Heavy objects could be moved easier if something round, like a log was placed under it and the object rolled over it. Logs or sticks were placed under an object and used to drag the heavy object, like a sled and a wedge put together. Later, humans thought to use the round logs and a sledge together. Humans used several logs or rollers in a row, dragging the sledge over one roller to the next. With time, the sledges started to wear grooves into the rollers and humans noticed that the grooved rollers actually worked better, carrying the object further.