Another theory is that people afraid of spiders were conditioned to feel that way by a traumatic experience with early in life (via The Conversation). If you're arachnophobic, think back to the very first time you remember having a fearful response to a spider. If you can actually remember the first time, you may be able to unpack what happened to make you feel that way. Perhaps you were bitten when you were a child or you pulled back your sheets to see a huge black spider crawling around where it didn't belong. Trauma literally rewires your brain and can cause anxiety and panic surrounding situations or, in this case, animals, that were involved in the traumatic event. Realistically, this could even be watching a movie or TV show in which spiders were portrayed as terrifying, and it stuck with you (remember "Arachnophobia" from the 90's? We bet that movie messed a lot of us up!).

Interestingly, if you are of European descent, you are more likely to be afraid of spiders than people whose families originate in other continents like Africa, Australia, and Asia (via Insider). This may be because in Europe, spiders were once incorrectly believed to carry plague and other diseases that they in fact do not carry. This ancestral fear may literally be wired into you. 

So next time you scream when encountering a spider, you now have a reason to give the people you startle.