Lampreys offer clues to the origin of our fight-or-flight instinct.

Lampreys look like something out of a horror movie, with their sucky mouths chock full of teeth, eel-like bodies, and parasitic behaviors. These “water vampires” represent a bit of an evolutionary fork in the road between vertebrates and invertebrates, and the scientific debate about just how closely related we are to these carnivorous fish has taken yet another turn.

Scientists found some evidence that lampreys have a rudimentary sympathetic nervous system–which is believed to control the fight-or-flight reaction in vertebrates. The findings are detailed in a study published April 17 in the journal Nature and could prompt a rethink of the origins of the sympathetic nervous system.

Lampreys are the closest living organisms scientists have to studying the fish ancestors that vertebrates evolved from some 550 million years ago. They belong to an ancient vertebrate lineage called Agnatha–or jawless fish. Some scientists believe that they represent the earliest group of vertebrates that is still living and can give us an evolutionary window into all vertebrate ancestors. Other scientists question the theories due to a lack of lamprey evidence in the fossil record.