A 2024 Harvard-led study collected 281 German cockroaches from 17 countries across 6 continents and analyzed their DNA; researchers found the pest evolved from an Asian cockroach about 2,100 years ago before hitching rides with humans around the world

For more than two centuries, the German cockroach has carried a misleading name. The insect was first formally described in Europe in the 18th century, leading to the long-standing assumption that the species originated there and then spread around the world. But its closest relatives are found in Asia, leaving scientists with a basic evolutionary puzzle: How did a cockroach apparently associated with Europe end up having such strong Asian roots?

A 2024 study published in Proceedings of the National Academy of Sciences (PNAS) set out to answer that question using genetics rather than historical records. Researchers collected 281 German cockroaches from 57 sites across 17 countries on six continents, then compared their genetic markers to reconstruct the species' ancestry and global movements.

“The German cockroach can’t even fly,” Qian Tang, lead author of the study and an evolutionary biologist at Harvard University, tells National Geographic. “They hitchhike in human vessels around the world.”

The cockroach's journey began about 2,100 years ago

The researchers found that the German cockroach, Blattella germanica, evolved from the Asian cockroach, Blattella asahinai, roughly 2,100 years ago. Their genetic analysis suggests that this transformation probably happened as the ancestral cockroaches adapted to living around human settlements in India or Myanmar, rather than in Europe's cities.

That discovery helps explain why the German cockroach is now so closely associated with human buildings. The researchers believe two domesticated lineages emerged, one associated with agricultural or peridomestic environments and another adapted to living inside buildings.

In other words, humans did not simply give the cockroach places to hide. Human settlements may have helped create the environment in which this particular species emerged.

Scientists read the cockroaches' genetic fingerprints

To reconstruct that history, the researchers looked at DNA from the insects. They sequenced the mitochondrial COI gene from 53 individuals and also generated genome-wide data from the full set of 281 cockroaches. The genome-wide analysis identified 158,216 single-nucleotide polymorphisms, enabling the team to compare genetic differences among populations worldwide.

They then used population-genetic tools to identify ancestral groups, examine how populations were related and test different scenarios for how the cockroaches could have spread.

The genetic picture revealed surprisingly little separation between many populations, consistent with a species that has spread relatively recently across the globe.

Two major routes took the cockroach around the world

The researchers reconstructed two major expansion routes. The first was a westward movement toward the Middle East about 1,200 years ago. The authors suggest that growing commercial and military activity during Islamic dynasties may have facilitated cockroaches' movement with people and supplies.

The second was an eastward expansion about 390 years ago, which the researchers associate with European colonial-era commercial activity between South and Southeast Asia. The species eventually reached Europe roughly 270 years ago, a timing that matches historical records from the 1760s.

Humans may have been the cockroach's greatest mode of transport

The study suggests that the German cockroach's worldwide success was closely tied to human technological and commercial expansion. Once humans developed better long-distance transportation and temperature-controlled buildings, cockroaches gained more opportunities to survive journeys and establish themselves in new places.

The researchers' conclusion turns the familiar story of the German cockroach on its head. It did not begin in Germany and conquer the world from Europe.

Instead, the evidence suggests that a cockroach lineage that emerged around human settlements in South Asia spent roughly 2,100 years hitching a ride with human civilization, eventually reaching Europe and then spreading across the globe.