[briandh]

(Not the person you were asking)

It depends on your heritage (I am of northwestern European and American descent, so my records are very good) but Ancestry makes it very easy, to their credit. A few minutes after you've added a new family member, Ancestry begins suggesting historical records and public trees (I try to avoid relying on the latter) that may reveal additional information about them, including who their family members are. You then mark which information to import.

So essentially you would start by inputting the family history you know, and then let Ancestry start making suggestions. It still takes plenty of manual intervention but I've taken their suggestions back to the 1500s in some cases.