Why do we say “It’s a dog-eat dog world”?

Well the word itself comes from the Romans, in 43 BC by the Roman Scholar Marcus Tarentius Varro, he observed humanity and how the society worked. He made a remark about how even a dog will eat a dog. His point actually was that humans are less principled in the matter of destroying their own kind than any other animal. By the 16th century, the phrase became a metaphor for competition among humans in the everyday world. Specially during the industrial revolution where the work was so big.

Marcus Terentius was a humble man, he lived in the farm of his family, he supported Pompey an opposing leader against the Caesar, Pompey ended up losing and everyone on his side penalized in some way, but not Marcus. Marcus asked pardon and he was given just that.