Regarding when the first bipedalism occurred, there are claims of about 7 million years ago, about 5-6 million years ago, and about 3.6 million years ago.
The University of Munich paper below challenges the notion that the function of the second bounce in the human foot is not clear, and that it is unusual to have a 90-degree angle between the foot and leg.
Phys. Rev. E 106, 064405 (2022) - Foot function enabled by human walking dynamics (aps.org)
The answer to this problem is that "the foot tries to remain stationary for as long as possible to maintain balance, but the ankle uses that downtime to slowly build up energy for its final release, and then the second bounce of the stride, when the knee bends just before the foot drops, provides the final push needed to throw the ankle into the next step.
This is a great engineering interpretation. However, as a whole, it seems to lack insight. The functions of the front and hind feet of a quadruped have been consolidated into two feet during the evolution of uprightness. The functions of the animal's four legs were consolidated into two feet.
In time, this evolution from quadrupeds to fully upright humans is the result of many small evolutions over a very long period of time. But if you compare the beginning and end of this evolution, it's pretty revolutionary. The balancing functions of the tetrapods' tails were largely transferred to our two arms, the jumping/landing/shock-absorbing functions of the quadrupeds were transferred entirely to our two feet, the attack functions of our forepaws and teeth were transferred to our hands, and the mouth, which was responsible for eating and attacking, was virtually reduced to eating.
The key reason for this rearrangement of functions, however, is that the evolutionary journey toward full uprightness required less and less force to support the moment generated by the forward protruding head and, conversely, more and more surplus energy. Surplus energy provided the capacity to think. It is the source from which fire, tools, writing, and language became available. It happened too long in time to be called a revolution, but in content it certainly seems to be the greatest of all revolutions.
#Bipedalism, #revolution
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