By the way, they really only use C14 dating for up to 60,000 years, so you're talking about the wrong method if talking about the million year evolution figures.
re: fossil and geological layer. The obvious assumption is that the earth around the fossil has decayed about the same time as the fossil itself, both having been subjected to the same sorts of events. They will adjust the calculations and use the right technique for whatever type of material they are dating. They cross test radiometric dates using different isotope pairs, which removes any "one off" error or circular reasoning that you talk about. The accuracy of these techniques has been reduced to 1-2% (a thousand or so years), and they are pretty hard to argue against. Since radiometric dating was invented we now have a range of different techniques, 10's of thousands of calculations done and compared and cross-checked with other researchers in the area, and no one is coming up with 6000-10000 year figures.