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.

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.

The German cockroach is "a creation of human-made environments," Edward Vargo, an entomology professor at Texas A&M University and co-author of a new study identifying the roach's origins, told The Washington Post.

The German cockroach can't survive in "temperate winters outdoors" and all species of cockroaches "prefer warm, moist places where they can feed on human and pet foods, decaying and fermenting matter, and a variety of other items," according to the Illinois Department of Public Health.

That's why it's pretty safe to say that if human-built establishments like houses, stores, restaurants, and other buildings didn't exist, neither would these pesky pests.

The researchers published their results last week in the peer-reviewed journal Proceedings of the National Academy of Sciences.

Where did the German roach come from?

Scientists have long understood that the species thrive indoors, but their origins remained a mystery.

Through DNA analysis, Vargo and his colleagues found that the species' closest relative is the Asian cockroach.

The German roach evolved from its Asian cousin about 2,100 years ago to adapt to "human settlements in India or Myanmar," the researchers reported in their paper.

With advancements in transportation and "temperature-controlled housing," the German cockroach made its relatively recent global spread, the researchers said in their report.

How the German cockroach took over

The German cockroach's adaption to warm environments, ability to rapidly breed, and unique resistance to insecticides make them a frustratingly common presence in households.

For example, in a lifetime, one adult female German roach can produce four to eight egg capsules containing up to 48 eggs each, according to Penn State's Department of Entomology. Do the math and that's between 192 to 384 roaches, if every egg survives to adulthood.

But German roaches didn't migrate thousands of miles across oceans and continents on their tiny insect legs. Their global spread coincides with advancements in human travel and housing, according to the study.

In particular, the researchers determined that the German cockroach's spread began along two routes, west and east of its origin in India or Myanmar.

The roach's westward spread likely occurred during times of increasing "commercial and military activities of the Islamic Umayyad or Abbasid Caliphates" about 1,200 years ago, the researchers reported. Meanwhile, the pest's eastward spread about 390 years ago was likely caused by "European colonial commercial activities between South and Southeast Asia."

Understanding the German cockroaches' origins could help other scientists understand how the species evolved to become so resilient against common insecticides. One study found that they're resistant to five types of common household insecticides.

"If we can know the origin of the species, we can try to identify the mechanism of this rapid evolution of insecticide resistance," Qian Tang, a research associate at Rowland Institute at Harvard who led the new study told the Post.