Now, I dont claim to be a scientist, an anthropologist, a paleontologist, nor a P.E. teacher, so simply ignore any small errors contained herein regarding the early history of humanity and mechanics of physical motion.
When the first Australopithecus suddenly found himself short of breath and made the decision to walk out of the primordial seathe fetid soup in which all manner of life was boiling out ofand onto what is now untouchable beachfront property, he did so at a leisurely stroll. As he slept the sleep of the just on the earliest, naturally occurring beach blanket (matted kelp), his vestigial gills dried into what we now call ears. On awakening, he suddenly realized he could hear, but he didnt feel all that comfortable with this new sense, or with what he was perceiving with it. Things he couldnt see were obviously though stealthily in motion. This First Man immediately became anxious, a little depressed, and, for the first time in human history, was suddenly worried if his parents had actually loved him or not. But, being unable to identify St. Johns Wort, and with a decade or more before the invention of psychotherapy, he had no choice but to rise up from the sand and walk away from the place of loud and frightening noise toward the place of slightly less noise, where stood thick stands of what we now call trees, but which would then have been called broccoli.
Upon entering the forest, he was immediately covered by teeth from head to toe. He felt great discomfort with this, so he turned and moved faster than anything ever had (that is, since emerging from the evolutionary stew 10 minutes before), until the last biting thing finally let go. This was the first time a two-legged human being had had to run. And believe him, it was no fun. No fun at all. He put saltwater on his wounds, which resulted in the first-ever human cry of excruciating pain scream. Not understanding the sensation, he tried to outrun the pain on the skin, but gave up in frustration after 300 feet (the origin of the hundred-yard dash). Alone in the world, he found himself breathless, bitten, and burning from the salt in his wounds. All these things he attributed the fast motion he had engaged in after his saunter into the broccoli.