How would an animal ever accumulate enough merit to warrant rebirth as a human?

i have only a basic knowledge of Buddhist thought but have hit a problem that i have not been able to locate an answer to. so any pointers would be appreciated... everything i read suggests that to be born as a human being is very propitious as it allows the degree of reflection/mindfulness required to be able to understand the teachings and make progression towards enlightenment. but if these fortunate circumstances are wasted and actions in your current life result in rebirth in a lower form, there seems no way that i can see to progress even to another human birth, let alone anything else, if progress is predicated on awareness/mindfulness/ability to understand the teachings etc how would something such as, for example, a dog ever be able to accumulate enough merit to warrant rebirth as a human? i have a dog and all he wants is food and walks, he never seems that reflective. seems like a game of snakes and ladders in which - other than when playing as a human - its going to be all snakes. thanks for reading.