through the outlet, the baby has to rotate so that it is facing downwards. Even after the twisting the opening is still too small hence why the baby must be born before it is fully developed. In some cases its cranial bones must squeeze together and overlap, compressing the skull a few millimeters so the infant can exit. Human birth is so complicated that before the era of modern medicine 1 out of every 5 births resulted in the death of the mother.

 
To the left is a phylogenic tree of our hominid family legacy. With so many various species of hominids discovered, and many more left to be discovered, it is difficult to know for sure which species descended from which branch of the hominid family tree. Thanks to known time periods of when each specimen lived, structural similarities, discovery locations, and genetics we can piece together a pretty accurate phylogenetic tree of our ancestors.