Too large a step in size might not be a step forward. Countless mutations continue to occur within the stock of a species when a good deal of time passes, passes without major environmental change. As more and more time passes, fewer and fewer of these mutations are adaptive. It makes sense, but no more than pedestrian sense, to suggest that such a stable environment might sometimes approximate the best of all possible worlds for a species. There is also a sense, equally un-profound, in which nature provides something approaching the best of all possible animals for each of the niches of the world.
Scientists are by no means in agreement about why and when adaptive change occurs.
In this case, the force that shifted our ape ancestor species to a new path would have been nothing less than the slow shifting of the giant tectonic plates beneath the crust of central Africa. The gradual sinking of a long and narrow strip of land produced the great Rift Valley. Along the western edge of that rift, a line of mountain peaks was slowly thrust upward. As a result, the rift and the area to its east became ever less humid. It gradually lost its forests and woodlands, and evolved more and more into grassy plains, open savanna.