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Every mammal started from quadruped stock. Hips are made with the spine coming in from one side, the tail in a straight line with the spine out the other, and the legs at right angles to that spine-tail line directly under the body.

The face is on the opposite end of the skull from where the spine attaches.

This had some bad implications for apes when they stood up. The tail has to go, it’s in the way of leg motion. No big loss, I guess.

With the legs forced to move under the center of a vertical body, that right angle between the spine and legs can’t remain, but it’s a lot of displacement to do. Standing apes started out with knees bent to allow the thighs to point more forward than down and the small of the back shifted forward to help conserve the straight spine-hip angle (oh my aching back!) The crouching vertical stance gave enough advantage (reaching higher fruit? seeing predators over tall grass? Or whatever) that the ones slightly better adapted got straighter legs and curvier lower backs.

At the other end, not much advantage to only up looking at the sky. To bring the face down to see ahead the top of the spine had to ‘c’ curve the opposite of the bottom of the spine leaving us with the ‘S’ curve that has made so many chiropractors rich. And the spinal opening didn’t start out under the center of gravity of the head, forcing the neck vertebra into yet another ‘c’ to at least more-or-less center the head over the shoulders (this didn’t quite work, most people’s heads are forward, off balance).

These are more or less essential adaptations to going from a horizontal lifestyle to a vertical one.

“S” spines are a result of standing up. Neanderthals have straight spines (and heads perfectly balanced by the occipital lobe). Therefore Neanderthals started out standing up. All mammals came from quadruped stock. Therefore...

Either my premises are wrong, or they ain’t from around here...