While at Parkminster recently, I had the opportunity to consult some of their books in the library. I came across a fascinating 3 volume work (folio, vellum bound) by J Bona from 1747, entitled Rerum Liturgicarum. It is a treasure trove of comments and references on the rite of Mass and on liturgical matters in general.
In Volume III, p.368ff., the author discusses the question of communion in the hand, giving more copious references than I have seen before to the early existence of the practice. He also traces its decline, saying that it probably ceased before Gregory the Great (d.604). In support of this, he cites a story in which St Gregory gave communion to a dumb man who miraculously recovered his speech. He gave communion into the mouth and this is not commented on as an innovation. (Gregory I Dialogues 3. seu 9 cap.3 PL 77.223/224)
He also cites the Council of Rouen (c.650) under Clovis, King of the Franks, which forbade communion in the hand. Earlier than this, the Councils of Saragossa (380) and Toledo (400) had stipulated that the sacred host must be consumed before the communicant left the Church. An early practice was for people to take Holy Communion to the sick at home. This had led to abuses and needed to be corrected.
There is a much-quoted text of Cyril of Jerusalem (d.387) speaking of the left hand as a throne for the right etc. (Mystagogical catechesis 5.21; PG 33.1125) This is often used as a justification for communion in the hand. The contemporary evidence of the correction of abuses shows that the text could equally be seen as an indication of the obvious need for a change in practice to ensure reverence. The insistence on Communion on the tongue was a natural next step.
Yet another case of something presumed "medieval" that was actually much earlier.