A fear of snakes and spiders is relatively common among adults, even when they live in places where seeing such animals is extremely rare. Are we born this way or does the fear have a social or cultural explanation. Six-month old infants who had never met these animals and who obviously had no idea of the danger they posed were studied. The infants were shown photos of snakes and spiders and as a comparison also photos of flowers and plants. They were then shown photos of harmless animals such as kittens or puppies and then photos of dangerous animals such as rhinos and lions. Finally, they saw photos of dangerous objects such as knives, and pistols. Infants’ reactions after viewing the photos were measured combining measures of pupil diameter variation, an expression of blood noradrenalin (stress hormone), and heartbeat. Photos of spiders and particularly snakes gave a clear stress response. The researchers conclude that fear of these animals has an ancestral origin and is not acquired culturally. Through natural selection we have developed an immediate fear response towards these animals as this improved our chances of survival. We do not fear rhinos or lions because these animals have only been around a few hundred thousand years (and unfortunately are already disappearing), whereas spiders and snakes have been around for about fifty million years and so encounters with them have had the time to affect our DNA. For the same reasons, infants are not afraid of pistols or knives for now. However, in countries where arms can be bought freely and where regular gun related massacres are quite normal, maybe within a few million years infants will be scared of guns.