How Does the Theory of Evolution Apply to Bipedal Primates?

We no longer walk on all fours, and this change is the result of evolution. There is no real satisfactory answer as to why this happened. It could be said that primates already had adaptations for standing on two feet, running upright and climbing vertically. We see examples of this in gibbons who do all of these activities and have the same amount of vertebrae as humans. All apes have these same abilities and chimpanzees are also sometimes bipedal. Whatever the case, whether it made carrying objects easier, conserved energy, provided for more solar protection or was a better presentation of one's sexuality, we do know that the lower limbs of primates adapted, becoming stronger.

Gibbon Jumping

The limb bones, joints and feet changed to support an upright body. It is believed that Homo sapiens respiratory system changed in conjunction with being in a tropical environment and with their increased mobility, bipedalism was most favorable for their survival, for running, trekking, etc. More sweat glands are also contained on the torso of humans and the top of the head that aid in cooling the body using sweat evaporation. Humans are also not as hairy as apes and so we don't need to pause to cool down.

The human calf is also built for energy conservation and acts as a spring when we walk or run. This adaptation allowed humans to see above tall grasses. {1}It also helped them to travel further as their hips also changed becoming less wide and more bowl shaped. The human brain also grew, becoming bigger and allowing us to become a dominant species. Chimps and humans share 96% of their DNA and new research suggests that humans and chimpanzees may have split off from a common ancestor 4 million years ago. {2}

Paranthropus Aethiopicus
Parathropus was one of the earliest robust species that lived about 2.5 million years ago and has only been found in east Africa. It had a smaller brain than other species like it so it may have been a transitional form from a species that came before it. It is believed that humans are Paranthropus' contemporaries. Another species, Australopithecus anamensis appeared a little over 4.2 million years ago for which we have no hominin fossils on record. However fossils of what may have been Australopithecus' immediate ancestor were found in Ethiopia. These teeth fossils look to be the transition between apes and Australopithecus. Dating back to 4.4 million years ago, these fossils could "represent the first stage in the evolution of bipedalism." {3} "Beginning around 2.5 million years ago or a bit earlier, there was a major forking in the evolutionary path of hominins. The australopithecines diverged into at least two very different evolutionary directions. One led to the paranthropoids and a genetic dead-end by about 1.4 million years ago. The other led to the first humans. It is likely that these diverging evolutionary paths were the result of exploiting different environmental opportunities." {3}

Resources:
{1} http://www.britannica.com/EBchecked/topic/275670/human-evolution/250597/Theories-of-bipedalism

{2} http://www.animalplanet.com/mammals/chimp.htm

{3} http://anthro.palomar.edu/hominid/australo_2.htm