Cave drawings
A drawing of a fish (if it is a fish) proves nothing in and of itself.
Archiologists are great at building an entire culture out of a single scrap of evidence.
'Look, we found a scrap of a jug with remains of a rabbit stew on it. This was obviously a community that depended upon the rabbit for food. They raised rabbits and ate them as part of a religious ceremony, and had an advanced pottery making trade, enabling them to create long-lasting jugs for cooking and storage...'