Dogs were the only animal to be domesticated by mobile hunter-gatherers. Humans and wolves were both persistent pack hunters of large prey, were competing in overlapping territory, and are both capable of killing each other. One study proposes how humans may have domesticated such a dangerous competitor. Humans and wolves are members of the large carnivore guild, and when there is abundant game the top members leave carcasses for the other members to scavenge. When game is scarce there is often conflict. Humans are unusual members of this guild because their ancestors were primates, therefore their ability to process meat is limited by the capacity of the liver to metabolize protein, and they can only derive 20% of their energy requirements from protein. High protein consumption in humans can lead to illness.