There are many theories about how bipedalism came to be the dominant trait that would lead humans into a successful survival, but the most important of these are physical characteristics that adapted as man began to walk upright.
The Australopithecines:
The Australopithecines, whose fossils "represent the first stage in the evolution of bipedalism", had brains about the size of chimpanzees. They were shorter than us had long arms and were most likely covered in fur. Most importantly, they were bipedal. These Australopithecines were different from their primate predecessors in two ways:
1). They walked on two legs.
2). They had smaller canine teeth.
These above mentioned teeth were important because long sharp canine teeth were used as weapons and because they no longer had them they needed other means to defend themselves.
In "The Descent of Man" Darwin stated "The free use of the arms and hands, partly the cause and partly the result of man's erect position, appears to have led in an indirect manner to other modifications of structure. The early male forefathers of man, were, as previously stated, probably furnished with great canine teeth; but a they gradually acquired the habit of using stones, clubs, or other weapons, for fighting with other enemies or rivals, they would use their jaws less and less."
We see this defense mechanism in the Chimpanzee who throws sticks and stones. However, their method of throwing is not very effective. They always throw underarm which is weak when compared to how humans throw. Chimpanzees also need their forelimbs and arms whenever they (and other quadrupeds) want to move easily and quickly.
The Australopithecines, which are related to the chimpanzee, were bipeds and probably able to move as quickly as modern men. They could use their hands and arms to throw and also to handle weapons and tools. Because of this, they would have been able to keep predators at a safer distance relying less on their sharp canine teeth that adapted for eating only. Although their is no evidence that they made artificial weapons, it is supposed that they probably used stones and clubs as weapons. This gave them a competitive advantage over other animals.
Even if the Australopithecines couldn't throw as well as humans, this ability to be able to throw was a fundamental factor in bipedal survival. It may seem like a small thing, but the action of throwing is quite complex. It involves the coordinating all the muscles of the arm, wrist, fingers, back and legs. And the actual evolution of these muscles, including the brain, were probably motivated by the necessity to develop better throwing capabilities.
Comparison of Chimpanzee & Human Hands:
2 Evolved Human Grips: Precision & Power
Primates typically have a diminutive thumb and long curved fingers while the human hand has a fully opposable thumb with shorter, straightened fingers. The evolution of the hand led to tools becoming hand-held weapons. During disputes, a club was swung to settle the disagreement.
As time went on, the best throwers/clubbers rose to dominate the male hierarchy which means that had more opportunity to breed and pass on their traits. The better fighters also obtained better lands and food, which continued to increase their reproductive success. And now, humans today can throw/fight better. NATURAL SELECTION!!!
All of this clubbing led to the need for hands to be free at all times for better success. As the Australopithecines continued in their bipedality, their legs and feet became more powerful. This increased use of walking upright eventually led to habitual bipedalism.
Though bipedalism was the wave of the future, it created problems for females that they hadn't faced before was quadrupeds. This was especially true of pregnant females who experienced more weight in their abdomen that was supported firmly between four legs. Because of this, females developed stronger stomach muscles and the shape of their pelvis changed. We see this in the Australopithecines who started walking upright. Their pelvis structure changed, giving them a new center of gravity. The shape of the pelvis was needed in order to birth these new bipedal offspring as they had developed larger brains, thus making their heads larger. (A change in pelvis structure=a bigger brain/head)
This wider birth canal reduced bipedal females' agility and speed which led them to depend more on males for protection.
Change in Pelvic Structure:
Time and necessity have favored humans walking on two legs instead of all fours. As we continue to go on living, it is interesting to think of how we will continue to change and evolve. Like the dinosaur, we may find that taking to the skies is better!
Resources:
href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571064/">
href="http://evolution-of-man.info/combined.htm">
href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571064/figure/fig01/">
href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571064/figure/fig03/">
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