The smell of a certain molecule in blood makes humans and mice recoil in horror, but drives wolves and blood-sucking flies into a ferocious frenzy. It’s a rare example of a scent signal that has the same effect across distant species. The molecule E2D forms when blood is exposed to oxygen in the air, and it has a metallic smell—interpreted as a warning by some species, and as a sign of food by others. A study by the Karolinska Institute in Sweden, published in Scientific Reports, isolated the molecule from pig blood and studied reactions to the scent across species. When wolves were given a piece of wood smeared with the molecule, they acted as they would with a fresh kill—licking, biting and guarding it. Humans, given a whiff, would unconsciously act like prey: lean back on their heels, sweat and become stressed. The study’s authors say the molecule probably evolved when humans were most similar to small, peaceful, insect-eating primates—before Homo Sapiens began to hunt.