The major orders of mammals developed at this time, including the Primates to which humans belong. Other primates include the tarsiers, lemurs, gibbons, monkeys, and apes. Although we have significant differences from other primates, we share an evolutionary history that includes traits such as opposable thumbs, stereoscopic vision, larger brains, and nails replacing claws. Approximately 20 million years ago central and east Africa was densely forested. Climatic changes resulting from plate tectonic movements and episodes of global cooling about 15 million years ago caused a replacement of the forest by a drier-adapted savanna mixed with open areas of forest.
Many primates can also swivel or turn their arms at the shoulder.
These two adaptations offer advantages to life in the trees. Primates have five digits on their forelimbs. They are able to grasp objects with their forelimbs in what is known as a prehensile movement. A second modification makes one of the digits opposable, allowing the tips of the fingers and thumb to touch. Placement of the eyes on the front of the head increases depth perception, an advantageous trait in tree-dwelling primates.