@ Doun
The earliest Bryce I have is my ggg grandfather Thomas Bryce who is in the Dalry (Ayr) PR as a weaver and the natural father of John Bryce (b.1818).
John's mother, Elizabeth Chalmers, was born in 1799, so presumably Thomas was of a similar age. There are no Bryce baptisms in the Dalry PR before John's, and apart from a farmer in Maybole, no Thomas Bryce baptisms in Ayrshire. So he's an incomer probably born 1795-1800.
There are 26 Thomas Bryce baptisms between 1780 and 1804 in ScotlandsPeople. I've eliminated 13 as potential candidates due to wrong occupation, early death, emigration, or marriage in home parish. He could very well have been Irish.
Rich