I think I've found a theory that we may be able to prove using DNA research.
Actual records of the first Joslin seem to be found in the UK using the Domesday Book. Family traditions tell us he came to England in 1066 with William, but beyond that we assume he was from the village of Josselin in Brittany.
My theory after doing extensive research and using the documents found in the Companions of the Conqueror and the Falaise Roll, is that eh very first Joslin was Joscelin Crispin of Tillieres France.
I long ago had an old document that linked our first ancestor to the Abbaye du Bec and some that even supposed he was the same person that founded the Abbaye.
The Abbaye was founded by Herluin Crispin and in the Falaise Roll he is documented as having brothers, William, Robert, Gilbert, Milo and Joscelin. Herluin stays at the Abbaye, Milo and the others show up in the Domesday Book. All except Joscelin are accounted for. BUT there is an entry for Joscelin of Brittany. My theory is that Joscelin of Brittany (our first recorded Joslin) is one and the same as Joscelin Crispin.
The names, dates and history make perfect sense as do the parallel naratives of the Priests that would follow Herluin. Herluin's Gilbert Crispin brother succeeds him at Bec and becomes an Abbott of Westminster. St. Gilbert of Sempringham is the son of Joscelin of Brittany, making him the nephew of the other Gilbert.
If this information is correct, it puts our ancestral home at Tillierese and not the village of Josselin. Although the name is the same, there is no proof whatsoever to tie the surname with the village in Morbihan.
I'd be very curious to see if we could find a company that would help us in this search. We'd need several males from both the Joslin and the Crispin families to see if there is one common male ancestor according to Paternal DNA.