Humans Walk Upright to Conserve Energy

No... just because something is more optimal does not mean evoluion makes it so. Evolution is a "good enough" system. If an animal can survive and breed, it is good enough. Humans needed to conserve more energy mainly because our brains were evolving and cost a lot more energy. We started walking upright as a way to save a lot of energy for our brain.

Also, energy efficiency measured on flat terrain is what matters here. Humans evolved from a group of hominids that left the forests for the savanna. With flat ground and no need to climb and jump from tree to tree, bipedalism worked better. Those animals with larger brains could think their way out of problems and survive better so those animals who could use less energy in an ambulatory means were at a better competitive advantage.

Also, lets look at the fact that we are looking as semi-bipedal creatures versus fully bipedal ones. semi-bipedalism is very bad for moving across flat ground, but when you have to rush into a tree to go after food, semi-bipedal ambulation is far superior.

Now, what about all those animals that are out on the savanna already? those animals are built for long distance running or high speeds, or weigh an enormous amount, or are very tall... Four legs offers the best methods for each of those conditions. strength, stability, bursts of speed or long distance endurance.

Well.. I see we mostly agree after reading more carefully. I think I made my point clear however as to why energy efficiency for the human condition in nature was important.

I still contend that if energy efficiency was the reason for human bipedalism, we'd be seeing a lot more bipeds out there. I suspect it had more to do with our history as a vision-centered predator, and the adaptation of the brachiating hand into a carrying/manipulating hand.

The test would not be to compare a semi-arboreal animal's efficiency with a human's, but rather to compare a quadruped's efficiency to a human's. The fish-nor-fowl aspect of the semi-arboreal animal is likely to make it less efficient overall, since it cannot specialize one way or the other.

I bet that four legs or two legs are the most energy efficient for savanna. Given that, and the fact that we were making the move to a carrying/manipulating hand on the savanna, so four legs was out of the question and at the same time, we needed more energy for our huge brains.