I posted a thread recently about my grandmother, who was adopted. I found that her birth certificate recorded the names of both her natural parents and I was able to work back from there. Here in Scotland there are only statutory death records from 1855 and birth records before that date normally only record children who were baptised so it can often be difficult to go back very far. I can only get back to about the late 1700's with my own ancestry from the town of Paisley where there are a lot of births and marriages not recorded and for those that are the information is often too sparse to confirm it relates to the person you are looking for, and not someone else of the same name, but as my wife's ancestors came from a rural area I have been able to trace some of my wife's lines back about a further hundred years.
Ancestry.com can be good if it finds a link to someone else's research which has brought them back to a common ancestor with you, but is not always reliable as some of them appear to have relied on a lot of guesswork.