We have discussed similar things in podcast a year or two ago. Being taller and leaner had advantages, we could see over the savanna grasses and we were better adapted to persistence running long distances with longer legs and a leaner build.
This Newsweek article discusses the suspected behaviorally driven 1.5 million year ago growth spurt helping us grow taller and lankier.
"An increase solely in stature would have created a leaner physique, with long legs and narrow hips and shoulders,” said lead author from Cambridge Manuel Will, according to Phys.org. “This may have been an adaptation to new environments and endurance hunting, as early Homo species left the forests and moved on to more arid African savannas."
And as we discussed when reviewing a Radiolab podcast, the taller and more hairless the person, the more surface area for heat radiation/sweating to keep us cooler during these persistence runs for big game food.
And with these growth changes, of course, our gait had to have changed.