Hominoids (a superfamily that includes modern apes and humans) split off from old world monkeys about 20 million years ago. The most likely common ancestor is Proconsul.
And before you go crazy because Alter Kaker and I gave you two different answers, my answer is considered the most recent ancestor to homo sapiens and Alter Kaker's answer is the most recent ancestor shared my homo sapiens and monkeys.
They are essentially two different answers to two different questions. Because I want to preempt any complaints.