"Female orang-utans are also brachiators - they are just less acrobatic about it than gibbons." (http://dml.cmnh.org/1995Nov/msg00347.html)
Brachiators began as four-footed monkey-like creatures in the Tertiary Era in Africa and Northern Europe. Eventually, the some of the monkeys began to use their arms to swing, and lost their tails, due to evolution. Without a tail, they ceased to be monkeys, and became apes with strong arms. Through the ages, the ape-like ancestors developed stronger arms and the shoulder blades moved from the side of their chests to the back of their bodies.
Most of these brachaiators were smaller than average apes, so were able to move through the trees easier than gorillas or orangutans, although female orangutans do brachiate through the trees occasionally. The brachiators held their bodies upright in the trees, and would sometimes go on the ground and walk upright. This helped them survive in the plains when the forests began to die, because they were so unfamiliar to the predators that they would not be attacked.