Good Morning, Robert,
I haven't been researching the Glenn lineage lately, but your inquiry is definitely of interest. My wife's mother was a Glenn and her lineage extends to a period of 1725-1754 wherein her sixth great grandfather, James Glen (1699-1777), was born in Stevenston, North Ayrshire, Scotland, He married Elspa Craig (b. 1705) on 9 March 1727 in Beith, Ayr, Scotland.
They had five children, one daughter and four sons: Jean Glen (b.1719), James Glen (b.1725), Robert Glen (b. 1738), David Glen (1754-1830) and Francis Glenn (1758-1831). It is notable that the surname would be spelt with or without a double "n". it seems that this family remained in Scotland throughout their lives. But it also appears that their third son, David Glen, may have left Scotland to emigrate to the British American Colonies, specifically South Carolina. Certainly, he married Elizabeth Smith (b.1730) of Laurens, South Carolina.
When I began researching the Glenn(Glen)(Glynn) lineage, two issues became obvious to me. First, Family Search had extended the lineage, most probably by others, clear back to Duncan MacDuff I, Earl of Fife (967-1060). Richard de Glen (1232-1291) was the last "recorded" Glen, but his father was John de Ness (b. 1180). The surname of Glenn does not appear again and this leads me to my second point.
As is in the case of both James David Glynn/Glen and Isabell Renfrew Glynn/Glen, there is an apparent lack of confirmed sources to verify this and older generations of the lineage. I am aware of the scant data that will require further examination of existing records and have been doing this on My Heritage. Others will use Ancestry, but My Heritage has been my primary source for the moment regarding other lineages that are in our family genealogical record.
To your point, there is nearly a ninety-three year difference between David Glen (154301598) and James David Glen (1450-1590). If that is the case, then how can one be a brother of the other?
Thanks for your inquiries,
Roy Edwin Atherton