Starting about 24 million years ago, the rain forests in East and Central Africa dwindled and became patchy. Apes that had spent almost their entire lives in the forest canopy had to walk from tree to tree.
Early human ancestors probably spent more time feeding on the forest floor, the researchers say. Compared to balancing on a springy branch, walking on the ground would have been easy.
The ancestors of chimps and gorillas, on the other hand, became more specialized at going up and down trees — holding on with both their hands and feet. They kept that same posture on the ground, which is why they often knuckle-walk, supporting their weight with their hands as well as their feet.It has been long thought that humans evolved because they alone could exploit the forest fringe environment while the monkeys took over the savannah and the higher apes took over the forest.