It is probable that Edward Ketcham is the progenitor of all Ketchams in America (with the exception of rumors of a native American Ketcham line). While likely, it is only probable because two caveats have been noted: the mysterious Henry Ketcham and the "Scottish Ketchams."
A Henry Ketcham is found in Ipswich, Massachusetts, in 1638. (Savage 1990, vol. 3, p. 14.) According to the published genealogies, we know nothing further of Henry Ketcham or his descendants (are there any out there?). While the presence of Henry Ketcham is troubling for a definitive answer to the question, Henry Ketcham is almost certainly related to Edward Ketcham. If Henry is not Edward's son, they are likely brothers (it is highly unlikely that two unrelated Ketchams would be found in Ipswich together in 1635). If that is the case, the progenitor of all Ketchams (assuming Henry's descendants are in fact among us), is Edward's and Henry's father -- an as yet unnamed "Ketcham" who is the progenitor of us all!
Some of the Ketcham genealogies also allege the existence of Scottish "Ketchams," a Scottish clan whose name (in Gaelic) is spelled differently than "Ketcham" but which sounds similar. None of the books give any further information, or the source of this story.
credit: John M. Ketcham