Modern chimpanzee	Homo sapiens

The question is why and when did this happen?

Our ancestors began walking on two feet about 7 million years ago, while some archosaurs started doing so about 240 million years before them.
Later, some bipedal theropod dinosaurs evolved into the first birds, about 150 million years ago.
Birds are best known for their flying abilities, but they spend a significant amount of time on the ground, walking on two feet.
Other theropod dinosaurs, like the tyrannosaurs, were also bipedal.
Bipedalism has evolved several times in archosaurs and four times in mammals, notably in kangaroos.

Knuckle-walking is a quadrupedal mode of locomotion used by some African great apes (gorillas, chimpanzees), as well as other mammals like anteaters and platypuses.
This is another example of parallel evolution; which is the development of physical similarities in different species.
To knuckle-walk, they support their body weight on their forelimbs (arms and hands), by walking on the curled-up knuckles (middle phalanges) of their front paws.

Knuckle-walking is very uncomfortable.
The skin on the upper side of the fingers of mammals is very different from the one on the palm side.
It is thin, and very sensitive.
This skin wasn't conceived to bear the body's weight.

The idea of a slow evolution of bipedalism is not plausible.
Walking upright is not something you do gradually.
You walk erect, or you don't.
Yes, the human body took millions of years to adapt to bipedalism.
But, these changes occurred in response to a sudden, initial shift in behavior.

Yet, chimpanzees and gorillas had been using knuckle-walking as a type of locomotion for several million years without any problems.

Then, about 7 million years ago, some chimpanzees decided to start walking on two feet.
This event marked the beginning of the human race.

After more than twenty years of research, I've come to the following conclusion:

One day, some chimpanzees started walking on two feet as a joke, and others followed suit.
Bipedalism became the new trend, the new fashion.
You had to walk that way to be accepted by the rest of the group.

The extra height gained by walking upright gave bipedal chimpanzees a certain dominance and authority over the other chimps.
They began feeling superior, and considering regular chimps as animals.

Every expert will tell you that it was to free their hands that the chimpanzees started walking upright.
But, while studying chimpanzees, I realized how similar their behavior is to humans.
Humans are jokers, pranksters and show-offs.
They'll do anything to prove their superiority, and attract attention.
So do chimpanzees.

Fashion has always been part of human culture.
It seems that humans inherited this behavior from our chimpanzee ancestors, because even modern chimpanzees follow trends.

This is just one example of outlandish behavior by some chimpanzees.
However, the other members of the chimpanzee community had to adopt them to avoid being rejected.

Our foolish ancestors didn't give much thought to the consequences of their actions.
That's probably because the change affected the female chimpanzees much more than the males.

Female chimpanzees carry their offspring on their backs.
Newborns quickly learn how to grasp their mother's fur while she moves around.
Young chimpanzees hitch a ride this way until they're able to walk on their own.
They remain in skin-to-skin contact with their mother for an extended period of time.
This creates a bond that no baby stroller can replace.

When the female chimpanzees began to stand upright, the babies couldn't cling to their mothers' backs anymore.
The mothers had to carry them in their arms.
You can't call that "liberating the hands".

But, the females really needed their hands to forage for food.
So, they had to put the baby down on the ground, while they were at work.
The young chimp/human would not participate in the search for food, and would not be taught how to select it.
But, the baby would often disappear from its mother's sight, and she would become anxious very fast.
I'm convinced that the mothers tried to stay in touch with their baby through sounds.
They would utter basic syllables, and prompt the baby to respond, to maintain vocal contact.
This is what led to human language.

The savannah hypothesis proposes that human bipedalism evolved as a direct consequence of our ancestors' slow transition from an arboreal lifestyle to life on the savannas.

• 1809 - Jean-Baptiste Lamarck proposed that an early ancestor might have abandoned an arboreal life for open plains, leading to an erect posture through generations of use.
• 1871 - Charles Darwin, in his work "The Descent of Man" envisioned early humans leaving the forest for the grasslands, suggesting bipedalism would free their hands for other purposes.
• 1924 - Raymond Dart argued that the transition to an open, competitive savanna environment in Southern Africa was essential for the evolution of bipedalism, and higher intellect in humans.

As you can see, the Savannah theory has been around for some time.
It remains the preferred hypothesis among paleontologists.
It is deeply ingrained in the collective imagination as being the cause of our bipedalism.

• 1974 - Orrorin tugenensis; this fossil belongs to an early human that lived approximately 6 million years ago, who was likely bipedal on the ground, but remained an adept tree climber.
• 2001 - The remains of Sahelanthropus tchadensis, the oldest known representative of the human lineage, which lived approximately 7 million years ago in West-Central Africa, suggest that it may have been capable of walking upright.