The answer is a lot of time, actually 65 million years of time.
Humans took a lot longer than 65 million years to evolve.
When the dinosaurs were around, there were also small burrowing mammals, which we evolved from. The burrowing allowed them to survive when most everything on the surface died. They had already evolved most systems we have today - such as respiratory, circulation, muscles, brain, etc. The last 65 million years just "shaped" the human body.