One of the most endearing things about dogs is how we can read their feelings on their faces. Turns out, it’s taken tens of thousands of years for them to develop that trait.

Our pet pooches evolved from wolves, but dogs’ faces are very different from those of wild canids. Mammals have two types of muscles: fast-twitch muscles that fire quickly but tire easily and slow-twitch muscles that take longer to react but are the heavy lifters. We have many fast-twitch fibers in our faces so that our thoughts register rapidly in our expressions. The same holds for dogs — unlike their wolf ancestors. Scientists think this adaptation favored dogs in allowing them to instantaneously communicate with humans.

Prehistoric people also favored the barking of dogs over the howling of wolves, another trait that depends on fast-twitch fibers.