Working like a dog : alternative

anyone knows the origin of "working like a dog"?

Someone asked that question here recently. Someone else answered it: basically, a sheepdog works from dawn till dusk for only room and board and affection. (From memory; I haven't found the original posts.)

In olden days, the two sawyers who worked on a tree sawing planks were the top-dog and the under-dog. A dog in those days was a manual worker.