One of my children's history textbooks claimed that Adams' peaceful concession to Jefferson following the US election in 1800 was the first peaceful transfer of power between rival executives in written human history. I don't have the book at hand to quote exactly, due to a move of house, but does this claim (or a similar adjacent claim) have merit?
At the time of the US's founding it was generally true in Europe that the executive of a state was a hereditary monarch, and transitions between rival dynasties were violent. The governors of the colonies which became the states which became the United States were mostly appointed by the Crown before the revolution, though I vaguely recall that some governorships were locally elected. I am murky on the interval where the federal government was described by the Articles of Confederation.
I am aware (as were the Founders, whose classical education was stronger than mine) of the Roman Cincinnatus, who assumed dictatorial powers in a crisis and abdicated afterwards. But that seems materially different from Adams, who until the autumn of 1800 was campaigning vigorously to remain in power. I don't know as much about classical Greek democracy, but it seems to have emphasized the deliberative body and had a relatively weak executive.