Mutations: Captchas initially featured familiar words and typefaces but soon evolved into gibberish with distorted backgrounds. Then the words themselves began to warp and in some cases gave way to pictures. Credit: Effat Emamian and Ali Abdi

Spammers use automated programs called bots to harvest online data, so in 2000, a group of researchers created a bot deterrent called the ­Captcha--the "completely automated public Turing test to tell computers and humans apart." The first Captchas required people to type in words displayed as images on a Web page in order to access a website.

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