[KenProspero]

Humanity is extraordinarily clever, using the tools that are provided to mold our environment.

Horses provide an incredible mix of speed and are incredibly versatile. However, I think there is little that is about them that is unique that would effect civilization in ways you claim.

For example, Hannibal crossed the Alps with Elephants in his battles with Rome. In agriculture, Oxen can be used to drag plows at least as well as horses. Oxen have also been yoked together to draw wagons -- i.e., used for transportation.

The Horse's main advantages are speed and agility. However, it is questionable whether speed is essential for societal evolution. In fact, one could argue that the speed and agility of Horses is incredibly useful in war, and that the destruction of war has hampered humanities progress.

In any event, though Horses are incredibly useful for many purposes, I don't think they are unique in enough that it would have hampered civilization significantly. My guess

12,000 years of progress with a horse -- 6,000 - 15,000 years. The shorter time because of less war potential (though man is so inventive that we probably would have come up with alternatives), so most likely 13,000-15,000 range.

Two more points -- I don't think Zebras are rideable in the same way that Horses Are.

If we're looking for evolutionary developments as evidence of God, I think that a better argument can be made for the opposable thumb than horses.