Uninvited dinner guests are the worst, and that’s especially true when those “guests” are cockroaches. Not that these skittering insects care at all. They've been joining us at suppertime for over 4,300 years, as fragments of pottery found in Japan recently revealed.

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Pottery fragments unearthed in Japan were found to contain impressions of cockroach egg cases. Inset: A scanning electron microscope (SEM) image of one egg case replica. Image: Hiroki Obata

While the style and artwork of ancient pottery can speak volumes about bygone eras, potsherds (unearthed pottery fragments) also offer up much tinier clues. These broken artifacts often contain impressions of seeds, shells, nuts and other organic material, says Kumamoto University archaeologist Hiroki Obata in a press release. All this helps us to see the past in more detail – detail as small as a cockroach egg.

When roaches are around, their egg capsules are the telltale clue, and in this case, finding them came as a surprise. Obata and his colleagues found the hidden hints while examining earthenware fragments found at Motonobaru, an archaeological site known to date back over 4,000 years.

After making a silicone cast of each fragment, the archaeologists examined every little divot and pockmark with a scanning electron microscope in order to see them in extreme detail. And on several of the pieces, the researchers found impressions of cockroach egg cases, leading them to the super-gross conclusion that the insects had laid their eggs right into the pottery.

The archaeologists were even able to pinpoint the culprit precisely: the eggs belonged to a smokybrown cockroach. Today, the species is found in the United States as well as Japan, but 4,000 years ago, it was thought to have been restricted to southern China. The pottery finds, however, suggest otherwise, meaning that the insect had made its way to Japan millennia earlier than previously thought.

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A smokybrown cockroach (Periplaneta fuliginosa) laying an egg capsule. Image: Toby Hudson, Wikimedia Commons

In fact, insects specifically identifiable as smokybrown cockroaches first appeared in Japanese literature and art in the 18th century, but this new discovery hints that the species might have crept in long before.

Such are the stories contained by our dishware. “Like little time capsules, potsherds are packed full of treasures which help to reveal the story about the living conditions of ancient humans,” Obata says.

Meanwhile, you might want to check your own kitchen to make sure cockroaches aren’t still keeping up their own time-honoured traditions of reproducing where you eat.