DNA analysis via Family Tree DNA shows ancestors come from Fordell (County
Fife, Scotland) and the family is part of Clan Henderson.

There are also persons named Henderson in Clan MacDonald and in Clan Gunn, but
they do not share any Henderson ancestors with this family.

The name Henderson was formerly Henryson, Henrysoun, and Henrisoun.  There are
dozens of other variations known, though not necessarily used in this family
line.

James Henryson/Henderson built Fordell Castle in 1450.  He is presumably
a relative.

We know Robert Henderson was married in 1826 in Edinburgh, which is just
across the Firth of Forth from Fordell.  He was born in Scotland but not in
Midlothian (where Edinburgh is), so it is likely he was born near Fordell.

Surnames came into use in the 1500s and 1600s.

I found a line in a Wikipedia article that seems to imply that there were
members of Clan Henderson, who would be ancestors of this family, living in
Fordell in 1217.  It gives no source for that, though.  It just states that a
charter referencing Fordell land was part of the "Henderson of Fordell" papers
and hyperlinks "Henderson of Fordell" to the Clan Henderson article.

Robert Henderson (1834) and his children emigrated from Edinburgh to the USA
in 1881.  They children and his sister Isabella (1830) were on the ship
Circassia from Liverpool to Boston.  We've seen the passenger manifest.  We
assume Robert came ahead, but we don't know when.

Bryan has a copy of a letter Robert Morgan Henderson sent to Lois Henderson in
1987 with many details of family history as he remembered it.  It is with
other papers concerning RMH's life and listed in Bryan's property index.

The file Henderson_haplotree.jpg came from a genealogist working with the
Henderson surname project of FamilyTreeDNA in 2017.  He sent it to Bryan after
Bryan upgraded his DNA test with Family Tree, which somehow allowed him to
place Bryan's family in the tree.

FamilytreeDNA's analysis of Bryan's DNA says his ancestry is

  82% England, Scotland and Wales
  10% central Europe
   8% Scandinavia

Bryan had lunch with several descendants of Ronald A Henderson (1914) (a few
second cousins once removed and a third) on July 14, 2006 at one of their
homes in San Jose.  Several members of the family had gathered and they had
heard that a cousin was living in San Jose and looked Bryan up in the phone
book.  Richard Douglas Henderson (1855) is the one who called.  Harold (1948),
Lois (1953) and Peter (1991) were there.

Bryan met Earl Henderson Jenner (~1928) in about 1983 on the University of
Washington campus.  Earl worked for IBM and Bryan was interested in working
there, so Bryan's father told him that Earl was coming to campus to recruit
and suggested Bryan meet him.  Bryan got a few minutes of his time at the end
of his day.  Earl said nepotism rules prevented him from helping Bryan in his
quest to work for IBM, so they just talked.  By the time Bryan was ready to
apply for a job at IBM, in late 1985, Bryan's father told him Earl had already
retired.