Cockroaches wouldn't exist without humans. We helped them become one of the world's worst pests, according to a new study.

• The 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. The German roach is the most common of the 70 different cockroach species in the US.

For 250 years, scientists didn't know where it came from and how it managed to spread to every continent on Earth except Antarctica. Now that mystery has been solved, and the answer is that it's largely our fault.