< Latin ancillāri-us (more correctly ancillār-is) of or pertaining to a handmaid, < ancilla: see ancilla n.
1. Subservient, subordinate, ministering (to).
2. lit. (after Latin.) Of or pertaining to maid-servants. rare and affected.
3. Designating activities and services that provide essential support to the functioning of a central service or industry; also, of staff employed in these supporting roles. Now used esp. of non-medical staff and services in hospitals.
1.†a. One who acts as an assistant or servant. Obs.—1
b. An ancillary worker. See sense A. 3.
2. Something which is ancillary; an auxiliary or accessory.
Robert Frederick: And why are researchers so interested in this?
David Grimm: Well, Rob, one of the things was just to really settle this debate about where the moms came from. But, sort of an ancillary finding here is this whole idea that we’re actually learning a lot about human culture by studying these horses.- 8 October 2010 Science podcast
Not quite sure if "ancillary" is really the word he wanted there: "auxiliary" probably would have been better, or even "additional".