What’s the origin of the term ‘top dog’?

‘Top dog’ means one who is dominant or victorious. When wooden planks were sawn by hand, two men did the job using a two-handed saw. The senior man took the top handle, standing on the wood, and the junior took the bottom, in the saw-pit below. The irons that were used to hold the wood were called dogs and that the bottom position was much more uncomfortable. The term ‘top dog’ originated from this practice.