Dog eat dog

In the year 43 BC, Roman scholar Marcus Terentius Varro observed humanity and remarked that even “a dog will not eat dog.”

His point was that humans are less principled in the matter of destroying their own kind than other animals.

By the sixteenth century, the phrase became a metaphor for ruthless competition.

And during the Industrial Revolution the expression “It’s a dog-eat-dog world” became commonplace. (source)