A German cockroach on a piece of bread.Schellhorn/ullstein bild via Getty ImagesThe German cockroach is one of the most common household pests worldwide.New research found that the species evolved to thrive in human dwellings about 2,100 years ago."It's a creation of human-made environments," a researcher told the Washington Post.If you ever saw a cockroach scuttling across your kitchen floor or a restaurant wall, chances are it was a German cockroach.