Arboreal to Terrestrial: Beginning approximately 9 million years ago, our ancestors began an evolutionary change from forests to woodlands to savannahs. This gradual transition from primarily arboreal (tree-dwelling) lifestyles to increasingly terrestrial (ground-based) behaviors was very gradual with traits specific for walking around carrying things gradually accumulating. This evolutionary journey spanned roughly 7 million years, culminating in species like Homo habilis, which by about 2 million years ago, had adapted to spend most of their time on the ground and carrying things like stone tools and likely walking sticks which were also used to hunt and fight off animals.

Analysis: The last common ancestor (LCA) of orangutans, which are predominantly arboreal, is dated to around 13 million years ago, whereas the LCA for gorillas, who are more terrestrial, dates to about 8 million years ago. This timeline suggests that around 9 million years ago, the ancestors of modern humans and other great apes began to adapt to life on the savannahs and open landscapes of Africa. As evolutionary paths diverged, each lineage developed distinct adaptations that favored survival on the ground—ranging from changes in limb structure and gait to the development of tools for defense and foraging. By the time of Homo habilis, evidence strongly indicates a lifestyle that was predominantly terrestrial, with occasional returns to the trees for foraging or safety.