Scientists have zeroed in on a molecule in blood whose very scent triggers alarm in humans but also beckons predators. In fact, one whiff of the E2D molecule causes animals of all kinds to act in "diametrically opposite" ways, suggesting that it's been around for eons and responses to it are hard-wired into the systems of everything from humans to horseflies, reports AFP. The molecule is believed to give blood its metallic odor, and researchers writing in Scientific Reports showed that it's E2D and not the blood itself that seems to cause such widespread reactions. In one experiment, they coated wood with a synthetic version of the molecule and found that wolves licked the wood and protected it as if it were fresh prey. Horseflies that feast on blood were similarly drawn to it.
Mice, on the other hand, were immediately freaked out and tried to flee. Researchers also tested the effect on humans, finding that people "recoiled" when presented with the scent, and their hands sweated more, per Healthworld. (Participants were unaware of the scent's connection to blood.) What's more, it turns out that humans are particularly good at picking up on the scent E2D—they can do so in less than one-part-per-trillion, with the typical human capacity in the parts-per-million or -billion range. "E2D seems to activate our entire general defense system," says one of the Swedish authors. "Modern humans are without doubt predators, but we probably evolved from a prey species, and some aspects of this characteristic remain." (In a bizarre medical case, a woman can't stop sweating blood.)