How fast could a hominid evolve to run?

This is question is related to the medieval world from my earlier question in which several types of intelligent hominids evolved in isolation, until they were rediscovered, conquered, and bred into a biological caste system by a dominant species.

BACKGROUND

To go with my farm-laborers I'd like a warrior hominid that can run incredibly fast. Ideally, they might end up in a military role comparable to light cavalry; moving rapidly over the battlefield, skirmishing where needed, and then breaking off. (Edit: They would prioritize speed over endurance, while still being able to run and fight long enough to be useful before tagging out.)

But first there are some obvious problems with this that need working out, which is why unlike the last time I'm devoting this entire question just to the issue of speed. First, bipedalism and a faster-than-human running speed isn't a common evolutionary strategy, so I need to justify that. Second, they also still need to be viable as close combat soldiers, so I'd like them to be tall and strong. Depending on the physique, this may not be conducive to speed.

I don't have a good grasp of the mechanics, but I anticipate one solution might be for them to drop to all fours while running. If so, that would be an workable but imperfect fix, because it creates all kinds of problems for holding weapons, especially large ones like spears. Happily, there are two large, fast, and 100% bipedal runners I can think of from the animal kingdom: the ostrich and the kangaroo. But I'm not sure what lessons there can be applied to a recognizably hominid physique.

QUESTION

• How fast could a human sized or greater intelligent hominid evolve to run? How would its physiology differ from our own? As a bonus: In what circumstances might such a creature evolve? Could they become fast enough to be a feasible competitor in a world with cavalry, or would I have to remove fast-moving mounts like horses from the world to make them more effective?

posted over 6 years ago

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